Following on from my previous post on my DIY timer I wanted to make something that I could mount to the wall and not have my wife be ashamed of it! In passing on one of the car forums I frequent I said that I could modify my original heating timer to be controlled by a google calendar for less than €100. As you know that timer died while experimenting so i was left having to make one from scratch if i was to come good on my claim. Thinking about it an arduino combined with an ethernet shield and touchscreen shield could come together to make a very nice controller with a tidy looking interface. The hardware for that should come in under €100. 20 each for the shields and arduino and a further 10 for a solid state relay. It was while browsing DX.com i came across this. A 4.3″ touchscreen android tablet for less than 35 euro delivered – mind blown, I knew what i had to do.
Hardware wise I needed a way of allowing the tablet to turn on the relay. I couldn’t use bluetooth as the tablet didn’t have it and it would be too unreliable anyway. The next choice for getting an output would be the vibration motor but again, the tablet didn’t have one. All that i was left with was using the speaker output to trigger the heater. To do this i had the software (detailed later) output two tones. Depending on the frequency of this tone we would know if the heating should be on or off. The speaker output is passed into an op-amp used as a comparator to give a nice 5v square wave that is input into an Attiny85 via one of the Boards.ie keyrings/dev boards i had made up. The attiny measures the incoming pulse and decides whether to turn the output on or not. The solid state relay can be directly controlled with an attiny pin. Here you can see the hardware during the prototype stage.