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Comment by ck2 | Hacker News Reader

So if you have a breadboard and a USB to serial adapter, you could build the bare bones version of this for about $6 and maybe break the bank at $15 if you had to buy everything.

I've got a usb to serial adapter just sitting here, definitely trying this project for just $6

Wish it only needed 5V supply, I have endless phone chargers laying around from 500ma to 2a

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If you don't have the cable, an arduino uno also works as a usb to serial adapter.

https://www.arduino.cc/en/Tutorial/ArduinoToBreadboard

To make 3.3V out of 5V you can use a voltage regulator.

If you want to do anything even semi-reliable with this thing you'll want a regulator on the board anyway. Wall warts are notoriously noisy.

This. New switch mode ones are crappy, really crappy!

I have a stash of old linear ones that aren't fortunately. Worth their weight in gold.

Thankfully, a circuit for dropping the 5v input to 3.3v is really easy to build. It only requires a pair of capacitors and a 3.3v regulator, only about $2 of materials total.

You can also use a voltage divider if you are willing to suffer a bit of leakage current. That's just 2 resistors.

The linear regulators we are talking about are basically voltage dividers with feedback. They aren't significantly more efficient than your design, though they are ton less noisy.

Because of that feedback, you get to use your load as the lower half of the divider, so the total current drawn from the source is the load current plus a few microamps for the regulator's internals.

In the passive voltage divider circuit, if the load varies its current draw, you have to have a resistor in parallel with the load so that the total current through both the load and the resistor doesn't vary significantly (where "significant" is derived from your tolerance for voltage variation).

While at peak current, the efficiency of a voltage divider is the same as a linear regulator, at lower currents a voltage regulator has significant benefits. This is because the quiescent current used by an unloaded voltage regulator can be very low (500nA for the TPS783!), while for a voltage divider to be sufficiently "stiff" its quiescent current should be about 10X the max load current. As a lot of microcontroller applications alternate between a low sleep current (several uA) and bursts of high run current (several mA), a voltage divider is significantly (>10000X!) more power hungry then a linear regulator in practice, even if the worst case efficiency for both is closer.

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