Action Potential Generator Circuit

Few biological cells are as interesting to the electrical engineer as the neuron. Neurons are essentially capacitors (with a dielectric cell membrane separating conductive fluid on each side) with parallel charge pumps, leak currents, and nonlinear voltage-dependent currents. When massively parallelized, these individual functional electrical units yield complex behavior and underlie consciousness. The study of the electrical properties of neurons (neurophysiologically, a subset of electrophysiology) often involves the development and use of sensitive electrical equipment aimed at studying these small potentials produced by neurons and currents which travel through channels embedded in their membranes. It seems neurophysiology has gained an emerging interest from the hacker community, as evidenced by the success of Back Yard Brains, projects like the OpenEEG, and Hack-A-Day’s recent feature The Neuron – a Hacker’s Perspective.

In pondering designs for complex action potential detection and analysis circuitry, I realized that it would be beneficial to be able to generate action-potential-like waveforms on my workbench. The circuit I came up with to do this is a fully analog (technically mixed signal) action potential generator which produces lifelike action potentials.

 

Cellular Neurophysiology for Electrical Engineers (in 2 sentences): Neuron action potentials (self-propagating voltage-triggered depolarizations) in individual neurons are measured in scientific environments using single cell recording tools such as sharp microelectrodes and patch-clamp pipettes. Neurons typically rest around -70mV and when depolarized (typically by external excitatory input) above a threshold they engage in a self-propagating depolarization until they reach approximately +40mV, at which time a self-propagating repolarization occurs (often over-shooting the initial rest potential by several mV), then the cell slowly returns to the rest voltage so after about 50ms the neuron is prepared to fire another action potential. Impassioned budding electrophysiologists may enjoy further reading Active Behavior of the Cell Membrane and Introduction to Computational Neuroscience.

The circuit I describe here produces waveforms which visually mimic action potentials rather than serve to replicate the exact conductances real neurons employ to exhibit their complex behavior. It is worth noting that numerous scientists and engineers have designed more physiological electrical representations of neuronal circuitry using discrete components. In fact, the Hodgkin-Huxley model of the initiation and propagation of action potentials earned Alan Hodgkin and Andrew Huxley the Nobel Prize in Physiology and Medicine in 1936. Some resources on the internet describe how to design lifelike action potential generating circuits by mimicking the endogenous ionic conductances which underlie them, notably Analog and Digital Hardware Neural Models, Active Cell Model, and Neuromorphic Silicon Neuron Circuits. My goal for this project is to create waveforms which resemble action potentials, rather than waveforms which truly model them. I suspect it is highly unlikely I will earn a Nobel Prize for the work presented here.

The analog action potential simulator circuit I came up with creates a continuous series action potentials. This is achieved using a 555 timer (specifically the NE555) in an astable configuration to provide continuous square waves (about 6 Hz at about 50% duty). The rising edge of each square wave is isolated with a diode and used to charge a capacitor*. While the charge on the capacitor is above a certain voltage, an NPN transistor (the 2N3904) allows current to flow, amplifying this transient input current. The capacitor* discharges predictably (as an RC circuit) through a leak resistor. A large value leak resistor slows the discharge and allows that signal’s transistor to flow current for a longer duration. By having two signals (fast and slow) using RC circuits with different resistances (smaller and larger), the transistors are on for different durations (shorter and longer). By making the short pulse positive (using the NPN in common collector configuration) and the longer pulse negative (using the NPN in common emitter configuration), a resistor voltage divider can be designed to scale and combine these signals into an output waveform a few hundred mV in size with a 5V power supply. Pictured below is the output of this circuit realized on a breadboard. The blue trace is the output of the 555 timer.

*Between the capacitance of the rectification diode, input capacitance of the transistor, and stray parasitic capacitance from the physical construction of my wires and the rails on my breadboard, there is sufficient capacitance to accumulate charge which can be modified by changing the value of the leak resistor.

This circuit produces similar output when simulated. I’m using LTspice (free) to simulate this circuit. The circuit shown is identical to the one hand-drawn and built on the breadboard, with the exception that an additional 0.1 µF capacitor to ground is used on the output to smooth the signal. On the breadboard this capacitance-based low-pass filtering already exists due to the capacitive nature of the components, wires, and rails.

