This is my first post here. I have an aftermarket backup sensor that needs to be disabled when I back a trailer. I want to be able to push a button to disable it, but have it be automatically re-enabled when the ignition is cycled. Right now the buzzer is powered by the reverse tail light, but I want the disable device to be controlled by the ignition. Reason being is when I've got a trailer, I may put it in R and D many times (cycling power to the unit), but I'll want the system to be silent until the next time I start the vehicle. So whatever controls it controls the power is NC until I press the button to open the circuit, but then it goes closed when ignition is OFF (or off then on).

I looked for a simple NC latching relay, but it doesn't seem to exist. All latching relays I found required a signal to toggle them between open and closed. The push button will give it the signal to open, but how can I give it a momentary signal to close once the ignition cuts off?

I suppose I can also use a timer relay, but I am hoping to do it the way I described.

  • \$\begingroup\$ Might be easier to just enable it with Rev. when you need it for simplicity, otherwise you have to keep cycling power when you need it after it is disabled. \$\endgroup\$ Commented May 11, 2021 at 19:51
  • \$\begingroup\$ Is this for a commercial vehicle? Please have a look at this before you decide that it's a good idea to disable the backup alarm. ksbw.com/article/… \$\endgroup\$ Commented May 11, 2021 at 19:54
  • \$\begingroup\$ It's a backup sensor alarm to alert me that I'm about to hit something. It's not a backup siren to alert people that I'm backing up. It's so I know how close I am to whatever I'm backing up to. \$\endgroup\$
    – idoseadoo
    Commented May 11, 2021 at 20:14
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    \$\begingroup\$ Ok, I see the issue. The sensor is falsing on the trailer. In that case, if there are backup lights on the trailer, sense current going into them and use that to disable the alarm. If you have a spare pin on the trailer receptacle connector, feed back the reverse lamp voltage from the trailer plug, then use that to open up the relay that powers the alarm. \$\endgroup\$ Commented May 11, 2021 at 21:38
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    \$\begingroup\$ Does this answer your question? 1 pulse toggle with simple non programmable electronics You can use an NC contact for your sensor instead of an NO contact. \$\endgroup\$
    – Dave Tweed
    Commented May 12, 2021 at 1:35

2 Answers 2



simulate this circuit – Schematic created using CircuitLab

Probably easiest to use a SPDT relay with the NC contact for your sensor and the NO contact wired back into the relay coil. D1 depends on your coil rating, the 4148 is just the standard one the schematic editor puts in. V1 is an ignition controlled supply that can handle your sensor.

Edit: if it's important that the sensor only have power if you are in reverse, use a DPDT relay and hook up the reverse light and sensor to the other pole.

  • \$\begingroup\$ Yes, the Reveres Light has to be on for the sensor to be on. From my understanding, the DPDT is two separate relays in one body. How is that different from just having two relays? Can you show me a diagram? Also, do you know if they use these in cars? It's quick and cheap for me to pull one from the local scrap yard. \$\endgroup\$
    – idoseadoo
    Commented May 11, 2021 at 20:12
  • \$\begingroup\$ I've edited the schematic to show how to hook up a DPDT relay in that configuration. DPDT is two electrically independent SPDT contact sets that are switched by a single coil. I cannot speak to whether or not you'd be able to pull one but they're not too expensive to buy new ($10-15). \$\endgroup\$
    – vir
    Commented May 11, 2021 at 21:41

The issue you're having is the backup alarm falsing on the trailer. So let's assume that the alarm is useless in that case, so we'll disable it when reverse is engaged. But we only want to do that when the trailer is connected.

Ok, so I came up with a solution.

This circuit is installed entirely at the rear of the vehicle, near the sensor itself.

  • With no trailer plugged in, the sensor works as normal, activating with the backup lamp wire.
  • With the trailer plugged in, the sensor is disabled even when in reverse.

Benefits: wiring is all local to the hitch receptacle and sensor. Nothing to remember, works automatically, no button to install, no chance of forgetting to reset the ‘mute alarm’ once you disconnect the trailer.

Here's the circuit (simulate it here):

enter image description here

How it works:

  • With no trailer plugged in and reverse engaged, the trailer reverse lamp isn’t detected, transistor never turns on and the relay is off. Power goes through the relay NC contacts to the alarm sensor as normal when reverse is on.
  • With the trailer plugged in and reverse engaged, reverse lamp is detected, transistor turns on and energizes the relay, opening the relay contacts and cutting power to the alarm sensor.

The diodes are 1N4004 type, and the transistor is a PNP, like 2N3906 or similar (very common.)

The 3 diodes in parallel can safely supply up to 3A to the lamp (36W.) Add more diodes if the load is more than that.

Bonus: You could do something fancy and add a second sensor to your trailer (assuming the sensor is wireless.) When plugged in, the vehicle sensor is off and trailer sensor is on.


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