I am designing a circuit to control a motorcycle horn (its operating frequency is 490 Hz, its resistance is 3 Ohm, and it consumes 1A on average as indicated by the power supply), and LED lights. The LED must be controlled by a high side switch, while the horn can be controlled by a low side switch.

The LED lights I am testing consist in multiple LEDs encased in a sealed commercially-available stoplight and plate light assembly. Additional components, which are probably present in the assembly, are not included in my schematic. The stoplight and illuminator are specified for 12 V, and at 12 VDC, consume about 100 mA. The plate LED turns on at a voltage of 2.8 V. The stoplight LEDs turn on at a voltage of 6.7 V.

The 12 V is supplied by a power supply (0 - 30 V DC, 0-5 A). I have also tested supplying 12 V with a DC/DC converter (5A, 12V, 50mVp-p ripple) connected to a 52V 40A battery.

When switching the horn on, the LED flickers on the whole time the horn is on. I think the issue is that V1 is spiking down by several volts when the horn is active. I have tried troubleshooting the issue by: i) adding a capacitor (470, 47, 0.1 uF) between GND and +12V ii) adding a snubber with R=10 Ohm and C=10 uF in parallel with the horn

But to no improvement. Can anyone suggest a design that would prevent the LED from flickering when the horn is active?

Thank you!


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ What sort of LED is it specifically? \$\endgroup\$
    – Andy aka
    Feb 10 '20 at 15:12
  • \$\begingroup\$ @Quentin Welcome to SE.EE Please update your question (using the edit link) so all relevant info is at one place and not scattered across comments \$\endgroup\$
    – Huisman
    Feb 10 '20 at 18:28
  • \$\begingroup\$ (1) Do you know (or can you test) what minimum voltage is required for the leds? Would e.g. -5% (11.4V) work? (2) What's the current rating of the 12V source? \$\endgroup\$
    – Huisman
    Feb 10 '20 at 18:36
  • \$\begingroup\$ Does the LED flicker the whole time the horn is on, or just the moment it starts or stops? You might try a cap across the base and emitter of Q2. \$\endgroup\$ Feb 10 '20 at 18:42
  • 1
    \$\begingroup\$ You only need 212­­uA current induced at base of Q3 to turn it on even with the input off. You have 4A of current in the horn. The horn current creates a field that induces current on nearby conductors. You need 85 dB of rejection from the horn trace to the Q3B trace. Otherwise it will turn on Q3. 60 dB of rejection is do-able. 80 dB is very difficult. You could try a large cap from Q3B to Q3E. But ultimately you've got to get the high current traces away from the low current electronics. \$\endgroup\$
    – scorpdaddy
    Feb 10 '20 at 19:20

After experimenting with placing a couple of capacitors at different points in the circuit, the solution I found working best is placing a 47uF ceramic capacitor between Q2E and Q2B AND a capacitor in parallel with the LED (47uF, electrolytic). It could work with smaller capacitances, but the next smallest I have is 0.1 uF, which is not enough to completely suppress the flickering.


Another thing to try is a diode from Q2E to the LED, followed by a cap across the LED.

However, you need to be aware that, at the LED current levels and the likely effects of the audio horn frequency, the value of the cap is likely to be large. As you discovered in your answer, you need a big cap.

Put it this way, your LED is flickering because the voltage across it is varying wildly, and it is varying at a frequency which is low enough for you to be able to see the flicker, which pretty much guarantees about 60 Hz or less. When you put a cap in to smooth the voltage, it needs to be able to store the charge needed to supply the LED when the voltage is low. This will probably be in the range of 10 msec or so, so you'll need more than a .1 uF cap.

An alternative is to put an inductor in series with the LED, but that will not likely do well, since the energy storage of inductors is much less than that of capacitors, so you'd need a physically large inductor compared the caps you're using.


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