A few improvements naturally come to mind when considering this completed, functional circuit:

  • Action potential frequency: The resistor/capacitor network on the 555 timer determines the rate of square pulses which trigger action potentials. Changing these values will cause a different rate of action potential firing, but I haven’t attempted to push it too fast and suspect the result would not be stable is the capacitors are not given time to fully discharge before re-initiating subsequent action potentials.
  • Microcontroller-triggered action potentials: Since action potentials are triggered by any 5V rising edge signal, it is trivially easy to create action potentials from microcontrollers! You could create some very complex firing patterns, or even “reactive” firing patterns which respond to inputs. For example, add a TSL2561 I2C digital light sensor and you can have a light-to-frequency action potential generator!
  • Adjusting size and shape of action potentials: Since the waveform is the combination of two waveforms, you can really only adjust the duration (width) or amplitude (height) of each individual waveform, as well as the relative proportion of each used in creating the summation. Widths are adjusted by changing the leak resistor on the base of each transistor, or by adding additional capacitance. Amplitude and the ratio of each signal may be adjusted by changing the ratio of resistors on the output resistor divider.
  • Producing -70 mV (physiological) output: The current output is electirically decoupled (through a series capacitor) so it can float at whatever voltage you bias it to. Therefore, it is easy to “pull” in either direction. Adding a 10k potentiometer to bias the output is an easy way to let you set the voltage. A second potentiometer gating the magnitude of the output signal will let you adjust the height of the output waveform as desired.
  • The 555 could be replaced by an inverted ramp (sawtooth): An inverted ramp / sawtooth pattern which produces rapid 5V rising edges would drive this circuit equally well. A fully analog ramp generator circuit can be realized with 3 transistors: essentially a constant current capacitor charger with a threshold-detecting PNP/NPN discharge component.
  • This action potential is not all-or-nothing: In real life, small excitatory inputs which fail to reach the action potential threshold do not produce an action potential voltage waveform. This circuit uses 5V rising edges to produce action potential waveforms. However, feeding a 1V rising edge would produce an action potential 1/5 the size. This is not a physiological effect. However, it is unlikely (if not impossible) for many digital signal sources (i.e., common microcontrollers) to output anything other than sharp rising edge square waves of fixed voltages, so this is not a concern for my application.
  • Random action potentials: When pondering how to create randomly timed action potentials, the issue of how to generate random numbers arises. This is surprisingly difficult, especially in embedded devices. If a microcontroller is already being used, consider Make’s write-up on the subject, and I think personally I would go with a transistor-based avalanche nosie generator to create the randomness.
  • A major limitation is that irregularly spaced action potentials have slightly different amplitudes. I found this out the next day when I created a hardware random number generator (yes, that happened) to cause it to fire regularly, missing approximately half of the action potentials. When this happens, breaks in time result in a larger subsequent action potential. There are several ways to get around this, but it’s worth noting that the circuit shown here is best operated around 6 Hz with only continuous regularly-spaced action potentials.

In the video I also demonstrate how to record the output of this circuit using a high-speed (44.1 kHz) 16-bit analog-to-digital converter you already have (the microphone input of your sound card). I won’t go into all the details here, but below is the code to read data from a WAV file and plot it as if it were a real neuron. The graph below is an actual recording of the circuit described here using the microphone jack of my sound card.

import numpy as np
import matplotlib.pyplot as plt
Ys = np.memmap("recording.wav", dtype='h', mode='r')[1000:40000]
Ys = np.array(Ys)/max(Ys)*150-70
Xs = np.arange(len(Ys))/44100*1000
plt.figure(figsize=(6,3))
plt.grid(alpha=.5,ls=':')
plt.plot(Xs,Ys)
plt.margins(0,.1)
plt.title("Action Potential Circuit Output")
plt.ylabel("potential (mV)")
plt.xlabel("time (ms)")
plt.tight_layout()
plt.savefig("graph.png")
#plt.show()

Let’s make some noise! Just to see what it would look like, I created a circuit to generate slowly drifting random noise. I found this was a non-trivial task to achieve in hardware. Most noise generation circuits create random signals on the RF scale (white noise) which when low-pass filtered rapidly approach zero. I wanted something which would slowly drift up and down on a time scale of seconds. I achieved this by creating 4-bit pseudo-random numbers with a shift register (74HC595) clocked at a relatively slow speed (about 200 Hz) having essentially random values on its input. I used a 74HC14 inverting buffer (with Schmidt trigger inputs) to create the low frequency clock signal (about 200 Hz) and an extremely fast and intentionally unstable square wave (about 30 MHz) which was sampled by the shift register to generate the “random” data. The schematic illustrates these points, but note that I accidentally labeled the 74HC14 as a 74HC240. While also an inverting buffer the 74HC240 will not serve as a good RC oscillator buffer because it does not have Schmidt trigger inputs.

The addition of noise was a success, from an electrical and technical sense. It isn’t particularly physiological. Neurons would fire differently based on their resting membrane potential, and the peaks of action potential should all be about the same height regardless of the resting potential. However if one were performing an electrical recording through a patch-clamp pipette in perforated patch configuration (with high resistance between the electrode and the internal of the cell), a sharp microelectrode (with high resistance due to the small size of the tip opening), or were using electrical equipment or physical equipment with amplifier limitations, one could imagine that capacitance in the recording system would overcome the rapid swings in cellular potential and result in “noisy” recordings similar to those pictured above. They’re not physiological, but perhaps they’re a good electrical model of what it’s like trying to measure a physiological voltage in a messy and difficult to control experimental environment.

