Building the electronic nicad

This site was last updated on February 15, 2008.

Here's how I layed out my breadboard. The voltage reference is to the left. The op amps are all in a single DIP package in the center. Q1 is at the top right and Q3 is at the bottom right. Q2 will be mounted remotely, to allow for adequate heat sinking. There's no particular reason to have an especially compact layout, so feel free to spread the parts out a bit. On the other hand, if you have better vision than I, you can certainly achieve a much denser packing.

Note that these sorts of breadboards are supposed to be terrible for any sort of high-speed electronics. I expect we get away with it here because we're not doing anything too critical.

Putting together the breadboard is a quick job. Adding all the meters, the pot, etc. expands the job considerably.

Having laid the parts out on a breadboard, it's easy to mount them more permanently on a PC board. I used an "experimenter's board" from Radio Shack that is drilled and etched to correspond with my breadboard.

I start by mounting the low-profile components. I use a socket for the op-amp IC to avoid overheating the chip while soldering. For reliability, it might be better to avoid the socket.

Here's the completed board.

I found it relatively quick to assemble the circuit boards. It was more difficult to build a neat enclosure (I ended up building mine into my pit box). Here, some cleverness in layout can improve the ease of use. Place the meters so they are easy to read (remember that you'll probably be standing over the pit box, looking down at it, wondering what's wrong). Make the voltage control hard to adjust (that is, difficult to accidentally bump out of the desired position). Position the power switch so that you can tell at a glance that it's "on".

Don't forget to mount Q2 on an adequate heat sink. If you make your enclosure out of aluminum, you might mount the transistor directly to the box.

Don't forget to make provision for recharging your batteries. Buzz Wilson keeps two sets of batteries in his field box, using a 3-position switch to choose between them. This gives him a backup if the first set runs down unexpectedly. Alternatively, you might devise a way to test the level of the battery; for example, a switch that lets you measure the voltage level when it's loaded. After all, we've already got a voltmeter lying around.

Using a Cat-5 cable, it's simple to use modular connectors (like those used for modern phones) to allow an easily removable cable.

Some people have had difficulties getting the circuit to work. In some cases, they made unfortunate parts substitutions; in other cases, they made wiring errors. In such cases, it might be good to first build on a breadbread; then build the final version. I'd work in stages.

I would build the first stage with parts R1, D1, R2, R3, R4, and C1. Then measure the voltages

across D1 - should be 2.50 volts
across R1 - should be your battery's voltage - 2.5
across R2 - about 0.77 volts
across R3 - about 1.08 volts
across R4 - about 0.63 volts
across C1 - from 0.63 to 1.72 volts

For a next step, I'd wire up A1, R4, A2, and R7, plus a jumper in place of C2. If correct, then the voltages at the output of each opamp should match the voltage across C1.

Note well that the wire shown across the bottom of the schematic, connecting D1, R4, C1, A3, and the voltmeter is _not_ ground. Instead, it is 1 of the 4 wires that run out to the glow-plug clip, and only reaches ground indirectly. through the heavy line drawn from the clip to ground.

For these early stages, it won't matter too much, but as you begin connecting op-amp's pins, you should distinguish between the true ground and the signal from the glow-plug clip.

If the measurements with A1 and A2 in place seem good, then I'd go ahead and add C2 in place of the jumper, add the wires running to the glow-plug clip, the voltmeter, and R5. Let's skip the booster for now, temporarily connecting the loose end of R7 to the top, heavy line running out to the glow-plug clip.

Then see how it works, without the glow plug. Hopefully the voltmeter will match the voltage across C1 and respond as you adjust the pot R3.

Next, I'd add the parts for the booster, Q1 and C3, Q2, the ammeter, and C4. Be sure to fix the connection of R7. The test it again, still without a glow plug. The ammeter should indicate 0 amps and the voltmeter should still match the voltage across C1 as you adjust R3.

If this all works, let's try it with a glow plug. First though, be sure you have a heat sink for Q2. Now repeat our tests, still without the glow plug, verifying that everything works so far.

Now, finally!, let's try it all with a glow plug. First, turn it on and turn the voltage down to 1 volt. The hook up the plug. The ammeter should indicate between 3 and 4 amps, the voltmeter should stay at 1 volt, and the glow plug should glow red hot. If you adjust R3, carefully!, the plug should glow brighter when the voltage is turned up and the current should increase. Not too hot, or you will burn out the plug.

Q1 and Q2 will get hot too, but hopefully not too hot.

If you blow hard on the plug, trying to cool it with your breath, you should see increased current through the ammeter, but the voltage should stay constant. As you unhook the plug, the voltage should stay constant, but the current should drop to zero.

If all that works, celebrate!