Dash Cam HOLD ON circuit

Thread Starter


Joined Sep 24, 2015
Last night someone hit my truck. No serious damage but I wish my dash cam had been on. Unfortunately it's only on when the truck is running. It HAS an internal battery, but that will deplete in short order if I try to run it in Motion Detection mode. So what I'd like to do is build and install the below circuit. It will not keep the camera running after I shut the truck off unless I push SW1 (hold) button while the engine is running. The camera will stay on until I step on the brake pedal. Obviously I won't be running it when the car is parked for hours on end; but when I run into the store for that quick pickup item, this won't harm the truck battery at all.

The plan calls for a relay - which I have plenty of them. However, I would like an electronic switch of some sort. As you can see, when energized, the relay will keep itself powered. The relay is chassis grounded through the incandescent brake light. If I step on the brake pedal and the brake lights light up, the relay is back-powered with positive feeds on both ends - the relay will drop out. So if at the store I push the button the camera will continue to record until I step on the brake pedal, which means when home the cam stops recording when I shut the truck off.

Condition 1) Engine off / dash cam off
Push SW1 - nothing happens.

Condition2) Engine on / dash cam on
Push SW1 - relay locks on - cam remains on

Condition 3) Engine on / dash cam on - step on brake pedal
Relay K1 drops out. Cam remains on until engine off.

Condition 4) Engine off (same as cond. 1)

What I would like to do is replace the relay with either transistor(s) or MOSFET(s). Any recommendations?
Last edited:


Joined Oct 29, 2013
Would it serve the same purpose and be easier to just install a delayed-off circuit, that keeps the camera running for a couple of hours after you turn the key off? The camera drain on a car battery should be insignificant for that amount of time.

Thread Starter


Joined Sep 24, 2015
MrSoftware hadn't considered that. Thanks for the suggestion. Will explore. Shouldn't need more than a half hour. A full hour may be useful when shopping with the wife.

[edit] but then when I park the truck I would have an hour of video inside the garage. I still think I'd want something I can set, like a trap, only when I want it. Maybe I can incorporate such a trap as set when pushing a certain button. Much less messing with the vehicle wiring too.