Dick Cappels wrote an article detailing his AVR data logger and voltage recorder:
The recorder includes the functionality of a data logger but also plays back recorded voltages and can perform a curve trace function.A simple file system provides for up to 128 different sequential files totaling up to 130,000 samples. At 1 sample per minute, that’s 90 days of continuous recording.
The voltage recorder is able to record one or two analog channels or positive analog voltage along with a one bit digital channel, sampled at 1 set of samples per second or 1 set of samples per minute. The analog channels are scaled from 0 to 2.5 volts in 250 even steps. The digital input is connected to the base of grounded emitter NPN transistor inverter with a threshold of about 0.5 volts.
The recorder can operate from batteries or from 12 to 18 volts applied to the power input connector.