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)

  • \$\begingroup\$ Are you asking to design it for you? \$\endgroup\$ – Eugene Sh. Jun 12 '18 at 17:06
  • \$\begingroup\$ Eugene; Yes, I need design help. I have a basic knowledge of circuitry and resistors, 555 timers, capacitors, etc. from my background as an EOD technician but I don't have the design power to figure this out on my own. \$\endgroup\$ – Wyle E. Coyote Jun 12 '18 at 17:16
  • 3
    \$\begingroup\$ @WyleE.Coyote the intent of this website isn't to provide design service. Instead, the community can provide guidance to help you design this yourself. If you have ideas on how to accomplish this, please add it to your question. \$\endgroup\$ – Daniel Jun 12 '18 at 18:01
  • \$\begingroup\$ In practice today this kind of thing is almost always done with a low end microcontroller. Especially as blinking a light has little utility - a useful task is likely a little more complicated, which means a far more complex dedicated circuit but typically only a little more code in an MCU. \$\endgroup\$ – Chris Stratton Jun 12 '18 at 20:22
  • 1
    \$\begingroup\$ Daniel - My apology. I did not realize this was the intent of this site. I had searched for info for over an hour and really liked the multiple responses I kept running into from this site - so I joined and threw my question out there. I'll work with staying within the confines of what this site is designed for. Thank you. \$\endgroup\$ – Wyle E. Coyote Jun 13 '18 at 22:49

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:

enter image description here

Link to LTspice circuit

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:

  • \$\begingroup\$ Dim - brilliant, thank you. I totally follow with everything you stated. I'll start working on this and let you know the outcome, and come back here with results and updates. Thank you! \$\endgroup\$ – Wyle E. Coyote Jun 13 '18 at 22:51

Not the answer you're looking for? Browse other questions tagged or ask your own question.