The temperature sensor switch is built across the two ICs LM35 and LM358. The relay in the circuit activates any electronic circuit connected to it when LM35 senses the desired temperature level. Furthermore, the circuit is very simple involving just the two ICs, a transistor, and a relay connecting other electronic devices/circuits.
Hardware Required
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | Temp. Sensor | LM35 | 1 |
| 2. | Operational Amplifier IC | LM358 | 1 |
| 3. | Variable Resistor | 10KΩ | 1 |
| 4. | DC Battery | 5V-12V | 1 |
| 5. | Diode | 1N4007 | 1 |
| 6. | PNP Transistor | 2N3906 | 1 |
| 7. | Relay | 5-12V | 1 |
IC LM35
| VCC | Positive supply of battery |
| OUT | Analog output voltage proportional to temperature |
| GND | Negative supply of battery |
Features
- Celsius calibration
- Range -55°C to 150°C
- Low Cost
- Operating voltage 4V to 30V
IC LM358
| PIN No | PIN Name | PIN Description |
|---|---|---|
| 1 | Output 1 | Output of operational amplifier 1 of the IC |
| 2 | -input 1 | Inverting input of operational amplifier 1 of the IC |
| 3 | +input 1 | Non-Inverting input of operational amplifier 1 of the IC |
| 4 | Ground | Negative battery supply |
| 5 | +input 2 | Non-Inverting input of operational amplifier 2 of the IC |
| 6 | -input 2 | Inverting input of operational amplifier 2 of the IC |
| 7 | Output 2 | Output of operational amplifier 2 of the IC |
| 8 | Vcc | Positive battery supply |
Features
- Large voltage gain: 100dB
- Very low current drain
- Unity gain through internal frequency gain
- Wide range of supply voltage: 3V to 32V
Circuit Diagram
Circuit Explanation
The IC LM35 is a temperature-sensing IC that detects temperatures up to 120°C. The 10KΩ variable resistor is set according to the desired temperature sensing level. Once the value of the variable resistor is set, LM35 when senses the desired level of temperature, thus, producing a signal sent directly to LM358. LM358 is a dual-operational amplifier IC. Moreover, LM358 works as a comparator of the input voltages.
LM358 receives the input signal and amplifies it. Furthermore, to operate the relay, transistor 2N3908 amplifies the signal. Moreover, the diode connected to the relay prevents voltage reversal. Also, the relay is selected so that its rating be same as the battery voltage. At last, the relay is activated as soon as it receives the signal and, thus, activates the electronic device or circuit connected with it.
Application
- Electronic Sensor circuits
- Audio Pre-amplifiers
- Small signal amplification
