I'm reversing engeering the dashboard of my motorbike (Yamaha T7 - 2019) and I found and a lot of circurity just to control the LED of a turn blinker.

I figure out this schematic:


Why did they put a resistor and capacitor in parallel with the LED?

  • \$\begingroup\$ Thanks... The 5V comes from a regulator in the dashboard. \$\endgroup\$ Feb 4 at 0:43

1 Answer 1


The capacitor is for suppressing voltage spikes that could damage the LED (it forms a low-pass filter with R4).

My idea for the purpose of R3 is either to increase the current a bit for discontinuity detection or to reduce the voltage above the LED in failure situations or both.

Where do the +5 V come from? Thought there is everything 12 V on a bike.

  • \$\begingroup\$ Nice try, but I doubt about the danger for voltage spikes here. I would say, OP has a good question \$\endgroup\$
    – Roland
    Feb 3 at 16:36
  • \$\begingroup\$ Sure. C1 is a bit too small to let the LED switch smoother. Therefore the best it can do is to be a return path for high frequency noise and to suppress spikes. As far as I know the supply net in cars (and bikes) might be noisy. Let's put a scope there. \$\endgroup\$
    – datenheim
    Feb 3 at 16:46
  • \$\begingroup\$ Modern dashboards are not the big clunky isolated circuits and lamps of the past. They have internal regulators and ics and cobs and leds. \$\endgroup\$
    – Passerby
    Feb 3 at 16:49
  • \$\begingroup\$ But ignition and so on are still there as in old ages. I can't prove why it was placed there. But at the plave it sits it can do what I described should there be any noise or spikes. Feel free to give a better explanation. \$\endgroup\$
    – datenheim
    Feb 3 at 16:56
  • \$\begingroup\$ Which component is more vulnerable for voltage spikes? The LED, or the transistor? I would say, the lead acid battery is a great protection against overvoltage, and for extra protection, I would put a capacitor between +5 and GND, not just over the led. \$\endgroup\$
    – Roland
    Feb 3 at 16:59

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