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My car flasher circuit can be simplified like so :

Factory flasher diagram

The factory flasher unit is a standard 2 pin that plugs in series with the switch and the load (indicator lights).

I'm attempting to make my own flasher, and came up with 555 timer outputting a square signal to a mosfet, flashhing the load when the switch is closed.

enter image description here

My problem here is that I have this ground pin on the 555 timer circuit which essentially makes my flasher unit 3 pins instead of 2 pins. Also in this configuration the flasher is constantly flashing in the background, but not visible until the switch is closed.

How could I make it into a 2 pin system so that only when the switch is closed, the 555 circuit starts flashing ? Can I connect 555 circuit "GND" to the output of the MOSFET through a Diode for example ?

Extra info : the load will be approximatively 5-8 Amps.

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    \$\begingroup\$ You also need to ensure that on power-on that the lamp turns on instantly not, for example, after one half-cycle of the flasher. At 100 kph you're traveling almost 28 m/s and that half-second delay could be significant in avoiding a collision. The thermal flashers are very clever as they also blink faster when a bulb blows or is disconnected. To do this electronically is a lot more complex. \$\endgroup\$
    – Transistor
    Jul 14, 2020 at 16:58

1 Answer 1

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Here's one idea.

enter image description here

Figure 1. Image source: EEV Blog.

This one uses D1 and C1 to maintain power to the timer circuit while it is shorted out by the MOSFET.

Be aware of a couple of safety points:

  • You also need to ensure that on power-on that the lamp turns on instantly not, for example, after one half-cycle of the flasher. At 100 kph you're traveling almost 28 m/s and that half-second delay could be significant in avoiding a collision.
  • The thermal flashers are very clever as they also blink faster when a bulb blows or is disconnected. To do this electronically is a lot more complex. The circuit of Figure 1 won't!
  • There is no audible indicator.
  • You may have trouble with the dashboard indicator lamps if they rely on a ground connection rather than +12 V.

I recommend the 2-pin electro-mechanical solution. The linked article has some interesting points made by various contributors.

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