Single Pushbutton Latching Relay Engine Start Circuit

Small engines often have two pushbuttons: A start pushbutton to operate the starter motor and a stop pushbutton to kill the ignition. These two functions can be combined into a single pushbutton via the use of a single coil latching relay that changes state each time the pushbutton is pressed. This circuit was requested by a reader.

Schematic

The following inexpensive relays are stocked by DigiKey – note that they have no minimum order:

30A Power Relay:
http://www.digikey.com/product-detail/en/1432786-1/

Single coil latching relay:
http://www.digikey.com/product-detail/en/G5RL-U1A-E-DC12/

How it works

Each time the pushbutton is pressed, the latching relay (K1) changes state – if its contacts pick up K2, the starter motor is powered and the engine starts – if the K2 contacts short the ignition, the engine stops. I am assuming that a 30A relay will do the job – operation is intermittent, so there should be no thermal issues – 40A relays are also available…

If the engine fails to start, the next time the button is pressed, nothing appears to happen – just the ignition becomes disabled. However the following pushbutton operation will crank the engine again.

Safety issue improvement

This circuit has one over on the stock kill switch pushbutton because it keeps the ignition permanently disabled, so if the engine is unintentionally cranked via another means, it cannot start.

Wiring variation

The top schematic has the pushbutton carrying the starter current, while in the lower schematic the pushbutton carries only the relay coil current – this allows use of a less expensive pushbutton.

Latching relay polarity

Per relay datasheet, observe coil polarity. If negative ground (rather than positive ground as shown), swap polarity. Note that the biased magnet magnetic circuit may click with reverse polarity, but will not change states – such can be a real sleeper.

Not tested

This works OK in my cranial simulator, but real life often turn up problems – be forewarned because Murphy never sleeps!

Assembly

Push-on connectors will do for the power relay, but the latching relay is PCB mount so wires must be soldered to the small terminals. This may take a little finesse, but is fully doable.

Good luck and let us know how you made out.