A Cheap Sonar Module

For less than $1, I bought a HC-SR04 sonar module on eBay.  It can't get cheaper than this.

I was a little disturbed first when I saw the circuit board.  Two of the ICs have no labels.  And the layout appears different than other HC-SR04 module.  A quick test seemed to confirm it actually worked.

A Lithium Ion Battery USB Charger

A simple linear battery charger that draws power from a USB port is designed for 14500 lithium ion battery (which is about the same size as a AA battery).  The circuit diagram is shown below with a Linear Tech LDO, but others LDO can be used.

The actually construction uses an Analog Device LDO, which features a comparator that is used to indicate the charging completion.