Electrical – Replacing push button by transistor in 555 circuit

^{simulate this circuit – Schematic created using CircuitLab}

This also has a retrigger wait time equal to the One shot time but cuts the time short with the switch. It also has 10ms debounce filtering.

• R2C2 = 10ms debounce time filter shown
• Adjust T=1/(R3)C1 [s] for time constant while pressed (e.g. 10 s shown)
• R2C1= retriggerable wait time to avoid repeat triggers (e.g 10s shown)

user must define all 3 values

Among experienced EEs it is a well known fact that electrolytic capacitors (like that 100 uF one) can have huge tolerances. These capacitors often have a 20 to 30 % higher value than their nominal value. As there caps are mostly used for supply decoupling that is usually irrelevant.

You are trying to make a (somewhat) precise timer with a timing of a few seconds. You cannot expect much precision from this circuit, the NE555 is not very well suited for longer timings. Most EEs would use a faster running clock and a counter, the CD4060 (14-stage ripple carry binary counter) is a candidate for that. You can make it monostable if you play with the reset.

To solve both circuit problems I would add a small (10 nF) capacitor in parallel with R2, this will force the Trigger to be slightly longer when the button is pushed. You could try a different combination of R1, C1 like 1 Mohm and 5.6 uF. That way the capacitor is smaller making it easier for the discharge transistor in the NE555 to discharge it.