# Adjustable 555 timer off duty delay

What I want to do is have a circuit with a 555 timer (which is what I have available) that works the opposite of what a monostable circuit should do—well, kind of. What I mean is, on a regular monostable circuit, when the trigger is connected, out turns HIGH and remains HIGH for an amount of time depending on the RC network, right? What I want to do is to have the trigger impulse start a countdown and only after the countdown, the output turns HIGH for—I don't know—a pulse, something between 0 and 1 seconds. How should I do this?

I need to be able to regulate the time it is off before turning on, the on time should stay the same.

The purpose is to "fool" an automatic door to think the close limit is on so the motor stops after the output goes high, this way I can open the door like 30% or 50%, depends on the time the output on the 555 is low.

If I'm reading this question correctly you need two monostable circuits.

The circuits are both negative edge triggered monostables.

The first negative edge comes from the switch, S1, (this could be replaced by an NPN transistor if you want it to be triggered by another device). C1 and R2 produce a short spike from +V to 0V on the negative going edge of the input signal. This starts monostable 1 with R3 and C2 controlling the length of the pulse (T1) When the switch is opened diode D1 limits the positive going edge spike amplitude to 0.6V above +V, but this will not retrigger the monostable.

When the first monostable goes high R4 and C2 only allow the positive edge of this pulse through. D2 limits this to 0.6V above +V. This positive edge will not trigger the second monostable.

When the first monostable output goes LOW the negative edge of the pulse triggers the second monostable and its output goes HIGH for a period controlled by R5 and C4.

By having two separate monostable periods you can independently adjust the initial delay (T1) and the operating pulse time (T2)

• Thank you man, that seems spot on what i want, im off of work now, but tomorrow morningmorning i will try this and i will post the results, but from my reading of the schematic, it seems right :) once again, thank you for your help, cheers ;) Commented Dec 5, 2013 at 19:36

If you want to use only one 555 IC, you can build a regular monostable with it but instead of putting a switch directly on port 2, do something like this:

simulate this circuit – Schematic created using CircuitLab

If you make sure that R1C1 is much bigger (10 times or so) than R3C3 it will delay the switching on of the monostable by about $R_3C_3ln(2/3)$ seconds.

• so if i change R3 or R1 for a potentiometer, will it allow me to regulate the time before on? Commented Dec 5, 2013 at 18:16

if your question is to turn on the motor for sometime when output pulse is low and you want to adjust this time.Here is the circuit.>http://www.electronicshub.org/adjustable-timer/.The first circuit suits you.But if you want to invert the output you should use a inverter IC,so that when the pulse is high it give low output and when it is low it gives high output..or you can simple use a transistor at to invert the output...