# Simple circuit to turn on when above certain voltage?

#### MikeA

Joined Jan 20, 2013
272
This is for in-vehicle purposes. Would like the circuit to sense when on-board voltage exceeds say 13V, which means the engine is running and the alternator is spinning, and then provide power to another low power (<1A) circuit.

When the engine is not running, the sensing circuit would cut the power to the secondary circuit, and not use much power for itself while the vehicle is parked.

Any ideas for a circuit that does not use any purpose built ICs?

#### dl324

Joined Mar 30, 2015
13,527
Use a comparator like LM393 to switch power using a MOSFET (high side or low side).

#### MrSalts

Joined Apr 2, 2020
723
Just use two diodes if your device can handle a bit lower voltage.

the 12 volt is used until the switch is closed. Then no current flows out of the 12VDC - only 13.5v.

#### MikeA

Joined Jan 20, 2013
272
Just use two diodes if your device can handle a bit lower voltage.
It can, but I'm not following how this works. This sensing circuit would have only 1 input, that alternates between 12v and 13.8v, depending on if the engine is running or not.

#### MrSalts

Joined Apr 2, 2020
723
so, you're asking for a circuit like used for the cigarette lighter on most import vehicles. The lighter (accessory power) only works when the key is turned to run position (I.e. engine running, typically).

#### MikeA

Joined Jan 20, 2013
272
so, you're asking for a circuit like used for the cigarette lighter on most import vehicles. The lighter (accessory power) only works when the key is turned to run position (I.e. engine running, typically).
In a way, yes. This sensing circuit would be completely autonomous. As @dl324 mentioned, a comparator was something that came to my mind first, but I was wondering if there is something more elegant, and something that would drain less when idle. Something in the 0.1ma range would be ideal, if I decide to have more than a couple around the vehicle.

#### dl324

Joined Mar 30, 2015
13,527
I was wondering if there is something more elegant
Just connect the device to a switched 12V circuit. There should be several fuse circuits that you can use.

#### crutschow

Joined Mar 14, 2008
28,201
If the vehicle has an alternator light, you could use the voltage from that to determine if the engine is running.

#### Juhahoo

Joined Jun 3, 2019
250
Motor oil pressure sensor (=oil pressure light) or as mentioned alternator light. Both turn OFF when motor is running.

#### MikeA

Joined Jan 20, 2013
272
If the vehicle has an alternator light, you could use the voltage from that to determine if the engine is running.
The reason for this circuit is I want to have devices around the car that only turn on by themselves, or can be turned on manually, when the engine is running. The purpose is to not drain the battery.

Tapping into a circuit that is only on when the engine is running, or sensing some other variable is not feasible as not tearing up the interior and running wires is the goal.

Say I want to put a fairly bright LED into the dome, but want it to work only when the vehicle is running. Running a wire up there would be tricky. But a little PCB the size of a penny would fit just fine.

#### ElectricSpidey

Joined Dec 2, 2017
1,970
Be aware the battery voltage will not drop to 12 volts as soon as you shut off the engine.

Maybe you could use a circuit to sense the "noise" on the line when the engine is running.

#### Tonyr1084

Joined Sep 24, 2015
6,459
Maybe I'm the only one who's lost here; you want a circuit to become active when the voltage exceeds 13V (engine running) OR a circuit that can be manually turned ON? Then you speak of a dome light that is on or can be turned on manually. That means it's always on when engine is running or on when you turn it on. Maybe I need to eat a snickers bar. What's the problem with using an accessory switched source? You don't want to tear up the dash and run wires? Then how are you going to run wires if you don't want to run them? Oh, wait - I think I see something in that question: You want something that is on an existing circuit that comes on when engine runs. OK. The mud and fog is clearing a little. But now I want to know "Why?" If you have a light that can be turned on manually via the wires that already exist OR turn it on automatically when voltage rises above 13V. If it can be turned on and off manually then if it's "OFF" how is it going to see a voltage? Even if the dome light is switched on the ground side, how are you going to sense the voltage without ground reference?

