Build Your Precise 12V Heating System Thermostat Circuit

This circuit is designed to regulate a heating system or central heating setup, maintaining a steady indoor temperature despite significant fluctuations in the external environment. Two sensors are required: one positioned outdoors to monitor the external temperature, and another placed on the return water pipe from the heating system, close to its entry point into the boiler. The output from the Relay contact must be connected to the boiler’s start-stop control input. This device, though uncomplicated, has demonstrated exceptional reliability; it was initially installed at my parents' residence over two decades ago. Although somewhat aged, it continues to function flawlessly and without any operational issues.

Circuit diagram

Parts:

Device purpose:

This circuit is intended to control a heating system or central heating plan, keeping constant indoor temperature regardless of substantial changes in the external environment. Two sensors are necessary: one situated outdoors, monitoring the outside temperature; and another placed on the returning water pipe from the heating system, positioned immediately before its entry into the boiler. The output signal from the Relay contact should be connected to the boiler’s start-stop control input. This device, despite its relatively simple design, has consistently proven to be exceptionally reliable; it was originally installed at my parents' home over twenty years ago. Although somewhat dated, it continues to operate effectively and without any operational difficulties.

Circuit operation:

When the Base-to-ground voltage of Q1 is lower than half the supply voltage (determined by R7 & R9), a voltage develops across R8, and the driver transistors Q2 & Q3 activate the Relay. When the Base-to-ground voltage of Q1 exceeds half the supply voltage, caused by a decrease in the value of the n.t.c. Thermistor due to a rise in temperature, no voltage appears across R8, and the Relay is deactivated.

More about components

The transistors Q1, Q2, and Q3 are fundamental components within this circuit. Specifically, Q1 is a PNP transistor, serving as the primary switching element controlled by the sensor signals. Q2 and Q3 are NPN transistors, functioning as driver transistors to energize the Relay. These transistors are critical for efficiently controlling the Relay’s operation, ensuring precise regulation of the heating system.

Notes:

Warning: This circuit involves mains voltage. Ensure all connections are secure and insulated. Incorrect wiring can result in electric shock or fire. Consult a qualified electrician if you are unsure about any aspect of this circuit.