Often a temperature controlled, eg in the fridge, in a terrarium, aquarium or for heating. Here, matching circuits (Links)

Here you will find Schematic Circuit Diagram for DIY and further information on Temperature Control
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Simple temperature controller with diode sensor	www3.telus.net/chemelec/Projects/Temperature-Sensor/Temp.htm
Two-step controller with Operationsverstaerker	www.interq.or.jp/japan/se-inoue/e_ckt25.htm