Monostable 555 Trigger release issues

Thread Starter

Jeff Chap

Joined Jul 4, 2015
1
Hi All,
There seems to be a short coming of the 555 in its Monostable form thats holding me back. The function of the circuit in question is to protect a solenoid from burning out when a push button is held down to long. The venerable 555 seemed like the perfect solution. So what is happening is when the trigger is held low the solenoid on the output closes and never releases. What I want to happen is when the trigger is held low the the output goes low and releases after 2 seconds. Is there a simple solution Im just missing? Or is the 555 just not the chip for this circuit. I can built it with a attiny85 but that seems like a big waste of silicone.
Thanks Jeff
 

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crutschow

Joined Mar 14, 2008
24,950
Yes, the 555 output will stay high if the trigger is held low longer than the one-shot period.
You need to modify the trigger input with a capacitor, resistor and diode (1N4148 or similar) as shown here:
upload_2015-7-4_10-5-57.png
The capacitor generates a short trigger pulse, so no matter how long the PB is pressed, the 555 will still time out.
The diode allows for fast recovery of the capacitor voltage after the PB is released.

Note that there may be silicone in the chip package but the chip itself is silicon. ;)

Edit: Arrgh, they beat me to it.
 
Last edited:

ian field

Joined Oct 27, 2012
6,539
Try placing a 10uF cap in series with your pushbutton switch to provide a pulse signal.
You beat me to it - but there is a bit more to it than that.

You can activate the solenoid for a short time by the capacitor charging current, or you can charge the capacitor then the button dumps the charge into the solenoid.
If you charge the capacitor first, the charging resistor must be too high to operate the solenoid on its own. Presumably if you use a discharge resistor and actuate the solenoid by capacitor charging current, the resistor would have to be permanently in parallel with the capacitor - so that would also have to be too high to actuate the solenoid, or you'd need changeover contacts on the button.
 
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