I am new in this kind of stuff so please excuse me for my lack of knowledge and English Language :)

I have an old car and I want to make a circuit with an LED that lights when car is switched on and doors locked, LED is turned on otherwise turned off , and when car is off and doors are locked, LED is flashing...

My car gives a pulse when trigger lock and one for Unlock so I need this pulse to turn LED On and stay on until Unlock pulse arrives. I am thinking of a latching relay ? or some sort of timer? or a D type flip flop ? What would be the best approach?

This Information will be helpful to reach the best Answer:

my car is : Peugeot 206 for year 2004

I want replace the Flasher Button from dashboard With two function Button like this:

Old Model

New Model

I do this, and attached wires,But the LED not works

  • \$\begingroup\$ Are "lock" and "unlock" pulses identical? \$\endgroup\$
    – Dejvid_no1
    Jan 2, 2016 at 9:48
  • \$\begingroup\$ No, it's different \$\endgroup\$
    – A.Rad
    Jan 2, 2016 at 9:55
  • \$\begingroup\$ you could use a latching relay \$\endgroup\$ Jan 2, 2016 at 9:56
  • \$\begingroup\$ What is latching relay?How it works? \$\endgroup\$
    – A.Rad
    Jan 2, 2016 at 9:58
  • \$\begingroup\$ This will be much easier if you update the question with more information. Do you have 12V available even when the car is off? What control signals (how many wires and voltage levels) do you have from the locking mechanism? \$\endgroup\$
    – Dejvid_no1
    Jan 2, 2016 at 10:01

3 Answers 3


The simplest solution is using a dual coil latching relay, like this one. No, I don't work at Fujitsu I just happened to use that relay a couple of times.

How does this work?

Instead of the usual single coil the relay has two coils: one to 'set' it and the other to 'reset' it. When a sufficiently long (some 20ms)pulse is applied to coil A the switch is set to position A, while when you excite coil B switch toggles to B position.

The component is quite pricey but I am sure you can find something better, since you only need to turn on a single LED.


Someone in the comments mentioned a latching relay, but the problem with those is that you need quite a bit of support circuitry to make them work. In your case, you'd need a micro to keep track of the pulses.

Then I realized, that there must be a product that has all that, so I googled for "pulse control relay" and found this product: MD-D1022A/12V (datasheet). It works from 12V and has a pulse input, so should be ideal for your application.

EDIT: As has been pointed out to me in the comments, you do have two separate pulses, so a two-coil latching relay would work. I will keep my answer, though, as it offers a different route that does not require any soldering and has additional functionality (i.e. works with a single pulse as well as with two separate SET/RESET pulses).

enter image description here

  • \$\begingroup\$ latching relays are dead simple when you have two different puses or pulses of different polsrity \$\endgroup\$ Jan 2, 2016 at 10:25
  • \$\begingroup\$ Looks like I missed the bit about him having two different pulses. Well, the board I link to has SET/RESET inputs, so it would work with that too. \$\endgroup\$
    – Armandas
    Jan 2, 2016 at 10:29
  • \$\begingroup\$ Latching relays do not need any additional circuitry at all. That's the point of using a latching relay. And keeping the contacts state on loss of power. \$\endgroup\$ Jan 2, 2016 at 11:43
  • \$\begingroup\$ @VladimirCravero I meant that you'd need additional functionality, if you only had a single pulse source. I've updated my answer to indicate that. \$\endgroup\$
    – Armandas
    Jan 2, 2016 at 12:14

it seems to me that you want a 2-coil latching relay that will operate from 12V connect the coils to the central locking circuit in the obvious way.


simulate this circuit – Schematic created using CircuitLab

power comes from the clock circuit which is always on and powers the radio's memory and the clock and similar always on accessories, the ignition circuit (which powers the engine electronics) is tspped to detect when the engine is turned on and that is used to stall the 555, the two inouts at the top are for the lock and unlock pulses from the central locking circuit the unusual D2 is to ensure that it won't blink when the engine is running.

  • \$\begingroup\$ Thanks, but i can't understand clock's mean, do you mean +12v from the battery?could you explain more? \$\endgroup\$
    – A.Rad
    Jan 2, 2016 at 10:38
  • \$\begingroup\$ yes clock is 12V from the battery, there will be a fuse somewhere to power the clock and the radio's station memory, tap into the circuit after that fuse. connecting directly to the battery is another option, but if you do that fit a fuse near the battery. \$\endgroup\$ Jan 2, 2016 at 20:34
  • \$\begingroup\$ here's a cheap-ish relay aliexpress.com/item/… \$\endgroup\$ Jan 2, 2016 at 20:41
  • \$\begingroup\$ Hi , could I disconnect the - of LED from discharge input of 555 IC in this circuit??? Because the _ contact of this LED in my car is connected to Car's - (GND) and we have only + contact for LED... \$\endgroup\$
    – A.Rad
    Jan 25, 2016 at 11:05
  • \$\begingroup\$ yes, if it goes to ground you could connect the led (and series resistor) to the output pin instead. \$\endgroup\$ Jan 26, 2016 at 8:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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