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\$ Commented Feb 4, 2023 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
    Commented Feb 3, 2023 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
    Commented Feb 3, 2023 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
    Commented Feb 3, 2023 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
    Commented Feb 3, 2023 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
    Commented Feb 3, 2023 at 16:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.