I have designed a basic application of a 555 in a monostable configuration to delay the LOW/off state of an output signal. (The output drives some power electronics through an optocoupler).
As per my simulations and breadboard tests, when the foot pedal is depressed and held the output should remain high until it is released where the output remains high for a further ~0-10s depending on the value of the adjustment R5.
The idea is that C7 discharges through Q4 until the pedal is released where it charges up to 2/3VCC and then turns the output LOW. Unfortunately, when I had this circuit board printed and I populated it, it behaves as expected only for the smallest values of the potentiometer (delays up to around a second), after which, at around 100k or so the output just stays high indefinitely until I turn the knob back down below this value.
I cannot think of any theoretical reason this should be happening. Is there something I'm missing?
Any advice is greatly appreciated, thanks.