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I am making an engine toggle switch for a recoil-start engine using two switches wired in series for safety. The engine will shut off only when two "kill" cables on the front have been connected. In the picture below, I have lengthened the wire to make it more noticeable and easier to work with.

Recoil Engine with lengthened kill cable

Unfortunately, the switches are built to be normally open and cause the engine to shut off when both switches are on (hereinafter called "switch box"). The ideal situation is that the engine status reflects the switch box status: both switches ON activates the engine and turning either switch OFF in the switch box shuts down the engine.

Thankfully I was able to find some answers on how to flip the switches to act normally closed from using concepts from this kill switch post. I also viewed a post about latching relays to potentially use for hooking up to the engine. I spent some time prototyping these ideas and it works exactly as I need it to. Below are the schematic and pictures of the functioning prototype, whenever the LED is ON the engine is OFF because a connection has been established:

Switch box engaged, LED OFF, Engine ON LED OFF means Engine ON Switch box disengaged, kill switch engaged, LED ON, Engine OFF

schematic

simulate this circuit – Schematic created using CircuitLab

For my question, I would have been able to hook up this circuit and been done with it except a recoil-start engine does not utilize an external battery like this prototype circuit and most of the engine kill-switch questions on this forum do. I've noticed through testing that the battery is crucial for making the circuit work. Given that the kill cables from the engine are wired with its spark plug, how would I be able to implement the Normally-Open to Normally-Closed wiring without having a battery? Is there some way to wire the same path the battery creates without using a battery?

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  • \$\begingroup\$ That can't be the right schematic... it shorts the battery out and doesn't do anything regarding the LED (not to mention the missing resistor for the LED). Most "kill" switches on motors without batteries are hooked up in series with the spark plug. Open the circuit, remove the path for the spark plug, engine dies. Just make sure the switch can handle the high current/voltage of the spark plug circuit. \$\endgroup\$ – Ron Beyer Jul 11 at 1:51
  • \$\begingroup\$ Thanks for the response. I believe I've fixed the schematic now unless the LED is reversed in the schematic to cause a short-out. The purpose of the switch box was to have an engine toggle button and a kill switch. If it comes down to it, I can use your solution although it may be confusing for anyone driving the engine since you have to turn a switch on to turn the engine off, especially that missle-launch style toggle switch that was meant to kill the engine when you push the large red flap down. I wanted to take advantage of the large flap and its ease of shut off for a kill switch. \$\endgroup\$ – Kharonos Jul 11 at 2:20
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    \$\begingroup\$ Most kill circuits are wired so that you need to close the switch so that the device runs. This is called "fail safe" because if the wire breaks the equipment stops. This is how yours should work too. If your big toggle cover doesn't work that way, you should be able to take the cover of and turn it around. \$\endgroup\$ – Ron Beyer Jul 11 at 3:02
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You don't want to be relying on a battery-powered circuit to actuate a relay that will kill the engine, if the battery is dead you lose the ability to stop the engine. Ideally a safety circuit is wired as Ron suggests so that is is failsafe and ceases operation of the circuit is opened, but that's not how stop switch on these engines work, they short a winding in the magneto that prevents the generation of a spark at the plug by diverting the generated current through the kill switch.

So you need the switches to be normally open such that if either one is closed it will kill the engine. To get the logic correct, you then wire the two in parallel.

If I remember right, the small rocker switch on my weed trimmer has the markings the wrong way around, it is biased to the '0' open side as you'd expect for a NO switch, and you push it to the '1' closed position to stop the engine. If you have the possibility of inverting the switches so that the markings or physical position are reversed, you may be able to achieve your intent without any additional componentry - the rocker on the small rocker switch typically will pop out of its housing with a screwdriver/knife blade pushed down either side to flex the body out enough. Looking at the photos - is the switched biased off? It's in the on position.

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  • \$\begingroup\$ Yes, the rocker switch with the cover is set to the off position when the cover is closed. I seem to have worded my explaination incorrectly regarding the switch box. I was hoping to have two switches, one to allow the engine to turn on and the other to to act as a kill switch after having also been flipped up. Basically, flipping both off-biased switches up turns on the engine if you disregard the manual start and switching either back to off kills the engine. I was wondering if this was possible with my current components but I do not mind using the given solutions. \$\endgroup\$ – Kharonos Jul 11 at 19:45

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