On/Off switch will turn relay on, but will not turn relay off.

Thread Starter


Joined Apr 18, 2024
I've included the the schematic. It works as intended when using a 12V incandescent bulb as a resistive load however when I connect the relay it will not turn off anymore.



Joined Mar 14, 2008
Check the 555 Q output voltage when driving the relay.
It may be pulled down by the relay load so the voltage going through R3 to C1 may not be sufficient to reset the 555 (needs to be above 2/3rd of the supply voltage).

If it's not, it might work better to connect the relay coil to the +5V and have the 555 act as a low-side switch to turn it on.
The 555's output is slightly better at sinking current than sourcing it.
You can also connect the pin 7 DIS pin to the Q output to improve the sink current capability and lower the output saturation voltage.
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Joined Aug 1, 2013
Check the relay datasheet for the coil current at 5 V.
Check the relay datasheet for the minimum guaranteed pull-in voltage.
Check the 555 datasheet for the minimum output voltage when operating on 5 V and driving the relay coil resistance.

See if the 555 can do what is required.



Joined Dec 31, 2017
Another suggestion in order to retain the functionality of the circuit is to use the output of the 555 to drive a npn transistor to activate the relay.
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