Comparator circuit to detect working alternator

I'm very new to electronics, but am trying to build a device that should be attached to a motorcycle (Arduino based sensor board) which should only draw power when the engine is running; specifically given the finicky nature of motorcycle batteries, and that my device will have an approximate two ampere draw.

I've been researching comparator circuits, but it seems as I want to avoid any draw on the battery when the engine isn't running.

I have self-imposed restrictions on specifically not interfacing with any of the bike's other circuitry, mostly because it's brand new, and I don't want to splice/hack into any wiring that might relate to the safety electronics, etc, etc; I can rely on a connection to the battery, and I'm hoping to detect 12v±1v when the motor isn't running, and closer to 14v when the alternator is doing it's job.

I heard a quip that I could use an opamp and a zener to somehow boost a 3v button cell to a suitable reference voltage to drive the comparator, that I might compare against 13v, for example.

The missing puzzle (in spite of lots of time spent with Google) is how can I use a rechargeable (I expect) button cell at 3v to compare against a 12-14(±1)v input.

Assuming that I detect the battery is currently charging from the alternator, I'll be stepping the voltage down to 5v using a LM2569, which is closer to the button cell's voltage, but doesn't help me to avoid drawing on the battery when the motor isn't running.

Use a zener diode to decrease the battery voltage so it comes in range of the inputs of your (rail to rail) opamp. Then compare that value with an (adjustable) reference voltage

simulate this circuit – Schematic created using CircuitLab

• ... but check the diode current at 12V and under; it will still draw some current (maybe 1 ma) from the battery when off.
– user16324
Sep 29, 2013 at 13:39
• @BrianDrummond - Good point, didn't think about that. Of course you can increase the zener voltage so it is not conducting when the motor isn't running, but it'll turn out a fragile balance to find the correct voltages. I guess this design needs proper verification in practice as several voltages are not well know. Sep 29, 2013 at 13:50
• I suppose I could combine this with a safety mechanism to detect a low voltage, and completely switch off the unit. Thanks for the advice! Sep 29, 2013 at 15:05
• Maybe feed it from the cigarette lighter socket. That usually gets switched off with the key. Sep 29, 2013 at 15:06
• Not many motorcycles with a cigarette lighter socket! But that would be a lot easier! There are some circuits that are only live when the ignition is on, but one simply cannot afford a 2A draw on a motorcycle battery when the ignition is on, but the engine isn't running. Sep 29, 2013 at 15:31

Simply use a relay exactly the same as a split charge system for an auxiliary battery on a vehicle.

Connect a "sensor" wire either to the sensor connection on the alternator (adding your ring crimp on top of the existing connection), or to the battery/alternator indicator light connection on the dash - which drives the coil of the relay. connect the contacts as you wish.

This gives zero current draw when inactive and doesn't require any other power source to function. Connection is by piggybacking a bolt on electrical connection so no modification to the existing wiring.

I have designed many things in the past where I have been bogged down in technical brilliance only to be blown out of the water by the most simple non-technical/low technical solution.

Remember - KISS - Keep It Simple Stupid!