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Frost alarm
About the device in general
The device is only 35x40 mm. The primary aim is to sense the temperature using thermistor as a detector.
It reacts only if the temperature is bellow a fixed value.
It is possible to calibrate the threshold temperature using micro-potentiometer from -15 to 45 degrees Cesium.
The device is capable of switching on and off a heater or any system with up to 250V/5A.
The circuit is based on operational amplifier 741 as a comparator. The sensing area is relatively small -1mm sq.
The electrical consumption is about 5 mA when the relay is off and 40 mA when the relay is on.
Application
The device is ideal for experimental purposes, where heather must be switch on. Initially this circuit has been developed as a home frost alarm. However, the high sensitivity (around 1 degree Celsius) and the small size make this device very competitive and useful for various purposes. The output is 12V/300mA and can drive powerful relay or other TTL electronic devices. The temperature hysteresys can also be adjusted from 0.5 degrees to several degrees Celsius.
The device needs a 12 V stable power supply. Small LED diode is connected to the output in order to allow visual control of the state.
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