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.
Best Answer
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: