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.

  • 3
    \$\begingroup\$ Look up monostable one-shot, the 555 can do it. Why do you want it analog? A uC will do this way better. \$\endgroup\$ 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)


Power Off Relay Drive


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:

555 Version

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)


555 Version Simulation

  • \$\begingroup\$ 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? \$\endgroup\$
    – reeeky2001
    Feb 2 '13 at 5:28
  • \$\begingroup\$ @reeeky2001 - Okay, no problem. I'll add a version which won't pulse on switch on, plus a 555 version a little later on. \$\endgroup\$
    – Oli Glaser
    Feb 2 '13 at 6:21
  • \$\begingroup\$ @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. \$\endgroup\$
    – Oli Glaser
    Feb 2 '13 at 11:21

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