Day / Night Electronic Thermostat

new.gif (1957 bytes)  7 Feb 04 - Added information about mounting the TC622 on a case.

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.

Day/Night Thermostat Schematic

The circuit is basically in 3 parts - A regulator, the light level sensor and the thermostat.

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 could 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 0C and 27C. When the light level rises causing RLA1 to operate the value of VR3 will be in series with R7 and VR2, giving an increase of up to 22k resistance and 27C switching temperature. The output FET (TR2) in this case drives RLA2 to switch whatever load is connected to the contacts. The second indicator (LED2) acts as a temperature reached indicator and is extinguished when the relay is operated. D2 is again fitted to protect TR2. The outputs of RLA2 can be used to switch almost anything, but in this case will connect to the heating elements. I actually mounted IC3 on the end of a cable (approx. 1' long) to allow positioning where required. You need to be careful, because the TC622 doesn't seem to like long cables. Because of the ripple problem above, I fitted C5 across the power pins of IC3 to help prevent any problems due to the long wires.

Day/Night Thermostat Board

If you want to know any more information or have a question please ask me.

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