My DIY Bench Power Supply

Another thing everybody needs (and has probably built) is a simple laboratory bench power supply. A lot of people use things like modified PC power supplies but I wasn’t in favor of this because I wanted something smaller, lower current, and cleaner (from an RF perspective). My needs are nothing particularly high power, just something to provide a few common voltages for digital logic and small RF circuits. This is what I came up with!

In the image above you can see an ordinary LED being powered directly from the a 5V hook-up. There is no current limiting resistor, so a lot of current is travelling through the LED, burning it up as I photographed it. The ammeter (blue number) shows it’s drawing 410 mA – whoa! The layout is pretty simple. Each red banana plug hook-up supplies a voltage (5, 5, 12, and variable respectively). Black hook-ups are ground. The black hook-up on the top left is a current-sensing ground, and current travelling through it will be displayed on the blue dial. The right dial shows the voltage of the variable voltage supply, and can go from about 3.5 – 30.5 V depending on where the potentiometer is set. All voltage outputs are designed to put-out approximately 1A of current.

I built this using a lot of (eBay) components I had on hand. I often save money where I can by stocking my workbench with components I buy in bulk. Here’s what I used:

  • 4.5-3.0V DC volt meter – $2.08 (shipped) eBay
  • 0-9.99 A ampere meter – $4.44 (shipped) eBay
  • L7805 5V voltage regulator – 10 for $3.51 ($.35 ea) (shipped) eBay
  • L7812 12V voltage regulator – 20 for $3.87 ($.19 ea) (shipped) eBay
  • LM317 variable voltage regulator – 20 for $6.15 ($0.30 ea) (shipped) eBay
  • 10k linear potentiometer – 10 for 4.00 ($.40 ea) (shipped) eBay
  • banana plug hook-ups – 20 for $3.98 ($.20 ea) (shipped) eBay
  • aluminum enclosure – $3.49 (radioshack)

TOTAL: $13.60

Does the variable voltage actually work? Is the voltmeter accurate? Let’s check it out.

I’d say it’s working nicely! I now have a new took on my workbench.

A note about the yellow color: The enclosure I got was originally silver aluminum. I sanded it (to roughen the surface), then sprayed it with a yellow rustoleum spray paint. I figured it was intended to go on metal, so I might as well give it a shot. I sprayed it once, then gave it a second coat 20 minutes later, then let it dry overnight. In the future I think I would try a lacquer finish, because it’s a bit easy to scratch off. However, it looks pretty cool, and I’m going to have to start spray-painting more of my enclosures in the future.

A note about smoothing capacitors. Virtually all diagrams of linear voltage regulators like the LM7805 show decoupling capacitors before and after the regulator. I added a few different values of capacitors on the input (you can see them in the circuit), but I intentionally did not include smoothing capacitors on the output. The reason was that I always put smoothing capacitors in my breadboards and in my projects, closer to the actual circuitry. If I included (and relied) on output capacitors at the level of the power supply, I would be picking-up 60Hz (and other garbage) RF noise in the cables coming from the power supply to my board. In short, no capacitors on the output, so good design must always be employed and decoupling capacitors added to whatever circuits are being built.

The input of this circuit is a 48V printer power supply from an archaic inkjet printer. It’s been attached to an RCA jack to allow easy plugging and unplugging.


Single Wavelength Pulse Oximeter

I want to create a microcontroller application which will utilize information obtained from a home-brew pulse oximeter. Everybody and their cousin seems to have their own slant how to make DIY pulse detectors, but I might as well share my experience. Traditionally, pulse oximeters calculate blood oxygen saturation by comparing absorbance of blood to different wavelengths of light. In the graph below (from Dildy et al., 1996 that deoxygenated blood (dark line) absorbs light differently than oxygenated blood (thin line), especially at 660nm (red) and 920nm (infrared). Therefore, the ratio of the difference of absorption at 660nm vs 920nm is an indication of blood oxygenation. Fancy (or at least well-designed) pulse oximeters continuously look at the ratio of these two wavelengths. Analog devices has a nice pulse oximeter design using an ADuC7024 microconverter. A more hackerish version was made and described on this non-english forum. A fail-at-the-end page of a simpler project is also shown here, but not well documented IMO.

That’s not how mine works. I only use a single illumination source (~660nm) and watch it change with respect to time. Variability is due to a recombination effect of blood volume changes and blood oxygen saturation changes as blood pulses through my finger. Although it’s not quite as good, it’s a bit simpler, and it definitely works. Embedded-lab has a similar project but the output is only a pulsing LED (not what I want) and a voltage output that only varies by a few mV (not what I want).

Here’s what the device looks like assembled in a breadboard:

I made a sensor by drilling appropriately-sized holes in a clothespin for the emitter (LED) and sensor (phototransistor). I had to bend the metal spring to make it more comfortable to wear. Light pressure is better than firm pressure, not only because it doesn’t hurt as much, but because a firm pinch restricts blood flow considerably.

An obvious next step is microcontroller + LCD (or computer) digitization, but for now all you can do is check it out on my old-school analog oscilloscope. Vertical squares represent 1V (nice!). You can see the pulse provides a solid 2V spike.

Here’s some video of it in action:

Out of principal, I’m holding-back the circuit diagram until I work through it a little more. I don’t want to mislead people by having them re-create ill-conceived ideas on how to create analog amplifiers. I’ll post more as I develop it.