I have a set top box. It has a 3 mm green LED (on the front panel) that lights up when I connect the dish coax cable (signal LED).

I measured the voltage across the LED. It has 0 V when the dish cable is disconnected and about 1.2 V (max) when I connect the dish cable.

I would like to add a buzzer to this LED. It should buzz when the green signal LED comes on.

I have a few of those tiny buzzers from old Pentium 4 PC motherboards. I connected one across the LED but I only managed to get a low humming sound (I guess it lacks a signal generator? 555 positive triggering?).

I want to use the voltage from the LED as a trigger (555 IC) and an external 5-12 V power supply to induce a loud 1200 Hz signal in the buzzer. I have a square wave generator which showed that the buzzer was at its loudest at 1.2 kHz.

Any help with a schematic would be greatly appreciated.

Added later:

enter image description here

Here pin 2 of 555 timer ic is the trigger. How do I trigger this circuit using the 1v coming from the led diode?

  • \$\begingroup\$ The trigger and threshold pins are inputs to the voltage comparators inside the 555 that make it oscillate in conjunction with the rising and falling voltage on the timing cap. It's not a "trigger" that allows activation using a external signal. I'd think more along the lines of manipulating the 555's reset pin... \$\endgroup\$ Jan 24, 2022 at 12:06

2 Answers 2


Probably the voltage that supplies the LED is low, 5 V. If no information about how is driven, you can try with an optocoupler in series with the LED, and check if voltage drop is not high. Also will isolate both circuits. Search for optocouplers similar to OMRON G3VM, and connect output MOSFET in positive supply to your buzzer. enter image description here

  • \$\begingroup\$ I added a schematic of 555 buzzer for reference. \$\endgroup\$
    – user141178
    Jan 24, 2022 at 11:50
  • \$\begingroup\$ This is interesting. Can I use 817? \$\endgroup\$
    – user141178
    Jan 24, 2022 at 12:16
  • \$\begingroup\$ It could also work, if existing LED current can sature output transistor (20 mA). In that case, voltage drop of output transistor will be about 0.2 V, and max current 50 mA. \$\endgroup\$
    – Bravale
    Jan 24, 2022 at 12:24

Buzzer is probably active, no generator needed. Check it, if it has "+" mark on case. But voltage is should be higher, some starts from 3V. External power supply 5V may help, just small transistor switch needed. Resistor around 1kOhm from plus diode to base, emmiter to minus PS and diode, collector to minus buzzer. Plus buzzer to plus PS.

  • \$\begingroup\$ Checked with correct polarity. It gives a LF hum. Sound is low. The buzzer is bare and has no internal driving circuitry so giving it straight 9v produces no tone except LF hum. \$\endgroup\$
    – user141178
    Jan 24, 2022 at 12:13
  • \$\begingroup\$ Working frequency of active buzzer 3kHz. Passive buzzer does not create any sound. Check the polarity. You may damaged buzzer with wrong polarity. \$\endgroup\$
    – user263983
    Jan 24, 2022 at 12:18

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.