I am trying to build an APC using the following schematic:

APC Schematic

And the output is having some issues.

Sound only comes out when the rightmost potentiometer is at 0 resistance, and in that position the rightmost 555 heats up rapidly. In any other spot, the speaker simply makes clicking noises.

One possible difference is that I am using TLC555 chips instead of LM555 as suggested in the diagram. Would that make a difference?


Using a TLC555 should not be a problem. The behaviour you observe is a failing of the APC circuit itself.

When the rightmost potentiometer is at 0 resistance, the 555 heats up rapidly because, internally, pin 7 shorts to ground during parts of the oscillation, to discharge the 0.1uF capacitor. With the potentiometer at 0 ohms, this results in a direct short across the power rails and a very large current flowing through pin 7 (as well as a direct short discharging the capacitor). The absolute maximum current through pin 7 for the TLC555 is 150mA. To ensure this isn't exceeded, you could try connecting 220 ohm resistors (I'm assuming a 9V supply) in series with both potentiometers, and also between pins 6 and 7 of the rightmost 555.

Most of the audio range of the circuit is contained within the lowest 20% or so of the resistance of the right-most potentiometer. Try reducing the value of this potentiometer, or connecting a 10k 'fine tuning' potentiometer in series with it, if you want more precise control.

  • \$\begingroup\$ I tried what you suggested and replaced the 500k potentiometer with a 50k one, but audio was still only heard at the lowest 5% of the potentiometer, and dropped down quickly when slightly higher than 0 resistance. In addition, when held one of these low resistances, the 50k potentiometer began to burn up. It seems like this circuit is supposed to have a greater diversity in range of the second potentiometer than it does, but for some reason this potentiometer doesn't seem to be contributing much to audio. \$\endgroup\$ – aftrumpet Mar 12 '14 at 15:40
  • \$\begingroup\$ Sounds like something else might be wrong too then; try connecting the output to pin 3 of just the left 555. You should get a tone which can be varied over a large pitch range. Try setting this to a high squeek, then reconnecting the right 555. Its possible that your right 555 has been damaged by excess current, though they are typically pretty robust. \$\endgroup\$ – Chris Johnson Mar 12 '14 at 16:35
  • \$\begingroup\$ I think I must have put the capacitor in the wrong spot or some other wiring problem, because I rewired the second 555 according to the diagram and it worked fine. Thank you for all of your help though. \$\endgroup\$ – aftrumpet Mar 12 '14 at 16:54
  • 1
    \$\begingroup\$ An interesting answer. How did you come up with a value of 220 Ω? Was this just a random low value that you chose? For a 9V supply I calculate that only 60 Ω is required: R=V/I=9/0.15=60. Also, would a minimal resistance be required for the astable (the first 555), as there is already a 1KΩ resistance between 7 and GND (although I guess as 7 goes to ground during the low of the oscillation, then, at that point, there would be a need)? Thirdly, wouldn't the resistor between pins 6 and 7 on the monostable (the second 555) affect its operation (as, by design, they are meant to be shorted)? \$\endgroup\$ – Greenonline Apr 17 '18 at 6:24

I had the same problem, I even burnt the 555 of the right on your figure. I was using a 8 ohms speaker. Then I added 100 ohm resistors in serie at the exit of the 555 to limit the output current just before the connection with my speaker and the issue was fixed.


I put together this exact circuit some time ago and it works perfectly for me. Here's a tutorial and demo video I made with background information on the circuit design, details on the construction process, and several demos.

Atari Punk Console | Tutorial and Demo


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