This project was an interesting exercise in analog land, and is completed sufficiently to allow me to move toward my initial goal: creating advanced action potential detection and measurement circuitry. There are many tweaks which may improve this circuit, but as it is good enough for my needs I am happy to leave it right where it is. If you decide to build a similar circuit (or a vastly different circuit to serve a similar purpose), send me an email! I’d love to see what you came up with.

 

UPDATE: add a microcontroller

I enhanced this project by creating a microcontroller controlled action potential generator. That article is here: https://www.swharden.com/wp/2017-08-20-microcontroller-action-potential-generator/


     

Logging I2C Data with Bus Pirate and Python

I’m working on a project which requires I measure temperature via a computer, and I accomplished this with minimal complexity using a BusPirate and LM75A I2C temperature sensor. I already had some LM75A breakout boards I got on eBay (from China) a while back. A current eBay search reveals these boards are a couple dollars with free shipping. The IC itself is available on Mouser for $0.61 each. The LM75A datasheet reveals it can be powered from 2.8V-5.5V and has a resolution of 1/8 ºC (about 1/4 ºF). I attached the device to the Bus Pirate according to the Bus Pirate I/O Pin Descriptions page (SCL->CLOCK and SDA->MOSI) and started interacting with it according to the Bus Pirate I2C page. Since Phillips developed the I2C protocol, a lot of manufacturers avoid legal trouble and call it TWI (two-wire interface).

Here I show how to pull data from this I2C device directly via a serial terminal, then show my way of automating the process with Python. Note that there are multiple python packages out there that claim to make this easy, but in my experience they are either OS-specific or no longer supported or too confusing to figure out rapidly. For these reasons, I ended up just writing a script that uses common Python libraries so nothing special has to be installed.

Reading data directly from a serial terminal

Before automating anything, I figured out what I2C address this chip was using and got some sample temperature readings directly from the serial terminal. I used RealTerm to connect to the Bus Pirate. The sequence of keystrokes I used are:

  • # – to reset the device
  • m – to enter the mode selection screen
    • 4 – to select I2C mode
    • 3 – to select 100KHz
  • W – to turn the power on
  • P – to enable pull-up resistors
  • (1) – to scan I2C devices
    • this showed the device listening on 0x91
  • [0x91 r:2] – to read 2 bytes from I2C address 0x91
    • this showed bytes like 0x1D and 0x20
    • 0x1D20 in decimal is 7456
    • according to datasheet, must divide by 2^8 (256)
    • 7456/256 = 29.125 C = 84.425 F

Automating Temperature Reads with Python

There should be an easy way to capture this data from Python. The Bus Pirate website even has a page showing how to read data from LM75, but it uses a pyBusPirateLite python package which has to be manually installed (it doesn’t seem to be listed in pypi). Furthermore, they only have a screenshot of a partial code example (nothing I can copy or paste) and their link to the original article is broken. I found a cool pypy-indexed python module pyElectronics which should allow easy reading/writing from I2C devices via BusPirate and Raspberry Pi. However, it crashed immediately on my windows system due to attempting to load Linux-only python modules. I improved the code and issued a pull request, but I can’t encourage use of this package at this time if you intend to log data in Windows. Therefore, I’m keeping it simple and using a self-contained script to interact with the Bus Pirate, make temperature reads, and graph the data over time. You can code-in more advanced features as needed. The graphical output of my script shows what happens when I breathe on the sensor (raising the temperature), then what happens when I cool it (by placing a TV dinner on top of it for a minute). Below is the code used to set up the Bus Pirate to log and graph temperature data. It’s not fast, but for temperature readings it doesn’t have to be! It captures about 10 reads a second, and the rate-limiting step is the timeout value which is currently set to 0.1 sec.

NOTE: The Bus Pirate has a convenient binary scripting mode which can speed all this up. I’m not using that mode in this script, simply because I’m trying to closely mirror the functionality of directly typing things into the serial console.

import serial
import matplotlib.pyplot as plt

BUSPIRATE_PORT = 'com3' #customize this! Find it in device manager.

def send(ser,cmd,silent=False):
    """
    send the command and listen to the response.
    returns a list of the returned lines. 
    The first item is always the command sent.
    """
    ser.write(str(cmd+'\n').encode('ascii')) # send our command
    lines=[]
    for line in ser.readlines(): # while there's a response
        lines.append(line.decode('utf-8').strip())
    if not silent:
        print("\n".join(lines))
        print('-'*60)
    return lines

def getTemp(ser,address='0x91',silent=True,fahrenheit=False):
    """return the temperature read from an LM75"""
    unit=" F" if fahrenheit else " C"
    lines=send(ser,'[%s r:2]'%address,silent=silent) # read two bytes
    for line in lines:
        if line.startswith("READ:"):
            line=line.split(" ",1)[1].replace("ACK",'')
            while "  " in line:
                line=" "+line.strip().replace("  "," ")
            line=line.split(" 0x")
            val=int("".join(line),16)
            # conversion to C according to the datasheet
            if val < 2**15:
                val = val/2**8
            else:
                val =  (val-2**16)/2**8
            if fahrenheit:
                val=val*9/5+32
            print("%.03f"%val+unit)
            return val
    
