Electronic – LED as Zener: is this a good way to lower the BOM cost


I had an old AM radio board designed to operate with a 3V CR2032 coin cell. I want to revitalize this old junk with as few parts as possible and convert it into a USB-powered AM radio with a built-in speaker. Parts I have for this project include three caps, two resistors, a blue LED, an 1N4728 3.3V Zener, a speaker and an LM386 audio power amplifier.


simulate this circuit – Schematic created using CircuitLab

My question is that can I get away without the Zener and use the blue power indicator LED (my bach have a forward voltage of about 3V) instead like this:


simulate this circuit

Best Answer

Yes, however you should bypass the LED with a capacitor such as 100uF electrolytic and you should expect the LED to vary in brightness a bit with the sound output.

You may not be able to get full sound power output while keeping the LED within maximum current specifications.

You could put a diode (eg. 1N4148) in series and use an emitter follower (eg. SS8050) to avoid both problems.

Or use a TL431 which costs less than CNY 50 per K in Shenzhen.