Trying to figure out how to trigger a monostable 555 on a 12VDC circuit, based solely on the presence or absence of a constant 12VDC source.

Actuate on initial power up and re-actuate on power down.

 

If it actuates on power down, then what is powering the 555? You need to give us a bit more data to work with.

 

There is a 12V always on and a 12V switched. In a automotive application, the switched is a tap on radio remote wire, only plan on using it as a signal wire.

Want to fire with radio coming on and going off, couple different brands of radios but all aftermarket.

 

Power up is easy, almost a standard circuit.

What you need is something that creates another pulse with the power supply goes low.

Something like this might work. Never built it, but I think it will work.

 

How wide do you want the output pulse to be?

 

bowez,

Any idea what the available current is on your switched +12V line? What's the length of the monostable's output? What is the load on the monostable's output?

Ken

 

Current available is 15A max. Output I'm looking for is about 4.5s, and will drive a 12V PC relay (about 270ohm coil)

 

Try this. It should do what you want.

 

Ron, tested your design on Yenka and I assuming I'm not following exactly right. Did have to deviate abit form design due to its available components, plus I didn't understand where to put U1d , U1c or C3 C4 circuit.

 

Ron, tested your design on Yenka and I assuming I'm not following exactly right. Did have to deviate abit form design due to its available components, plus I didn't understand where to put U1d , U1c or C3 C4 circuit.

You had a wiring error. see below. C3 and C4 connect between +6V and ground, as I showed. You need them to absorb current transients from the 555.

 

Thank you very much, I can get it the rest of the way I think.