    
# the speed of sequential commands is determined by this timeout
ser=serial.Serial(BUSPIRATE_PORT, 115200, timeout=.1)

# have a clean starting point
send(ser,'#',silent=True) # reset bus pirate (slow, maybe not needed)
#send(ser,'v') # show current voltages

# set mode to I2C
send(ser,'m',silent=True) # change mode (goal is to get away from HiZ)
send(ser,'4',silent=True) # mode 4 is I2C
send(ser,'3',silent=True) # 100KHz
send(ser,'W',silent=True) # turn power supply to ON. Lowercase w for OFF.
send(ser,'P',silent=True) # enable pull-up resistors
send(ser,'(1)') # scan I2C devices. Returns "0x90(0x48 W) 0x91(0x48 R)"

data=[]
try:
    print("reading data until CTRL+C is pressed...")
    while True:
        data.append(getTemp(ser,fahrenheit=True))
except:
    print("exception broke continuous reading.")
    print("read %d data points"%len(data))

ser.close() # disconnect so we can access it from another app

plt.figure(figsize=(6,4))
plt.grid()
plt.plot(data,'.-',alpha=.5)
plt.title("LM75 data from Bus Pirate")
plt.ylabel("temperature")
plt.xlabel("number of reads")
plt.show()

print("disconnected!") # let the user know we're done.

Experiment: Measuring Heater Efficacy

This project now now ready for an actual application test. I made a simple heater circuit which could be driven by an analog input, PWM, or digital ON/OFF. Powered from 12V it can pass 80 mA to produce up to 1W of heat. This may dissipate up to 250 mW of heat in the transistor if partially driven, so keep this in mind if an analog signal drive is used (i.e., thermistor / op-amp circuit). Anyhow, I soldered this up with SMT components on a copper-clad PCB with slots drilled on it and decided to give it a go. It’s screwed tightly to the temperature sensor board, but nothing special was done to ensure additional thermal conductivity. This is a pretty crude test.

I ran an experiment to compare open-air heating/cooling vs. igloo conditions, as well as low vs. high heater drive conditions. The graph below shows these results. The “heating” ranges are indicated by shaded areas. The exposed condition is when the device is sitting on the desk without any insulation. A 47k resistor is used to drive the base of the transistor (producing less than maximal heating). I then repeated the same thing after the device was moved inside the igloo. I killed the heater power when it reached the same peak temperature as the first time, noticing that it took less time to reach this temperature. Finally, I used a 1k resistor on the base of the transistor and got near-peak heating power (about 1W). This resulted in faster heating and a higher maximum temperature. If I clean this enclosure up a bit, this will be a nice way to test software-based PID temperature control with slow PWM driving the base of the transistor.

Code to create file logging (csv data with timestamps and temperatures) and produce plots lives in the ‘file logging’ folder of the Bus Pirate LM75A project on the GitHub page.

Experiment: Challenging LM7805 Thermal Shutdown

The ubiquitous LM7805 linear voltage regulator offers internal current limiting (1.5A) and thermal shutdown. I’ve wondered for a long time if I could use this element as a heater. It’s TO-220 package is quite convenient to mount onto enclosures. To see how quickly it heats up and what temperature it rests at, screwed a LM7805 directly to the LM75A breakout board (with a dab of thermal compound). I soldered the output pin to ground (!!!) and recorded temperature while it was plugged in.

Power (12V) was applied to the LM7805 over the red-shaded region. It looks like it took about 2 minutes to reach maximum temperature, and settled around 225F. After disconnecting power, it settled back to room temperature after about 5 minutes. I’m curious if this type of power dissipation is sustainable long term…

Update: Reading LM75A values directly into an AVR

This topic probably doesn’t belong inside this post, but it didn’t fit anywhere else and I don’t want to make it its own post. Now that I have this I2C sensor mounted where I want it, I want a microcontroller to read its value and send it (along with some other data) via serial USART to an FT232 (USB serial adapter). Ultimately I want to take advantage of its comparator thermostat function so I can have a USB-interfaced PC-controllable heater with multiple LM75A ICs providing temperature readings at different sites in my project. To do this, I had to write code to interface my microcontroller to the LM75A. I am using an ATMega328 (ATMega328P) with AVR-GCC (not Arduino). Although there are multiple LM75A senor libraries for Arduino [link] [link] [link] I couldn’t find any examples which didn’t rely on Arduino libraries. I ended up writing functions around g4lvanix’s L2C-master-lib.

Here’s a relevant code snippit. See the full code (with compile notes) on this GitHub page:

uint8_t data[2]; // prepare variable to hold sensor data
uint8_t address=0x91; // this is the i2c address of the sensor
i2c_receive(address,data,2); // read and store two bytes
temperature=(data[0]*256+data[1])/32; // convert two bytes to temperature

 

 

This project lives on my growing GitHub page for microcontroller projects:

https://github.com/swharden/AVR-projects/

 


     

Raspberry Pi RF Frequency Counter

I build a lot of RF circuits, and often it’s convenient to measure and log frequency with a computer. Previously I’ve built standalone frequency counters, frequency counters with a PC interface, and even hacked a classic frequency counter to add USB interface (twice, actually). My latest device uses only 2 microchips to provide a Raspberry Pi with RF frequency measurement capabilities. The RF signal clocks a 32-bit counter SN74LV8154 ($1.04 on Mouser) connected to a 16-bit IO expander MCP23017 ($1.26 on Mouser) accessable to the Raspberry Pi (via I²C) to provide real-time frequency measurements from a python script for $2.30 in components! Well, plus the cost of the Raspberry Pi. All files for this project are on my GitHub page.

img_8773

The entire circuit is only two microchips! I have a few passives to clean up the RF signal (the RF input is loaded with a 1k resistor to ground, decoupled through a series 100 nF capacitor, and balanced at VCC/2 through a voltage divider of two 47k resistors), but if the measured signal is already a strong square wave they could be omitted. The circuit requires a gate pulse which typically will be 1 pulse per second (1PPS) and can be generated by dividing-down a 32.768kHz oscillator, a spare pin on a microcontroller, a fancy 1PPS time reference, or like in my case a GPS module (Neo-6M) with 1PPS output to provide an extremely accurate gate.

schem

The connections are intuitive! The I2C address is 0x20 when A0, A1, and A2 are grounded. GPB(1-4) control the register select of the counter, and GPA(0-7) reads each bit of the selected register. The whole thing is controlled from Python, but could be trivially written in any language.

img_8777

Here’s a quick summary describing how the code works: First I send bytes to address 0 and 1 to set all pins of GPIO A as inputs, and GPIO B as outputs. Note that only 4 of 8 pins are used for the output, so technically 4 extra pins could be used for things like blinking LEDs or controlling other devices. I then set the register select pins by sending a value to 0x13 (GPIO B), and read the entire GPIO A bus (INTCAPB, 0x18). For address details, consult the datasheet. I do this 4 times (1 for each byte of the 32-bit counter), do a little math to turn it into a frequency value, and compare the current value with the last value and take the difference to display as the measured frequency.

screenshot

An advantage of this continuously running mode is that no clock cycles are lost, so a gate which accidentally fires a bit early due to jitter and cuts-off a cycle will compensate for it on a subsequent read. This is shown above, as a very stable 10MHz frequency reference is measured with this method. A “slow” 1PPS clock tick causes a reading slightly higher, compensated-for by the next reading being slightly lower. In this way, clock sources which are extremely accurate but suffer from low precision (like GPS time sources) are able to maximize the long-term measurement of frequency. Combining this frequency measurement technique with the ability to generate an analog voltage with a Raspberry Pi will allow me to perform some interesting experiments with a voltage controlled crystal oscillator.

Useful Links:

 


     

Generating Analog Voltage with Raspberry Pi

I recently had the need to generate analog voltages from the Raspberry PI, which has rich GPIO digital outputs but no analog outputs. I looked into the RPi.GPIO project which can create PWM (which I wanted to smooth using a low pass filter to create the analog voltage), but its output on the oscilloscope looked terrible! It stuttered all over the place, likely because the duty is continuously under software control. I ended up solving my problem with a MCP4921 12-bit DAC chip (about $1.50 on eBay). It’s controlled via SPI, and although I could have written a python program to bit-bang its protocol with RPi.GPIO I realized I could write directly to the Raspberry Pi SPI device using the echo command. Dividing 3.3V into 12-bits (4096) means that I can control voltage in steps of less than 1mV each, right from the bash console!

img_8696

Video: The Problem (RPi PWM jitters)

Video: My Solution (SPI DAC)

Hardware Connection

There’s very little magic in how the microchip is connected to the Pi. It’s a straight shot to its SPI bus! Here’s a quick drawing showing which pins to connect. Check your device against the Raspberry Pi GPIO pinout diagram for different devices.

img_8701-1

Controlling the DAC with a Bus Pirate

Before I used a Raspberry Pi to control the DAC chip, I tested it out with a Bus Pirate. I don’t have a lot of pictures of the project, but I have a screenshot of a serial console used to send commands to the chip. One advantage of the Bus Pirate is that I can type bytes in binary, which helps to see the individual bits. I don’t have this ability when I’m working in the bash console.

serial

I’m less familiar with the Bus Pirate, but this was a good opportunity to get to know it a little better. It look me a long time (requiring I pull out the logic analyzer) to realize that I had to manually enable/disable the chip-select line, using the “[” and “]” commands. When I set up the SPI mode (command m5) I told it to use active low, but I wasn’t sure how to reverse the active level of the chip-select commands, so I just did ]this[ instead of [this] and it worked great.

frompi

This is the signal probed when it was controlled by the Raspberry Pi, but it looked essentially identical when values were sent via the Bus Pirate. The only difference is there was an appreciable delay between the “]” commands and each of the bytes. It worked fine though.

