power off switch turns on led in car

[MrBill]

I'm trying to add a simulated alarm indicator in my car. I have a "security" light in my instrument cluster and want to use this for the indicator. What I hope to do is when the key is off, the light will flash in the dash, when the power is on, the light won't flash. Is this possible?

 

[RadioRon]

Yes, this is easy to do. You would need to build a module that has connections of:
- ignition (12V on or off with key)
- 12V battery (always on)
- ground
- lamp output

You need a flasher circuit to cause the lamp to turn on and off. This can be done with a 555 timer IC. The output of the timer would feed a transistor switch that is connected to the lamp.

Power supply for the circuit would come from 12V battery, and you need to find a way to turn the 555 IC on when ignition is at 0V and off when ignition is at 12V. I think this can be done by having the ignition input feed a transistor switch which then feeds 12V to the 555 (or perhaps just feed the output of the transistor switch to the reset input of the 555).

 

[Pommie]

Could a 12V LED with inbuilt flasher be connected between a constant live and a switched live. When the switch is on then the LED will be connected to 12V at both ends and when the switch is off the switched end would be grounded by all the normal equipment (EMU, radio, clock etc). This would probably work better on older cars, in newer fully computerised ones, a 150Ω 1W resistor from the switched live to ground would help.

Mike.

 

[mneary]

a 150Ω 1W resistor from the switched live to ground would help

Might make that 2W or bigger; 13.8V to 150 ohms is about 1.3W. Keep it from anything that can melt or burn in either case.

 

[Pommie]

You are correct, I did a quick V²/R but used 12V - figured 144/150 was about right. With 13.8V it should be a higher wattage or alternatively make it 200Ω. It's only to provide a path to earth when the ignition is switched off.

Mike.