Thermostat Controller for Hot Water System

In the various solar hot water projects, I've referred to the cold water inlet valve "turning itself on when the sun shines"... and haven't described exactly how this works.  Here's how:

I use a second-hand (of course!) hot water cylinder thermostat which you can pick up for free if you talk to a plumber nicely.  These thermostats generally get dumped with the old cylinder when someone installs a new hot water cylinder in their home.  Here's what one looks like:

View of thermostat
 
It consists of a long copper probe (which normally slides into a copper pocket in the hot water cylinder), and an adjustable switch to set the temperature at which you want the thermostat to turn on or off.  The adjustment is usually within the range 40 to 80 degrees Centigrade, which is perfect for our application.

The quick ones among you will have worked out already that this thermostat won't work in our situation, because:
-  We want the thermostat to turn ON when the sun shines (i.e. when it gets hotter than a certain temperature)
-  This thermostat is designed to turn ON when cold, and to turn OFF when it gets hotter than a certain temperature.

Hmmmmm.....

Ok, no problem.  We design a simple circuit consisting of a transistor and a relay, to "invert the logic" of the thermostat.  Like so:

Circuit diagram
The components are self-explanatory for the most part. 
T1 is an NPN switching transistor (e.g. BC547A)
The 100 ohm resistor is a "Pull-down" resistor which keeps point B at 0 volts while the thermostat switch is closed.
D1 is an LED (Green or Blue is good - it turns on when the output of the circuit is on)
D2 is a "freewheeling" diode (1N4002 or similar) to protect the circuit from back-EMF when the relay turns off.
The Relay needs to be sized for the job it needs to do.  (My one only needs to turn on a washing machine water inlet solenoid, about 5 - 10 Watts, but if you need to turn on a circulation pump, you'll need a bigger one!)  The 12V supply needs to be sized accordingly too.  I use a 12V "plug-pack" which provides 2 Amps. (That's 24 Watts). 
Incidentally, this circuit works fine on any voltage between about 10V and 24V.  My 24V washing machine solenoids work perfectly well on 10V.

The wires going to the hwc thermostat switch can be as long as you like (up to about 20 metres), so the thermostat can be installed inside the solar panel with the copper probe in the hot air-space under the clear plastic cover, and the circuit itself can be enclosed in a nice little box out of the weather with wires coming out of it, and a little green light to tell you when it's working.  Nice.

If you're interested, the circuit works like this:
When the temperature rises, the hwc thermostat switch opens, which causes point B in the circuit to go from 0 to 12V.  This turns the NPN transistor ON, and current flows through the Relay coil.  This turns the relay ON, which switches the output from 0V to 12V, and the solenoid (or motor) turns ON.  Easy!

If you don't know how to build a circuit like this, just ask any 12-year-old electronics geek, and he or she will be thrilled to build one for you.

Have Fun!

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