I'm trying to make a Visual Metronome, i have a prebuilt one already (bicycle light) but it has a very high frequency of lighting up (2 flashes in 1 second).

I have a circuit for metronome, to which i added an LED, alongside the speaker, and it works fine, both the sound and light work as expected, giving 60 BPM output. (with change in C from 22 uf to 10 uf)

Orignal Metronome Circuit:

enter image description here

Visual and Sound Metronome, by Adding an LED:

enter image description here

Now, i am trying to make just a visual metronome from this, without the sound... That is, only with a blinking light, at 60 Blinks per minute...

The problem i am facing is that, if i remove the piezo speaker, i get a constant glow of LED, or get a blink in many minutes, and it only starts to blink correctly when i add the speaker back. I have tried adding a resistor across the LED, but that did not help.

I did see this question on EE, SE, and tried the circuit suggested by Jasen, but it didn't work for me... ( 3V LED Pulse Circuit )

So, i am wondering what alteration i need to make to the circuit, for the LED to blink, without the speaker added, to make it just a Visual Metronome, maybe with a couple of LEDs at a later stage.

This does not blink or blinks in minutes:

enter image description here

I do not have simulation experience, so i am unable to troubleshoot this with a program.

Thank you.


This circuit has several problems, including the problem of rapidly draining the battery. At a minimum you need to swap the 2N3906 emitter and collector. As shown it might happen work, but if so then it will still be very inefficient. Also you need to put a resistor on the collector of the 2N3904 so power isn't shorted to ground when the transistors are on. Regarding the LED, you will need a series resistor if the LED's typical forward bias voltage is notably below the 3V (so it doesn't drain the battery unnecessarily) and a parallel resistor so that the output voltage is pulled low when the transistors are off, to provide proper feedback through the cap. I strongly suggest using a timer IC.

  • \$\begingroup\$ Yes, the metronome is a fully working circuit. I have made a few and even gifted them to friends. I will try the above points and see what happens.. TY \$\endgroup\$
    – vibhu
    Jun 26 '18 at 15:01
  • \$\begingroup\$ Reversing EC to CE on 2n3906 made the sound very faint. Also, a value of 20 Ohms on the collector of 2n3904 was the maximum i could use, after which the length of the beats became too short for use. Would a 20 ohms resistor be enough to save power (not shorting to ground)...? \$\endgroup\$
    – vibhu
    Jun 26 '18 at 15:26
  • \$\begingroup\$ When i add a parallel potentiometer on the LED, it dims the LED, otherwise it is bright... Also, i think the LED i am using is a 3mm LED, with operating voltage of 2.2V... It glows fine across the CR2032 battery. I am not going the IC way for now, as i have a learning curve to cross when it comes to transistors... \$\endgroup\$
    – vibhu
    Jun 26 '18 at 15:42
  • \$\begingroup\$ Reversing E-C should have made the sound stronger. Maybe the part is mismarked (a common occurrence). I do think this would be draining the battery, but it would also depend on the duty cycle. \$\endgroup\$
    – Digiproc
    Jun 27 '18 at 14:47
  • \$\begingroup\$ It had become very inaudible by changing E to C on PNP.. Also, thanks for the suggestion of putting a resistor between PNP's B to NPN's C, i realised, the the short beat produced was equivalent of changing the capacitor value, a smaller value of C made the beat shorter, just like increasing R between 06's B and 04's C... Hence, a small C could also save power... \$\endgroup\$
    – vibhu
    Jun 28 '18 at 5:28

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