555 monostable w/ post-output pulse trigger lockout issues

See attached below:



This circuit, after power-up, works fine (in the TINA SIM) with a PB switch trigger resulting in the charging of C1 which initially draws down the trigger input to ground, and then a timed output pulse. Any subsequent trigger(s), well after the end of the output pulse, are ignored (since C1 retains its charge, holding trigger high). Re-setting the simulation resets cap charges back to zero.

On my breadboard tests (with a discharged C1), the circuit acts as in the SIM. When I remove power and then re-power the circuit, however, C1 still retains its charge and, as a result, grounding C1 won't trigger the circuit and no output pulse is generated.

If I short C1 (manually) while in power-down, then repower the circuit, it behaves as it should.

I've tried various resistive discharge arrangements but, as I expected, that didn't work.

Is there a circuit mod that will fully discharge C1 at power down without something mechanical (i.e., relay contacts released into their NC state, thereby shorting C1 at power-down, for instance)?

Any suggestions greatly appreciated.

CBB

Add a pull-up (my R3). Make (my) C5 much smaller. Add D2 to prevent the Trig pin from being driven positive when the PB turns off.

Mike,

Thanks for responding.

I'll give it a shot in the AM.

CBB

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