This paper aims to enable schools to make their own low-cost temperature logging instrument and to learn something about its calibration in the process. This paper describes how a thermistor can be integrated into a simple potential divider circuit which is powered with the sound output of a computer and monitored by the microphone input. The voltage across a fixed resistor is recorded and scaled to convert it into a temperature reading in the range 0-100 °C. The calibration process is described with reference to fixed points, and the effects of non-linearity are highlighted. An optimised calibration procedure is described which enables sub degree resolution and a software program was written which makes it possible to log, display, and save temperature changes over a user determined period of time.