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I have made a monostable multivibrator and an astable multivibrator using 555 timers using a popular circuit design found on the web.

The monostable (out via pin 3) is used to control the astable (via its pin 4). When I push the momentary switch on the monostable the astable should flash two LED’s until the mono times out. And it works well if I use seperate power supplies to each multivibrator. (The zero volt rails are commoned together).
However, when connected together to a single power supply as in the circuit below, the mono will no longer time. Holding the switch closed will operate the astable ok until the button is released.

They only seem to work together if I use separate power supplies for each multivibrator.

I have changed resister and cap values on the mono to no avail. Any ideas or suggestions would be much appreciated. enter image description here

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Most likely when the LEDs turn on or off they cause some sort of transient on the supply voltage, what can make the monostable misbehave. Pretty significant transients can be created when long power supply cables are used due to the series inductance.

A negative transient on the supply may cause the voltage on pin 5 of the monostable to also drop. Once the voltage on pin 5 becomes lower than the voltage on pin 6, the monostable will reset itself and disable the astable circuit.

Things you can try to fix this:

  1. Add a bulk capacitor across the positive and negative terminals of your power supply close to your circuit. This is a common practice in any kind of circuit to mitigate problems like this. Smaller decoupling capacitors next to each 55 IC can also help.
  2. Add a capacitor between pin 5 of the monostable 555 and ground to filter the threshold voltage used in the internal comparator. Inside the 555 there are 3 ~5kΩ resistors in series. The external capacitor will form an RC filter, where the equivalent R is ~3kΩ (5kΩ//(5kΩ+5kΩ)).
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  • \$\begingroup\$ OK, thanks for your reply. I will try the cap on the supply rails. I have a 0.01 microfarad cap between pin 5 and ground already. Thanks. \$\endgroup\$
    – Richard
    Apr 26 '19 at 9:01
  • \$\begingroup\$ You could also try to use bigger capacitance on pin 5. \$\endgroup\$
    – joribama
    Apr 26 '19 at 9:05
  • \$\begingroup\$ Thanks joribama, I placed a 1000 microfarad electro across the 12 volt supply rails close to the circuit board and this fixed my issues. All working as expected. Thanks for your help. \$\endgroup\$
    – Richard
    Apr 27 '19 at 1:30
  • \$\begingroup\$ I’ve been there. You’re welcome. \$\endgroup\$
    – joribama
    Apr 27 '19 at 8:36

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