# Generate DC pulse after switch turned off?

I've got a power source (say 12V DC) that I'm monitoring with a relay. I want to send a pulse (over 5V DC) to another relay for 60 seconds after the first power source has been turned off.

A practical example would be: A computer has been turned off and the power supply is no longer providing power, turn on a relay to turn on a light for 60 seconds (or send a 60 second 12V DC pulse to the relay).

I'm having a hard time finding a circuit for this. I'm fine using transistors/diodes/caps/sub \$1 ICs/etc. but want to keep it analog.

• Look up monostable one-shot, the 555 can do it. Why do you want it analog? A uC will do this way better. Jan 31 '13 at 18:37

Insta is right, a 555 is simple to setup for this and costs less than a dollar. I was thinking you were including something like the 555 in your considerations, but just in case here is a possible transistor only solution (parts shown and values are guidlelines, circuit is rough and could be improved, but it's gives the idea - time will vary with components chosen, so you may need to tweak things a bit, add a pot instead of the R1/R7, etc)

Circuit:

Simulation:

L1 is your relay coil, and is powered for roughly 60 seconds on the 12V supply dropping out. C2 may need to be increased if the +12V supply drops slowly.

555 Solution

Here is a version based around the trusty old 555 timer IC:

Depending on how fast the 12V supply drops, you may need to increase C3 (if it drops slowly) Again values are guidelines, you can tweak as necessary. The R3/R4 divider stops the voltage going negative at the trigger input, which might cause issues.
The diode (D1) can probably also be included in a similar fashion with the first circuit (in series with R5) to stop the power on pulse you mentioned (you would also need a large value resistor from the input side of C2 to +5V)

Simulation:

• I built this in Orcad Cadence,and while there is an initial pulse from the circuit after the voltage is turned on initially, I think I'll still use the circuit. All the 555 monostable circuits I looked at didn't seem to have this behavior. Perhaps you could link me to one? Feb 2 '13 at 5:28
• @reeeky2001 - Okay, no problem. I'll add a version which won't pulse on switch on, plus a 555 version a little later on. Feb 2 '13 at 6:21
• @reeeky2001 - I added a 555 version. If you have any problems when you actually build whichever circuit you choose, just let me know and I'll help to get it working. Feb 2 '13 at 11:21