I (am seeking design help with building) a vehicle 12v circuit with blinking LED that turns off after 30 seconds, that plugs into the cigarette lighter socket. It stays off until the vehicle power is turned off and then turned back on again. Basically, every time I get into my vehicle and turn it on, I want the LED to blink for 30 seconds and stay off until the next time I get into the vehicle and start it. Thank you for your assistance. Ron (edited above thanks to Eugene's question)
Ok, I'll bite. Whole designs are not what this site is for, as others said, but I have serious stuff to do that I wasn't ready to do yet... And since you know some 555 stuff, let's go.
Here is something that should work:
Basically, there are two stages: one 555 that provides the turn-on timing (30 seconds monostable), and another that provides the blinking (~0.8s astable).
The circuit is actually pretty basic. The only tricks are:
- Starting the first stage with a small RC network (R3+C4), so that the 555 sees a trigger pulse when the 12V supply is provided, and then nothing until the power disappears and reappears again. The output of the first stage will therefore stay on for only 30 seconds each time power is applied.
- Chain the output of the first stage to the RST pin of the second 555, so the output of the second stage is low outside of the active period of the first stage.
Note that you can use a NE556, which is two 555 in a single package.
However, I advise you to be careful about powering the circuit directly from the car supply. The car battery voltage can go pretty high in some situations (load dump), and you will probably need some protection, because NE555 can only stand 18V max, which is far from sufficient in an automotive environment. Random relevant links about this: