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Wireless Thermostat

Wireless Thermostat

Over the mains carrier current remote control for central heating. The thermostat transmits a 100 KHz carrier over the mains wiring when it’s too cold. The receiver detects the 100 KHz carrier and fires the boiler.
The LM35 is an easy-to-use, low-cost 3-pin temperature sensor in a plastic TO92 package. To respond rapidly to changes in room temperature, the sensor needs to be in good thermal contact with the air. Mounted in a 3-way turned-pin strip underneath the circuit board, it protrudes through a ventilation slot in the back of the case. I tried mounting it inside but the response was too slow. The pins are connected using fine gauge (29 SWG) wire to minimise thermal inertia. 
The LM35 produces an output of 10mV per degree Celsius. This is compared to the roomstat setting by a 741 op-amp comparator. The roomstat forms a potential divider between voltages of approximately 50 and 400mV corresponding to a temperature range of 5 to 40 degrees C. The comparator functions as a Schmitt Trigger with positive feedback via resistor R3 providing hysteresis. 

The amount of hysteresis depends on the Thevenin equivalent source resistance of the potential divider. Unfortunately, it’s not constant. Series resistor R2 helps to mask the spread. Hysteresis can be increased by increasing R2 or reducing R3. With the values shown, it’s about 0.7 degrees at the roomstat midpoint. It varies from 0.5 to 0.9 over the full range. 

A simple improvement would be to buffer the roomstat potentiometer output using half of a DIP 8 packaged dual op-amp such as the LF353N as a voltage follower – the other half being the comparator. This would yield a constant and easily programmable amount of hysteresis.

Article by Andrew Holme at http://www.holmea.demon.co.uk/

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