# Thread: How to pass 12 volts only....

1. ## How to pass 12 volts only....

I have the new 7inch HB lillput monitor and have the reverse light wire connected to the auto switching wire in the however i think there must be a bad earth.
When i either put my foot on the brake or turn the indicator on the monitor will switch momentary then back again. I placed a multimeter across the wire and found that i have a maximum of 0.3V passing when i do this. Obviously i get 12v when i place the car in reverse.

So without trying to sought out the earthing problem (which i think is the reason) is there a way to pass only 12v to the switching wire instead of the 0.3v that the brake and indicator creates?
I know it will be some sought of circuit but how

James

2. Are you saying it is 0V with no brake or indicator, and 0.3V with brake or indicator? (And obviously 12V when in reverse.)

That wire will be grounded through the reverse bulbs when NOT in reverse.

The 0.3V might be measured because of ground voltage drops with the stop & indicator bulb currents.

3. You can use a relay maybe?

4. Originally Posted by almera
You can use a relay maybe?
+1 this is going to be way easier then trying to troubleshoot what is causing the wierd problem on the car side.

5. Connect 12 volt relay to + side of reverse light, ground relay to a convenient ground.

Connect 12 volt line to one side of relay and your backup light indicator to the other side.

When the car goes into reverse, the relay contacts will close. Power will flow across the relay contacts and signal the monitor to switch.

You need an SPST NO (normally open) relay for this.

6. If the clicking noise gets annoying, you could always use a diode. A cheap rectifier diode, forward in series will drop the voltage by roughly 0.7V but knowing that it only takes 0.3V or possibly with an unknown current to trigger, you could also use a zener diode in reverse. Say a 5.1V which would trigger only above 5.1 volts, blocking anything below.

7. I like the zenor diode idea, could you please provide details on how to do this and the exact type of diode required.

Many Thanks

James

8. Just a common zener from radio shack would do. I think it's 1N4733A from their site.

The sense on the monitor is probably very low power, but I'm sure it's enough to drive the zener. All you need to do is put the diode in the line. Cut the wire, connect the cathode side of the diode to the wire going to the car, the other to the wire going to the monitor. The band is the cathode.

9. Yeah, a zener would work. Good call and solid state!

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