Controlling the DAC with Console Commands

Once the hardware was configured, the software was trivial. I could control analog voltages by sending two properly-formatted bytes to the SPI hardware device. Importantly, you must use raspi-config to enable SPI.

# set analog voltage to minimum value (about 0V)
echo -ne "\x30\x00" > /dev/spidev0.0 # minimum

# set analog voltage to something a little higher
echo -ne "\x30\xAB" > /dev/spidev0.0 

# set analog voltage to maximum value (about 3.3V)
echo -ne "\x3F\xFF" > /dev/spidev0.0

Helpful Links:

 


     

VHF Frequency Counter with PC Interface

Projects I build often involve frequency synthesis, and one of the most useful tools to have around is a good frequency counter. Being a budding programmer and data analysis guru, I love the idea of being able to access / log / analyze frequency readings on my computer too. Commercial frequency counters can be large, expensive, and their calibration is a chicken-and-egg problem (you need a calibrated frequency counter to calibrate a frequency reference you use to calibrate a frequency counter!). For about the cost of a latte I made a surprisingly good frequency frequency counter (which directly counts >100 MHz without dividing-down the input signal) by blending a SN74LV8154 dual 16-bit counter (which can double as a 32-bit counter, $1.04 on mouser) and an ATMega328 microcontroller ($3.37 on Mouser). Although these two chips are all you need to count something, the accuracy of your counts depend on your gate. If you can generate a signal of 1 pulse per second (1PPS), you can count anything, but your accuracy depends on the accuracy of your 1PPS signal. To eliminate the need for calibration (and to provide the 1PPS signal with the accuracy of an atomic clock) I’m utilizing the 1PPS signal originating from a GPS unit which I already had distributed throughout my shack (using a 74HC240 IC as a line driver). If you don’t have a GPS unit, consider getting one! I’m using a NEO-6M module ($17.66 on Amazon) to generate the 1PPS gate, and if you include its cost we’re up to $22.07. Also, all of the code for this project (schematics, C that runs on the microcontroller, and a Python to interact with the serial port) is shared on GitHub! You may be wondering, “why do GPS units have incredibly accurate 1PPS signals?” It’s a good question, but a subject for another day. For now, trust me when I say they’re fantastically accurate (but slightly less precise due to jitter) if you’re interested in learning more read up on GPS timing.

 

pc frequency counter schem

This is the general idea behind how this frequency counter works. It’s so simple! It’s entirely digital, and needs very few passive components. sn74lv8154 is configured in 32-bit mode (by chaining together its two 16-bit counters, see the datasheet for details) and acts as the front-end directly taking in the measured frequency. This chip is “rare” in the sense I find very few internet projects using it, and they’re not available on ebay. However they’re cheap and plentiful on mouser, so I highly encourage others to look into using it! The datasheet isn’t very clear about its maximum frequency, but in my own tests I was able to measure in excess of 100 MHz from a breadboarded circuit! This utilized two cascaded ICS501 PLL frequency multiplier ICs to multiply a signal I had available (the 11.0592 MHz crystal the MCU was running from) by ten, yielding 110 MHz, which it was able to measure (screenshot is down on the page).

neo-60 gps 1pps

The 1PPS gate signal is generated from an inexpensive GPS module available on AmazonI’ve hinted at the construction of this device before and made a post about how to send output signals like the 1PPS signal generated here throughout your shack via coax using a line driver, so I won’t re-hash all of those details here. I will say that this module has only VCC, GND, and TX/RX pins, so to get access to the 1PPS signal you have to desolder the SMT LED and solder a wire to its pad. It requires a bit of finesse. If you look closely, you can see it in this picture (purple wire).

IMG_8207

I first built this device on a breadboard, and despite the rats nest of wires it worked great! Look closely and you can see the ICS501 frequency multiplier ICs I wrote about before. In this case it’s measuring the 10x multiplied crystal frequency clocking the MCU (11 MHz -> 110 MHz) and reporting these readings every 1 second to the computer via a serial interface.

ss

Frequency measurements of the VHF signal are reported once per second. Measurements are transmitted through a USB serial adapter, and captured by a Python script. Note that I’m calling this signal VHF because it’s >30 MHz. I am unsure if this device will work up to 300 MHz (the border between VHF and UHF), but I look forward to testing that out! Each line contains two numbers: the actual count of the counter (which is configured to simply count continuously and overflow at 2^32=4,294,967,296), and the gated count (calculated by the microcontroller) which is the actual frequency in Hz.

