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Topic: Picking a relay (Read 1 time) previous topic - next topic


I need a relay to catch a 24mv signal and open up a 12v line.  I am really not sure what would be the best way to go, or what kind of relay would close for such a low voltage, any suggestions would rock!


The 24mv is coming from my car's ecm, a 12v source is not problem, I just a relay to switch when the control line has a signal going, it read 0 otherwise


Sep 19, 2010, 09:06 am Last Edit: Sep 19, 2010, 09:10 am by jolyrojr Reason: 1
Its only a smallish part of my arduino project, but i cant find another way to read if the doors are open.  It is kind of scary if I could accidentally damage something, Ive had good luck so far though.

As far as I can tell, the signal comes from the bcm(its bcm, did i say ecm before?), as a result of any doors opening, upon further investigation. that line seems to act as a ground, because it looks like it may be going from the ecm to the dome, and opening the ground when it doesnt need the light? in which case i dont know


Sep 19, 2010, 11:23 am Last Edit: Sep 19, 2010, 11:25 am by mpeuser Reason: 1
Back to the question: There are no electro-magnetic relays for <2 Volts. As your voltage is also far below a diode's forward voltage, a simple transistor amplifier will not work. An opamp with symmetric power supply is the easiest way (e.g. +/- 5 Volts will do).

For all other cases you have first to make sure what the source of this signal really is (output impedance!!).

I wonder about the nature of this signal, too.


Thanks for the responses, I have been doing a bit more research, and some testing.  The only references I could find about that wire were in reference to car alarms, and multiple people post that you need a relay to use the wire.  And as I assumed the wire was sending a low voltage signal to coordinate the dome activity, a relay made sense.

Upon inspection, I found the wire in questions probably acts as a ground, having lower voltage than the power wire.   It looks like it may be the same wire actually running to the dome switch, but I dont get why it would be useful to open the ground to the dome when the doors are closed if you can still turn the dome on anyway, so basically, Im still confused on this particular circuit, and finding documentation is not bearing fruit. :(


I remember from Installing a few car alarms years ago that you usually get instructions with the unit (car alarm) that will show you
where to pick up the door signals for various makes of cars
if you could obtain some of that literature or talk to a proffessional car alarm installer they could help you, also if you just need to activate a relay when one of the doors are open  maybe you could just tie
a relay in to the positive side of the dome lite circuit and ground the other side of the relay coil.


also if you do use a relay and you are connecting this relay to a
microcontroller you should make sure you install a kickback diode
across the relay coil so as not to spike the controller when the coil collaspes.


OOPs I quess the relay would actually spike back in to the car
dome lite cicuit but at any rate its a good policy to install the diode


The dome light will activate a photo transistor that can then activate a relay. You would not even need a microcontroller for that.

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