# Filtering out everything below 12v on a car reverse light circuit

First of all I've like to mention that I'm by no means an electrical engineer. I'll try to explain what I mean with the title.

I have installed a headunit which has an option for a reverse camera. The headunit has a trigger wire which takes anything from 0 to 12 V.

I know an alternator makes it be 14.4 V approximately but for easyness sake we'll keep it at 12 V. If the trigger Wire notices a signal, it triggers the backup camera display in the headunit and a 12 V+ signal on a different wire.

The way the camera is now connected is as follows. I tapped in to the switched positive lead from the reverse lights and connected it to the trigger wire. The switched 12 V+ from the headunit, I connected to the camera positive lead. The camera negative lead goes to the reverse light negative lead. The issue with this is that BMW and possibly other brands as well, use a lower voltage test signal to see if the reverse lights still work (so they can show a notification on the dashboard) this triggers the trigger wire on the headunit and that will show the reverse camera.

This is not desired behaviour since I only want it to turn in when it gets more than let's say 12 V. I have a bunch of PNP and NPN transistors laying around as well as resistors and capacitors.

I'd like to avoid using a microprocessor to identify the signal. Is it possible to create the logic I'm after with just basic electronical components and how would I do this?

• You might get away with just a voltage divider in the trigger wire. If that works depends on 1/ the camera trigger voltage level and 2/ the rear-light voltage test signal level. This would be easier to answer if can find out what those voltages are. – Oldfart Oct 13 '19 at 15:36
• "...a lower voltage test signal to see if the reverse lights still work" - Exactly what voltage is this on your vehicle? "trigger wire which takes anything from 0 to 12 V" - What voltage does it need to trigger? (must be > 0, right?). "Is it possible..." - almost certainly, but to figure out how we need to know more about this headunit. Where can we get information on it? – Bruce Abbott Oct 13 '19 at 16:09
• Well, the test voltage for the rear lights is supposedly 5 volts, but there is no clear documentation about it since bmw doesn't really share that and I can't trigger the check myself. The only way I would know is to do some sort of logging on an Arduino with a voltage divider in the analog port and check how much that reads when it happens. My first course of action will be testing how much volts it takes to trigger the headunit. It's a Chinese no brand headunit so information is again not available. I'm sorry to be so vague but this place is my last resort to make some sense of all of it. – Kenny Geens Oct 13 '19 at 17:02
• Put a timer relay to disconnect the camera trigger for 15 or 20 seconds ie the duration of the lamp test. – Solar Mike Oct 13 '19 at 17:14
• ... or use a multimeter, probably easier than using an Arduino – vicatcu Oct 14 '19 at 1:05