This screenshot shows that my ~11.05 MHz crystal is actually running at 11,061,669.4 Hz. See how I capture the 0.4 Hz unit at the end? That level of precision is the advantage of using this VHF-capable counter in conjunction with a 10x frequency multiplier!

Once I confirmed everything was working, I built this device in a nice enclosure. I definitely splurge every few months and buy extruded split body aluminum enclosures in bulk (ebay), but they’re great to have on hand because they make projects look so nice. I added some rubber feet (cabinet bumpers from Walmart), drilled holes for all the connectors with a continuous step drill bit, made a square hole for the serial port using a nibbler, and the rest is pretty self-evident. Labels are made with a DYMO LetraTag (Target) and clear labels (Target, Amazon) using a style inspired by PA2OHH. I tend to build one-off projects like this dead-bug / Manhattan style.

IMG_8277

IMG_8282

I super-glued a female header to the aluminum frame to make in-circuit serial programming (ICSP) easy. I can’t believe I never thought to do this before! Programming (and reprogramming) was so convenient. I’m going to start doing this with every enclosed project I build from now on. FYI I’m using a USBTiny ISP ($10.99, Amazon) to do the programming (no longer the BusPirate, it’s too slow) like I describe here for 64-bit Windows 7 (although I’m now using Windows 10 and it works the same).

IMG_8330

The front of the device has LEDs indicating power, serial transmission, and gating. Without a 1PPS gate, the device is set to send a count (of 0) every 5 seconds. In this case, the TX light will illuminate. If a gate is detected, the TX and GATE LEDs will illuminate simultaneously. In reality I just drilled 3 holes when I really needed two, so I had to make-up a function for the third LED (d’oh!)

IMG_8286

The back of the device has serial output, frequency input, gate input, and power. Inside is a LM7805 voltage regulator, and careful attention was paid to decoupling and keeping ripple out of the power supply (mostly so our gate input wouldn’t be affected). I’m starting to get in the habit of labeling all serial output ports with the level (TTL vs CMOS, which makes a HUGE difference as MAX232 level converter may be needed, or a USB serial adapter which is capable of reading TTL voltages), as well as the baud rate (119200), byte size (8), parity (N), and stop bit (1). I just realized there’s a typo! The label should read 8N1. I don’t feel like fixing it, so I’ll use a marker to turn the 2 into an 8. I guess I’m only human after all.

IMG_8297

I should have tried connecting all these things before I drilled the holes. I got so lucky that everything fit, with about 2mm to spare between those BNC jacks. Phew!

IMG_8316

This is an easy test frequency source. I have a dozen canned oscillators of various frequencies. This is actually actually a voltage controlled oscillator (VCO) with adjustment pin (not connected), and it won’t be exactly 50 MHz without adjustment. It’s close enough to test with though! As this is >30 MHz, we can call the signal VHF.

IMG_8318

You can see on the screen it’s having no trouble reading the ~50 MHz frequency. You’ll notice I’m using RealTerm (with a good write-up on sparkfun) which is my go-to terminal program instead of HyperTerminal (which really needs to go away forever). In reviewing this photo, I’m appreciating how much unpopulated room I have on the main board. I’m half tempted to build-in a frequency multiplier circuit, and place it under control of the microcontroller such that if an input frequency from 1-20MHz is received, it will engage the 10x multiplier. That’s a mod for another day though! Actually, since those chips are SMT, if I really wanted to do this I would make this whole thing a really small SMT PCB and greatly simplify construction. That sounds like a project for another day though…

IMG_8335

Before closing it up I added some extra ripple protection on the primary counter chip. There’s a 560 uH series inductor with the power supply, followed by a 100 nF capacitor parallel with ground. I also added ferrite beads to the MCU power line and gate input line. I appreciate how the beads are unsecured and that this is a potential weakness in the construction of this device (they’re heavy, so consider what would happen if you shook this enclosure). However, anything that would yank-away cables in the event of shaking the device would probably also break half the other stuff in this thing, so I think it’s on par with the less-than-rugged construction used for all the other components in this device. It will live a peaceful life on my shelf. I am not concerned.

IMG_8340

This is the final device counting frequency and continuously outputting the result to my computer. In the background you can see the 12V power supply (yellow) indicating it is drawing only 20 mA, and also the GPS unit is in a separate enclosure on the bottom right. Click here to peek inside the GPS 1PPS enclosure.

IMG_8344

I’m already loving this new frequency counter! It’s small, light, and nicely enclosed (meaning it’s safe from me screwing with it too much!). I think this will prove to be a valuable piece of test equipment in my shack for years to come. I hope this build log encourages other people to consider building their own equipment. I learned a lot from this build, saved a lot of money not buying something commercial, had a great time making this device, and I have a beautiful piece of custom test equipment that does exactly what I want.

