In this post, I outlined a circuit I was building to activate a solenoid with voice using a small Arduino sound detection module (this one).

The output pulses from that module are too short to drive the solenoid directly so I had to build a monostable 555 circuit to stretch the pulses to around 50ms.

The problem is if I try to run both boards from a single 5v plugpack the sound module stops working, even with the solenoid disconnected.

The sound module output pin is active low so I connect that directly to the 555 trigger pin 2. I connect the grounds together and run power to the sound module only. It works perfectly and the output LED turns on and off rapidly in time with normal voice input.

The moment I connect the +5V power from the sound module across to the 555 circuit the sound module stops working. The output LED immediately goes hard on. I adjust the trimpot on the sound module so the LED just goes off, but the module is then too insensitive to respond to voice. I have to blow hard on the electret mic to trigger it.

The plugpack I am using is a 1.8A Google Home Mini power supply but I have tried various others with the same result.

If I run each module from a separate 5v plugpack, with the two circuit grounds connected together and the sound module output running directly to the 555 pin 2 it works perfectly and drives the solenoid properly.

I tried installing a 1000uF capacitor across the inputs to smooth out any ripples that may have been occurring in the power supply but no change. The current draw of the 555 circuit is around 1.5mA and even less for the sound module.

The 555 timing capacitor is 10uF and the timing trimpot is set to around 4.5K ohms. 10nF cap between 555 ctrl pin 5 and gnd. Mosfet connected to output pin 3 with flywheel diode across mosfet drain and +5V. 10k (or 47k) pull-up resistor on 555 input pin 2 (so I can test it without the sound module connected). 10k pull-down resistor on mosfet gate (not needed and works without it as 555 output pulls it low when off.

Can anyone suggest why sharing a common power supply should cause problems and what I can do to try to fix it please?

Sound module circuit diagram

555 circuit diagram


  • \$\begingroup\$ Connect that 510K ohm resistor between base and collector. \$\endgroup\$ – analogsystemsrf Feb 3 at 23:06
  • \$\begingroup\$ I found part of the problem. I was previously running the power to the audio module first then to the 555 module. I connected the power to the 555 first then to the audio module and the audio module now works perfectly and triggers the 555 as required. When I connect the solenoid to the mosfet output however the original problem returns. (Analogsystemsrf - It is a 10k resistor and it is a mosfet not a transistor.) \$\endgroup\$ – user2605793 Feb 4 at 4:53
  • \$\begingroup\$ I put a scope on the power supply. The voltage sags by around half a volt when the solenoid is on (it draws 1 amp), which would no doubt upset the comparator in the audio module. What I do not understand however is why in the steady state with the solenoid off the function of the audio module changes completely when one power supply is used. \$\endgroup\$ – user2605793 Feb 4 at 5:44

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