# quick automotive relay question

Thread Starter

#### leptontec

Joined May 7, 2009
3
I have a momentary on/off switch for my car.
I want to use this switch to turn on and off auxiliary lighting.

So I need something that will take the pulse signal from the switch and stay on there is another pulse from when I push the switch again.

I think this is called a latching relay, but I know nothing about electronics. I've tried googling to learn but it's way over my head. Can anyone suggest what I need and/or where I might find it?

The lights came with a relay, but I assume it's for a steady on/off switch.

Thanks so much!

#### beenthere

Joined Apr 20, 2004
15,819
What kind of current will the contacts need to pass?

Thread Starter

#### leptontec

Joined May 7, 2009
3
It's in a car so 12v. There are 4x55 watt lamps I will power, so I guess 20-30 amps? Or probably the better way to do it is to connect whatever I use here to the relays that came with the lights...in that case this wouldn't use very much current at all. Whichever way is cheaper/easier.

Will a latching relay draw a lot of power in addition to everything else?

Thanks for helping me learn this.

#### beenthere

Joined Apr 20, 2004
15,819
Not if it's an impulse latching relay - it only draws current while changing state. Look at a P & B S89R11DAC1-12. Digi-Key carries them as part # PB513 @ $45.38. Sorry the contacts are only rated to 15 amps, though. You could use a smaller latching relay to control a big VKP-11F42, which is good for 40 amps. A Panasonic S2EB-L2-12V might do the latching, but it's still$16.34.

Thread Starter

#### leptontec

Joined May 7, 2009
3
Wow, that's really expensive. I already have a VKP-11F42, so the smaller one will work.
So I guess my question is which smaller one will work?

thanks much beenthere. i promise i tried googling this all morning before i resorted to asking for help. i'm just very confused

#### BillB3857

Joined Feb 28, 2009
2,506
Could you use two push buttons? One for on, another for off? If so a simple start/stop circuit arrangement should work for you.

#### onecraftydude

Joined May 7, 2009
5
Hook up your lights like the directions say. Then add a switch between the push button and the relay. When the switch is on the lights will work with the momentary switch and they can be shut off by the second switch.

#### CDRIVE

Joined Jul 1, 2008
2,219
There's always a solid state solution to latch and unlatch a relay with a single push switch but without a good parts stash you'll end up spending more than the cost of the latching relay mentioned. The cheapest solution would be a Push-ON - Push-OFF switch but I think you're using an existing switch?

#### tkng211

Joined Jan 4, 2008
65
From the specification of the relay, the current passing through the coil at 12V is less than 150mA
You may try to build the attached circuit. Yet I haven't tested it. You can replace the NPN transistors 2N2222 by any small signal transistor with Ic max>500mA, Vce sat<0.5V, HFE>20 @Ic=150mA.
Good Luck!

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#### CDRIVE

Joined Jul 1, 2008
2,219
The attached schematic is a Bi-Stable latch that will do what you desire. I spiced this using discrete components but there are also a number of IC configurations that can be employed.

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#### eblc1388

Joined Nov 28, 2008
1,542
You may try to build the attached circuit. Yet I haven't tested it.
Interesting circuit. I have simulated it and it works.

Originally I was thinking even with Q1 in the OFF state, Q1's collector can never go higher than 0.65V as this will be limited via the Vbe of Q2. Therefore it might not be possible to turn Q1 ON again from the charge on 100uF capacitor.

This assumption turns out to be wrong as Q2 Vbe is more likely to be 0.92V due to the high base current and so it is sufficient to charge up the capacitor instead.

In the following simulation, a voltage control switch is used to simulate the user push button at t=2,5 and 8 second.

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