I am mixing audio tones from two 555s.


simulate this circuit – Schematic created using CircuitLab

The basic circuit is working fine so I have not shown all the little capacitors and resistors, etc.

I want to control the volume of 555 #1 only.

That is, I want to be able to set 555 #1 to 0 volume but still hear 555 #2.

I tried using a potentiometer as shown in the diagram. But if I set it to 0 resistance, all the current goes to ground and I cannot hear 555 #2.

I also tried using a large 256K pot at the output of 555 #1. This reduced the volume but the sound was still just audible.

Is there some way to isolate the two branches?

Is there some method with transistors?

Do I have to use two separate power sources?

What other options are there?


Wire output 1 to one end of the pot, wire output 2 to the other end of the pot and take your "mixed" output signal from the wiper. You might need to increase the pot value to 200 ohm to avoid loading the 666 timers up too much.

  • \$\begingroup\$ 666? Was that intentional? The number of the beast... \$\endgroup\$
    – Steve G
    Aug 24 '17 at 11:09
  • \$\begingroup\$ I tried this with a 250K pot and it basically works. However the signal it is still audible even at max resistance. So I would probably have to use a very large pot. Is there a way to get volume to 0 without just adding more resistance? \$\endgroup\$
    – thatsagoal
    Aug 24 '17 at 11:30
  • \$\begingroup\$ If the output impedance from each 555 (666 LOL) is zero then you would get perfect attenuation at the opposite ends. You could use a stereo ganged pot to obtain better (perfect) mixing of course but it's getting more complex. \$\endgroup\$
    – Andy aka
    Aug 24 '17 at 11:35

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