Thermostat? Do It With A 555!

Stephen Woodward built a simple thermostat using a 555 timer IC, leveraging its internal comparator with a thermistor for bang-bang temperature control. By tying the threshold pin to Vdd, the trigger pin acts as an inverting analog comparator. The setpoint is determined by a resistance ratio, eliminating the need for a precise voltage reference. The circuit supports both heating and cooling modes, includes adjustable hysteresis via a resistor, and can handle significant power output — all with just two resistors, a thermistor, and a battery.