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Return to BlogTemperature Controlled Electronic Relay by LM35
Posted Jan 27, 2012 at 11:10 am
Figure 1
The project has been constructed for applications that require automatic temperature control while providing highly accurate thermal control circuit.
The LM35 temperature sensor is the heart of the circuit as it is factory-calibrated in the Celsius or Centigrade scale with a linear Degree to Volt conversion function. This one-chip temperature sensor switches a miniature relay ON or OFF depending on the temperature detected. The relay gets activated when the LM35 detects a temperature higher than the pre-set level set by VR1. The relay is de-energized when the temperature falls below the pre-set temperature. Any AC or DC 12V supply or 100mA battery can power the circuit.
The change in temperature from 0ºC to 100ºC changes the output voltage at Pin 2 from 0V-100V. in order to build a complete thermostat, the design needs to be provided with a precision voltage reference like TL431 and an accurate comparator such as the LM358. A variable voltage divider is formed by the preset VR1 and resistor R3, which sets the reference voltage Vref. The reference voltage is being buffered by the op-amp to prevent the divider network from being loaded. The calibration process requires no special instrument.
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