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I'm building a circuit for my motorcycle to replace a damaged indicator module. It runs off a NE555 to turn on another NE555 in astable mode to flash the bulbs. The switches on the bike are push switches, push once for on, push again to turn off, and they are connected to ground when pressed. That's the issue, I can get the 555 to turn on and off when using a simple push button switch (SW2 in the image), but not sure how to connect to the bike switch (green and black drawn on switch).

I appreciate if anyone has an opinion on this.

555 Circuit flasher schematic enter image description here This is the full circuit. It's fully working, just don't know how to use the motorbike switch with it.

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  • \$\begingroup\$ So what you have drawn works, but you are unsure how to use the motorcycle switch? If the motor cycle switch only has one wire coming from it, then that means it is connected to the ground through its mounting. So you would only need to connect the wire as you have shown with the green line. \$\endgroup\$
    – Aaron
    Commented Sep 19, 2022 at 14:43
  • \$\begingroup\$ Just to clarify a few more points. Is the switch you drew a toggle? ie. It stays on or off, it's not a push button and is not momentary. The motorcycle switch is only a momentary push button that is normally open and connects to ground when pressed? \$\endgroup\$
    – Aaron
    Commented Sep 19, 2022 at 14:44
  • \$\begingroup\$ Correct, the motorcycle switch has one wire from it. I tried conecting with the green line but it only turns on, and won't turn off with another push. Also correct, the motorcycle switch is only a momentary push button that is normally open and connects to ground when pressed. I want to remove switch SW2 and use only the motorbike switch. Many thanks \$\endgroup\$
    – Grant Lee
    Commented Sep 19, 2022 at 15:17

2 Answers 2

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From your descriptions it sounds like you need to replace a momentary switch between two non-grounded points with a momentary switch that is permanently connected to ground.

You could do it with a relay. Connect one side of the relay coil to +12 VDC and the other side to the switch, then use the normally open contacts in place of the original circuit switch. Pressing the button will momentarily close the relay and the relay contacts will trigger your circuit.

An automotive relay should work, they often have built in spike diodes across the coil so mind the polarity.

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  • \$\begingroup\$ That sounds great. Would I be able to use transistors to do the same thing? \$\endgroup\$
    – Grant Lee
    Commented Sep 19, 2022 at 18:34
  • \$\begingroup\$ @GrantLee Hmmm... possibly, but it's trickier with transistors, there needs to be the correct DC biases for a transistor to work. A relay is simple and gives you an actual contact closure so it works like the original switch. If you're worried about space you could use a micro-miniature relay since this application isn't switching very much current. \$\endgroup\$
    – GodJihyo
    Commented Sep 19, 2022 at 19:01
  • \$\begingroup\$ Perfect. Thanks for all your help. \$\endgroup\$
    – Grant Lee
    Commented Sep 19, 2022 at 19:45
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If the point of the switch that you have drawn is to turn on/off the 555 circuit, and your switch can only connect to ground, then the simplest option would be to insert the switch between the ground connection to the circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Not to turn of the circuit, but to switch pin 3 (output) to high/low to turn off the other 555 circuit. It works as it is on a seperate PSU, but I need it to use the push switch on the motorcyle. Thanks \$\endgroup\$
    – Grant Lee
    Commented Sep 19, 2022 at 14:30

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