Microcontroller code (AVR-GCC), schematics, and a Python script to interface with the serial port are all available on this project’s GitHub page



Afterthought: Using without GPS

One of the great advantages of this project is that it uses GPS for an extremely accurate 1 PPS signal, but what options exist to adapt this project to not rely on GPS? The GPS unit is expensive (though still <$20) and GPS lock is not always feasible (underground, in a Faraday cage, etc). Barring fancy things like dividing-down rubidium frequency standards or oven controlled oscillators, consider having your microcontroller handle the gating using either interrupts and timers precisely configured to count seconds. Since this project uses a serial port with a 11.0592 MHz crystal, your 1PPS stability will depend on the stability of your oscillator (which is pretty good!). Perhaps more elegantly you could use a 32.768 kHz crystal oscillator to create a 1 PPS signal. This frequency can be divided by 2 over and over to yield 1 Hz perfectly. This is what most modern wristwatches do. Many AVRs have a separate oscillator which can accomodate a 32 kHz crystal and throw interrupts every 1 second without messing with the system clock. Alternatively, the 74GC4060 (a 14 stage ripple counter) can divide 32k into 1 Hz and even can be arranged as an oscillator (check the datasheet). It would be possible to have both options enabled (local clock and GPS) and only engage the local clock if the GPS signal is absent. If anyone likes the idea of this simple VHF frequency counter with PC interface but doesn’t want to bother with the GPS, there are plenty of options to have something almost as accurate. That really would cut the cost of the final device down too, keeping it under the $5 mark.

Update: Integrating Counter Serial Output with GPS Serial Output

The NEO-M8 GPS module is capable of outputting serial data at 9600 baud and continuously dumps NEMA formatted GPS data. While this isn’t really useful for location information (whose frequency counter requires knowing latitude and longitude?) it’s great for tracking things like signal strength, fix quality, and number of satellites. After using this system to automatically log frequency of my frequency reference, I realized that sometimes I’d get 1-2 hours of really odd data (off by kHz, not just a few Hz). Power cycling the GPS receiver fixes the problem, so my guess it that it’s a satellite issue. If I combine the GPS RX and counter in 1 box, I could detect this automatically and have the microcontroller power cycle the GPS receiver (or at the least illuminate a red error LED). I don’t feel like running 2 USB serial adapters continuously. I don’t feel like programming my AVR to listen to the output from the GPS device (although that’s probably the correct way to do things).  Instead I had a simpler idea that worked really well, allowing me to simultaneously log serial data from my GPS unit and microcontroller (frequency counter) using 1 USB serial adapter.

img_8401

The first thing I did was open up the frequency counter and reconnect my microcontroller programmer. This is exactly what I promised myself I wouldn’t do, and why I have a nice enclosure in the first place! Scott, stop fidgeting with things! The last time I screwed this enclosure together I considered adding super glue to the screw threads to make sure I didn’t open it again. I’ll keep my modifications brief! For now, this is a test of a concept. When it’s done, I’ll revert the circuitry to how it was and close it up again. I’ll take what I learn and build it into future projects.

img_8402

I peeked at the serial signals of both the frequency counter (yellow) and the GPS unit output (blue). To my delight, there was enough dead space that I thought I could stick both in the same signal. After a code modification, I was able to tighten it up a lot, so the frequency counter never conflicts with the GPS unit by sending data at the same time.

img_8403

I had to slow the baud rate to 9600, but I programmed it to send fewer characters. This leaves an easy ~50ms padding between my frequency counter signal and the GPS signal. Time to mix the two! This takes a little thought, as I can’t just connect the two wires together. Serial protocol means the lines are usually high, and only pulled down when data is being sent. I had to implement an active circuit.

fullsizerender-2

Using a few components, I built an AND gate to combine signals from the two serial lines. For some reason it took some thought before I realized an AND gate was what I needed here, but it makes sense. The output is high (meaning no serial signal) only when both inputs are high (no serial signals on the input). When either signal drops low, the output drops low. This is perfect. My first thought was that I’d need a NOR gate, but an inverted AND gate is a NOR gate.

img_8404

Here’s my quick and dirty implementation. A reminder again is that this will be removed after this test. For now, it’s good enough.

img_8405

After connecting the GPS serial output and frequency counter serial output to the AND gate (which outputs to the computer), I instantly got the result I wanted!

serial-combine

RealTerm shows that both inputs are being received. It’s a mess though. If you want to know what everything is, read up on NEMA formatted GPS data.

combined-python

I whipped-up a python program to parse, display, and log key information. This display updates every 1 second. The bottom line is what is appended to the log file on ever read. It’s clunky, but again this is just for testing and debugging. I am eager to let this run for as long as I can (days?) so I can track how changes in satellite signal / number / fix quality influence measured frequency.