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open loop solar water heater that wont freeze Answered

(first paragraph is blah blah,  skip if you wish) As some of you know, there are two approaches to a solar water heater.  One is a close loop with a coolant type liquid that wont freeze.  The problem is that you must build and exchanger of some sort to gather the heat.  Expensive and troublesome.  The second is an open loop in which you circulate the water.  Possibility of freezes. 

I have a plan for an open loop system that will latch on to an existing tank and supplement the heat but wont freeze.  What i need is a circuit that will shut off the pump when the sun is down and a water thermostat i can stick in the existing tank that will also shut the pump off.   I think they should be one circuit somehow and should be 12v DC so the pump can be solar powered. They should also be as cheap as possible.

I'm not good with circuits so i could use some help.  Perhaps there is some way to reverse a dusk-to-dawn switch.  Please help.


Use exactly that a dawn to dusk switch a visible light phototransistor that turns on the circuit during daylight and a thermostat to turn on the pump when the sun warms up the water. My Instructable photo components testing has a circuit that will work or a simple photo transistor to an SCR or a transistor can turn on the circuit to power the thermostat and pump.


I have a number of photo circuits I can tweak to sute the specks of the thermostat, pump, and solar panel.

I looked at you very thorough and incredibly in depth instructable. Im afraid i was unable to pull out the information i was looknig for. Is there a phototransitor that i can buy that can be directly used or do i need to create one somehow. Your help is greatly appreciated,.

You are amazing. What is the grid symbol on the left and what are the symbols on the right. I think I understand the diodes, resistor, and the transistors.

Well, the grid on the left is the solar panel. So that dawned on me. But the symbol on the right next to the 1n4001 seems confusing.

A relay to turn on the thermostat and pump.
Can be replaced with an SCR.

Also, does anyone know of a thermometer i could use inside the tank? I need to shut off the pump when the water hits temp.