When I moved to Edinburgh last year I set up an engineering group at a local secondary school. The children are building solar water heater to my design for a children’s retreat in Bulgaria. I need to say, they are doing a very neat job of it too, all galvanised sheet steel building and flawless soldered pipe joints.

Obviously, as they’re just 15 years old, its taking them some time to complete the work but they’re well underway, and its great to see them developing as individuals and realising that they can achieve impressive results if they put their minds to it.

As its somewhat beyond their current ability, I paid for and constructed the controller unit myself, and this movie shows its testing. Due to the radicly different system needs of a Bulgarian retreat with no electrical supply Vs a metropolitan Derby home with electricity, gas and internet, the entire system is significantly different (And easier) as compared to the Mark 1. The future Mark 3 will be an amalgam of both.

Reliability, simplicity and robustness are the key words here. The most important issue with the Raspberry Pi control system on the mark 1 was the boot-up time and the need to “Know” the software. The mark 1 controller system was vastly superior in terms of ability, adaptability and data presentation, but it’s not appropriate for a helpless croft in Bulgaria. A variation of it will be used on the future Mark 3.

This mark 2 system is analog, based on simple LM324 Op amps. External controls are just one switch – Off or On. There are three indicator lights. The whole is in a self contained IP67 box, with cable glands. Connections are by standard DIN springs mounted on rails.

There are, furthermore, some internal controls for setting the system up. Cut-in and hysteresis are flexible, as is the dump load.

A tiny rechargeable battery keeps the circuits running when the PV panel isn’t working, and the PV panel charges the battery and operates the pump.

A big benefit of the Mark 1 system over any other controller I know is that it may vary the rate of the pump. The Mark 2 may also do so, but it does so passively – The varying voltage on the PV cell should change the pump rate. This may require some setup, but the theory is there.

Finally, I’ve gone for the less efficient closed loop glycol circulation system for this particular build, again for simplicity. Lack of computer control and power on site means its not feasible to utilize a drain down system as I did on the Mark 1. The students built a heat exchanger inside a water cylinder, and the pipes is looking good.

I’m proud of what the children have attained, and once they’ve completed, I will clear it with the instructor and get a video made up.

Any questions, let me know as ever!