I have two 555s in astable mode producing two audio tones that are sent to a speaker. The idea is to mix the tones. The power source is a single 9V battery. The problem is that the two 555s interfere with each other.

I added some decoupling capacitors across the VCC and GND pins of each 555 but it looks like I will need 1000uf to separate their signals perfectly. Small capacitors in parallel seem to have no effect.

  1. Is 1000uf excessive? It feels excessive.
  2. Are there any other common methods for decoupling?
  3. Could I avoid the problem entirely if I used an LM358 op amp as an oscillator instead?

EDIT: More information

  • I have not tried the CMOS 555s but that is probably the easiest solution.

  • I am using a summing amplifier (op amp) which sums the signals but does not stop the 555s from interfering with each other.

  • 2
    \$\begingroup\$ If you mix (sum) the two tones, why don't you expect them to interfere with each other? \$\endgroup\$ – The Photon Aug 13 '17 at 16:05
  • 3
    \$\begingroup\$ Why not just use a summing amplifier? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 13 '17 at 16:08
  • 1
    \$\begingroup\$ If there is enough draw on the rails to cause interference between 555s, you have something seriously wrong with your circuit. Post a schematic. \$\endgroup\$ – uint128_t Aug 13 '17 at 19:09
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    \$\begingroup\$ Try CMOS 555s. Bipolar ones have a well known bug whereby they draw large current spikes while switching. Maybe 1000uF isn't so excessive. (Alternatively, run each off its own 5V LDO regulator to isolate them) \$\endgroup\$ – Brian Drummond Aug 13 '17 at 19:12
  • \$\begingroup\$ How is the signal connected to the speaker? Is the speaker also connected to the same battery? If yes, that could be drawing a significant amount of current from a battery that has relatively high impedance, and that could cause large voltage ripple. But like uint128_t mentioned, there must be something else causing the interference you are referring to. Post schematic and a bit more info. \$\endgroup\$ – Lucas Aug 14 '17 at 0:48

Do this


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks and the small capacitors help a little. But I think I will have to use the CMOS 555. \$\endgroup\$ – thatsagoal Aug 15 '17 at 16:17

I found a solution (suggested by Brian Drummond - thanks).

I used a LM7805 5V voltage regulator just after the 9V battery and all my mutual interference problems disappeared.

You still need around 100uf around each 555 as decoupling capacitors but this is not so bad.

The LM7805 is not an LDO regulator but it did the trick.

I'm not sure why it worked exactly, since it seems like the 555s should be impacting the circuit even more if there is less voltage.

If anyone knows why this works please leave a comment.

  • \$\begingroup\$ Please, add schematics to your answer, and to your question as well. \$\endgroup\$ – Chupacabras Aug 17 '17 at 15:03
  • \$\begingroup\$ The spikes of 555 will be proportional to VDD, perhaps even VDD^2. For CMOS, the current is K/2*W/L*Vgs^2 and Vgs is proportional to VDD. \$\endgroup\$ – analogsystemsrf Aug 18 '17 at 3:15

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