Day / Night Electronic Thermostat

Posted on Feb 7, 2014

This design was created at the request of my father. It is designed to operate a 240V a. c. heater, in his Orchid box, to maintain a higher temperature during daylight hours than darkness. Regulator - This is a very simple regulator used here to obtain a stabilised 12V supply. The 78M12 regulator used is rated for 0. 5A, but any 78 12 regulator coul

Day / Night Electronic Thermostat
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d be used if it provides sufficient current for your circuit. I had problems with the circuit due to ripple on the output, from the mains supply, and I fitted C4 to remove it. An option is to fit LED3 to indicate power with R9 acting as a current limiter. Light Level Sensor - An ORP12 (LDR1) light dependant resistor is connected to a very simple circuit to swap between day an night settings. The unit will trigger once the light level drops below a certain level set by VR1. When dark enough, TR1 will cause the relay (RLA1) to operate. Optionally LED1 can be fitted to indicator daylight mode with R6 acting as a current limiter. D2 is fitted to prevent the back EMF generated when RLA1 is switched off from destroying TR1. When the unit triggers, TR2 is energised and provides an additional voltage to TR1. This introduces hysteresis in to the unit and stops it switching back and fore if the light level fluctuates around the trip point. Thermostat - The basic principle of this unit is the same as my original thermostat designs. The main difference is that during darkness RLA1 will not be energised and therefore VR3 will be short circuited by the relay contacts. The resistance in the circuit during darkness will be R7 and VR2 and therefore between 91k and 113k, giving switching temperatures between 0 °C and 27 °C. When the light level rises causing RLA1 to operate the value of VR3 will be in series with R7 and VR2, giving an...

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