I would like to make an alarm that goes off about 1 minute after it has been triggered. To do this, I want to delay the circuit using a 555 timer in monostable, but I can't do this as they turn it on for a certain amount of time and then off. Is there a way I could set up the 555 so that this is reversed? (I can't use a NOT gate as then it would be on while not being triggered) Or maybe another way this can be done?
Electronic – Can you set up a monostable 555 timer so it reverses the output
555delaytimer
Best Answer
I think if you use a second 555, also connected as a monostable, but replace the timing capacitor with a short (and you don't need the timing resistor)-- triggered by the first one, that should work.
You could also use a D flip-flop such as a 4013 with the D tied high, but you'd need an inverter on the clock and a power-on reset circuit.
Or, something like this:
simulate this circuit – Schematic created using CircuitLab
C1/R3/D1 provides a semi-reliable power-on reset (100% reliable if the power turns on cleanly every time after a relatively long 'off' time where +V falls to near 0V).
C2 and R4 provide an edge trigger from the 555 output falling edge.
R1 and R2 protect the CMOS inputs