555
I would like to create a circuit with a button. When this button is pressed, a circuit should be closed for a fixed amount of time.
Right now, I have managed to get the desired effect using an arduino, but that seem way overkill for my needs.
I have found similar questions, like this one on EE, but my requirements are different. I need the circuit to be closed for 1 to 50 milliseconds (the value would be selectable using a potentiometer). Is the 555 timer suitable for this kind of durations?
The 555 can't be reduced under 50% duty (just with resistors, see below) since the discharge resistor has to be at less than half the value of the charge resistor (otherwise the junction of the two cannot reach the low trigger of 1/3 the supply voltage - see datasheet pg.12)
The various ways to do this would be to: either add a diode across the lower charging resistor, or add a second inverting transistor between the output and the TIP122. Or you could use a PNP transistor to drive your load.
Diode option (ignore part number for NPN shown):
Simulation:
Or something like this:
Simulation:
A 555 isn't really required for a simple task like this. You can make the LED latch with another transistor and a few extra components:
When the voltage at the junction of the photoresistor and R4 rises high enough, Q1 turns on, turning on the LED. Q1 pulling low also turns on Q2, which charges C1 and latches Q1 in its ON state. Discharging C1 with the switch turns the LED off, and assuming it is sufficiently light (the resistance of the photoresistor has decreased), Q1 and the LED will remain off.
Best Answer
Yes, you could use NE555 in a monostable one-shot configuration. There is a lot of posts/docs about it. Just use google.
Another solution, simpler, is something like this:
For a 5V supply, if you vary R1 between 10Ohm and 1kOhm, you get a one-shot pulse that powers RLoad for a time between ~1ms and ~50ms (this is not tight tolerance). The load shouldn't be much more than 50mA, otherwise, you need to adjust other parameters.
There is one thing to take care, however, is that you can run into missed or shorter pulses problems if the switch is closed/opened/closed too fast (200ms of opening time between two button presses seems to be enough to guarantee you won't run into those problems).
Note: The load can be powered by a voltage different from the switch voltage without effect on the delay time.