Wednesday, 20 December 2017

Raspberry Pi User Group Meeting - November 19, 2017

We have a few core people who show to our meetings at the New Glasgow Library. This meeting was no different that way.

What was new was the free Stepped Tone Generator / APC kit we have to offer. The Stepped Tone Generator is actually a circuit taken from "Engineer's Mini Notebook, Vol.1 - Timer, Op Amp & Optoelectronic Circuits & Projects," by Forrest Mims III. APC or Atari Punk Console is the name that was given to the circuit by some electronics designer who made it their own. The circuit's sound is pretty lo-fi so imagine something that might come from an old 80's video game, thus the APC name. The circuit's central component is the 555 timer. I am writing another post with this circuit so you can also reference that post as well.

We built one of the kits that I had to make sure all the components worked and they did. I also had another circuit of the same that I had made before and had taken to the meeting. We tried to get it to work but found it did not. Well, ... time to troubleshoot.

Stepped Tone Generator / APC Schematic:


Anytime something does not work in your electronics projects is an opportunity to learn and to practice. Now, it had been sometime since I had to troubleshoot anything past something like a misplaced power supply lead to one of my circuits. It took a little time to figure out what was wrong. Even then, it was only after we had fixed the circuit had I realized what must have been wrong.

We started out checking all the wires to see if any were missing or misplaced. Then we started using the DMM (digital multi-meter). We swapped speakers and even IC's. While using the DMM it was seen that the voltage level on the middle lead one of the pots was not as it should be.

We are using the square blue trimpots for the kits because they are 1) more convenient to use in solderless breadboards and 2) they cost a lot less per item. To use a metal potentiometer would require at least leads soldered on to the lugs and then, if it were not mounted on something, it would hang free, useable but subject to disconnecting easily. Below are some examples of trimpots. Note that one is on its side and how that relates to the problem I found as described below.

The point is that little blue trimpots can look like they are correctly inserted into a solderless breadboard when they are not. Without really realizing what was wrong with the pot, it was removed, reinserted into an unused portion of the solderless breadboard and leads inserted into appropriate holes to allow DMM leads to take measurements. The pot was fine. It was reinserted into the circuit and presto, the circuit worked.

After the meeting, I realized that yes, the problem was with the potentiometer. It must have been inserted sideways so that two of the leads were connected and the third lead connected to the rest of the circuit to follow. However, the lead that was supposed to be connected to the power rail was not. So no power was supplied to the variable voltage divider found on the left side of the schematic.

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