This pulse oximeter is so simple and cheap to build it’s almost criminal. The most obvious way to monitor the output of the sensor is to use an oscilloscope. The poor-man’s stand-in for that is a ...

The LM324 low-voltage quad op-amp is decades-old design that it refuses to die – because it subtler characteristics are just so useful. For example, it works single-supply from 3V to 30V, its inputs ...