Temperature Controlled Switch | Full Electronics DIY Project

This simple temperature controlled circuit triggers automatically when the ambient temperature goes beyond a set limit of, say, 50 degrees centigrade. This temperature setting can be changed as per requirement through the potentiometer in the circuit.

The circuit has three parts: the sensor, main control unit, and relay driver. It works on 230V AC mains. A bulb used in the circuit acts as a load for demonstration purpose. When the room temperature goes beyond the set limit the bulb switches on automatically. The author’s prototype is shown in Fig. 1.

Fig. 1: Author’s prototype on a breadboard

Circuit and working

Circuit diagram of the temperature controlled switch is shown in Fig. 2. The circuit comprises step-down transformer X1, bridge rectifier BR1, 5V voltage regulator 7805 (IC1), temperature sensor LM35 (IC2), op-amp LM35 (IC3), 5V single changeover relay RL1, and a few other components. Capacitor C1 connected across the supply terminals minimises any ripples or noise signals in the voltage.

The circuit needs 5V DC to operate, which is derived from the 9V, 500mA secondary side of transformer X1. The 230V AC mains is connected to the primary of X1 via CON1 in the circuit whose 9V AC secondary is connected to bridge rectifier BR1 for rectification. The rectified and filtered voltage (by C1) is given to IC LM7805 to get regulated 5V for the circuit. Glowing of LED1 indicates that 5V is being supplied to the circuit.

The IC LM35, heart of the circuit, is a 3-pin device like a transistor with pins for VCC, GND, and OUTPUT. It gives variable voltage at the output based on temperature up to 150 degrees Celsius. This popular and inexpensive temperature sensor is generally used in digital thermometers.

For every one degree centigrade rise in temperature there is 10mV increase in voltage at the output pin. Let us say, at 0°C the output of sensor is 0V. When the temperature rises to 10°C the output will become 100mV; at 25°C it will become 250mV.

Temperature sensor LM35 senses the temperature and gives the signal to the controller built around IC LM358. The data received by the controller is then given to relay driver circuit built around driver transistor T1 and relay RL1. On receiving the signal from the controller, the relay is tuned on/off. This process repeats forever.

Calibration of LM35 sensor should be done as we have used op-amp LM358 to compare the output voltage of LM35 with the reference voltage. Since the circuit has been set for threshold voltage of 50°C, to trigger the op-amp we need to set the reference voltage to 0.5V, as at 50°C the output of LM35 will be 0.5V. The reference voltage is at pin 2 of LM358.

Working of the circuit is simple. When power is switched on, LED1 glows and the circuit is ready to use. As soon as some heat is provided near LM35 sensor, the temperature at pin 3 of IC3 goes higher than the reference voltage at pin 2, so the output of op-amp IC3 at its pin 1 goes high and relay RL1 energises. As a result, the light bulb gets disconnected from the 230V AC supply and switches off.

When temperature at pin 3 of IC3 goes lower than the reference voltage at pin 2, the output of op-amp IC3 at its pin 1 goes low and relay RL1 de-energises. As a result, the light bulb gets connected to 230V AC and switches on.

Parts List
Semiconductors:
IC1	– LM7805, 5V voltage regulator
IC2	– LM35 temperature sensor
IC3	– Op-amp LM358
BR1	– 1A bridge rectifier
LED1	– 5mm LED
T1	– BC548 npn transistor
D1	– 1N4007 rectifier diode
Resistors (all 1/4-watt, ±5% carbon):
R1	– 1-kilo-ohm
R2	– 10-kilo-ohm
R3	– 680-ohm
VR1	– 10-kilo-ohm potmeter
Capacitor:
C1	– 1000μF, 35V electrolytic
Miscellaneous:
Load	– 230V AC, 20W bulb
RL1	– 5V, SPDT relay
CON1-CON3	– 2-pin connector
X1	– 230V AC primary to 9V, 500mA secondary transformer

The bulb can be replaced with any other appliance, such as a fan or an air cooler, which will switch on automatically when the room temperature goes beyond a limit. The circuit can also be used as a fire alarm circuit by setting the reference temperature very high, like 100°C, and connecting an electric bell or siren as the load. After using a higher current rating relay, the circuit can be designed to automatically switch on even an air-conditioner when room temperature becomes high.

Fig. 3: Actual-size PCB layout for the circuit

An actual-size, single-side PCB layout for the circuit is shown in Fig. 3 and its component layout in Fig. 4. After assembling the circuit on PCB, enclose it in a suitable box. Fix LED1 and CON2 in front side of the box and the load on the rear side of the box.

Download PCB and Component Layout PDFs: click here

S.C. Dwivedi is an electronics enthusiast and circuit designer at EFY

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