one shot help

Thread Starter


Joined Nov 16, 2009
i would like to know how do i connect the output of the one shot to a relay as i wanted to activate the relay for about 5sec when the one shot is triggered.

the one shot circuit is attached with this message.



Joined Jul 17, 2007
The formula for the time delay in that circuit is Ts=1.1RC, or Time_in_Seconds=1.1 x Resistance x Capacitance.
Since your cap is 1uF and your resistor is 1 MEG, your time delay will be 1.1 seconds.
If you want closer to 5 seconds, then you will need to use a 4.55 MEG resistor.
Or, you could change to a 4.7uF cap - a little too long - and the cap would be larger and more expensive to change than the resistor.

To trigger the relay, connect a 100 Ohm resistor from pin 3 of the 555 to the base of a 2N2222 resistor.
Connect the emitter of the transistor to ground.
Connect the collector of the transistor to the ground side of the relay coil.
Connect the other side of the relay coil to +V.
Note that you will also need a diode connected across the relay coil; cathode towards +V.