Sorry, I have more questions than answers for you. Perhaps some clarity will come with greater information that is precise and accurate. Right now I just don't fully understand the need or the desired solution.

#### BobaMosfet

Joined Jul 1, 2009
1,898
In a way, yes. This sensing circuit would be completely autonomous. As @dl324 mentioned, a comparator was something that came to my mind first, but I was wondering if there is something more elegant, and something that would drain less when idle. Something in the 0.1ma range would be ideal, if I decide to have more than a couple around the vehicle.
@MikeA - "drain less when idle" you say-- it's not voltage that is drained, but current. Few things will draw less current than an OpAmp or comparator (choose carefully)...

#### crutschow

Joined Mar 14, 2008
28,201
Below is the LTspice simulation of a comparator circuit using a low-cost, programmable TL431 voltage reference as a comparator to turn a MOSFET on when the battery is above 13.5V and off when it drops below 13V.
Potentiometer U2 allows adjustment of the trip point.
R4 adds some hysteresis to make the turn-on voltage (green trace) about 1/2V higher than the turn-off voltage, which prevents oscillation at the trip point.

The circuit takes about a half mA (blue trace) when the load is turned off. That is much less than the normal self-discharge current of a vehicle battery and would take years to drain it.

The M1 P-MOSFET can be just about any that has at least a 50v voltage rating, and a low enough on-resistance that it dissipates less than 1W with your load current.

It can be constructed on a small PCB or perfboard.

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#### dcbingaman

Joined Jun 30, 2021
498
Here is another possibility:

This can handle up to a 2 Amp load with no issues. I would recommend the caps for filtering as the noise on the 12V of a car is significant. Recommend breadboarding it first, you may have to fine tune the parts some. Q2 is a 2N3904, forgot to change that.

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#### LowQCab

Joined Nov 6, 2012
1,340
Here's my 2-cents ........
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#### dcbingaman

Joined Jun 30, 2021
498
Here is another version. The quiescent current less than 1mA. You may use other part numbers for M1. This one can handle up to 5 Amp loads.

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#### crutschow

Joined Mar 14, 2008
28,201
Be aware the battery voltage will not drop to 12 volts as soon as you shut off the engine.
True.
But it shouldn't take long to drop below 13V for even a small drain on the battery, leaving it still essentially fully charged.

#### MikeA

Joined Jan 20, 2013
272
You want something that is on an existing circuit that comes on when engine runs. OK. The mud and fog is clearing a little. But now I want to know "Why?" If you have a light that can be turned on manually via the wires that already exist OR turn it on automatically when voltage rises above 13V. If it can be turned on and off manually then if it's "OFF" how is it going to see a voltage? Even if the dome light is switched on the ground side, how are you going to sense the voltage without ground reference?
So the issue I have is a vehicle which is not driven enough and the battery is always somewhere below 60%. The engine just doesn't run enough, and there's lots of parasitic load from factory gadgets when the vehicle is parked. That's in addition to self discharge of a lead acid battery. I also have a bunch of aftermarket equipment, that taps into constant on circuits around the vehicle. For example the vehicle has 360 degree parking camera system, but in the dark I can't see anything on the sides of the vehicle. So I added lights under the doors to light up the area when it's dark.

The dome light(s) has its own timer circuit, so when the vehicle turns off, it will keep the dome on for several minutes, and probably even indefinitely as long as doors keep opening and closing and the car thinks someone is loading or unloading things. So, whenever anyone uses the car, I'd like to have all those accessories, all powered differently, to only be operable when the engine is running (well, it's a hybrid, so when the vehicle is not turned off).

So in the case of the dome, it has an off, auto, and on. But no matter what, I don't want it to be on when the vehicle is off. The circuit will sit before any accessory and will prevent it draining the lead acid battery to any degree.