Super easy 5V to 3V conversion

dc/dc convertervoltage-regulator

What is the absolutely simplest way for a 10 year old child (not really) to get a 5V (350mA) phone charger to power an LED (3.0V) and a clock (1.5V)?
The clock is meant to run on a 1.5V battery, but since I want to install a small LED, I thought why not run both light and clock on the same power supply.

I need it to be not bigger than a cigarette pack (preferably smaller.)
I don't understand electronics jargon so please treat me like a child.

I originally thought I could just solder on some resistors but there's no info online I could use to be found.

Let me know if there's anything else I forgot to include.

Best Answer

Rather than use words, here's a design you can play with. That should please your inner ten-year-old, or your actual ten-year-old.

Try this (simulate it here): enter image description here

What's going on?

I've set up the sim with a 3V LED. The 510 ohm provides about 4.2mA to the LED. This is a good current to start with for most LEDs. If you want less or more brightness, adjust the load resistor.

The NPN transistor is set up as a voltage follower to make a crude linear regulator. The two resistors set the base at about 2.15V, which gives about 1.6V at the emitter. The transistor action will maintain this voltage even with variable current to the clock. This should be fine for a clock designed to work on a 1.5 (1.7 ~ 1.2V) battery.

You can use most common switching NPN types like the 2N3904 or 2N2222 for this, it's not too fussy.

And finally, the second 510 ohm on the NPN is for safety: it protects the transistor if the output to the clock is shorted, because, you know, ten-year-olds. It otherwise has no effect.

This can fit easily in a 1 sq. in space with through-hole components. The transistor is cheap - about a penny in high volume, a bit more from Digi-Key or Mouser in onesey-twosies. Buy a bunch in case you fry them.

Here's an improved version that uses a Darlingtion pair of transistors. This has better output regulation owing to the additional gain provided by the second NPN. They can both be the same kind, e.g., the 2N3904.

Simulate it here. enter image description here

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