I'm now getting to grips with mark II propagator, this time its a bit more complicated having thermostat control and a plate warmer as a heating element.
The Plate warmer comes from aldi €17.99 and claims to be 60 watts (it might be about half that) its essentially a heating pad wrapped in aluminium foil and with a fabric cover. It claims to be able to warm plates to 65- 70 degrees C after 20 minutes but then its supposed to be used as a stack. I am testing it open and temperature seems much lower. dimensions are 22cm by 1 metre with a wire loop running up and down just the once.
For temperature control I bought a room thermostat these go for as little as €8 plus vat mine was a little more at €12 total. These are very simple devices you can set the dial to between 5 and 35 degrees C and they open a relay at the set point no power required. This will need to be in close contact with the heat pad to avoid measuring air temperature instead of soil temp. my first version there can be a 15 degree difference between air and soil temperature. I will have to monitor the system when its all together to ensure i don't cook the roots.
The wiring mods will be simple enough i just need to switch the live with the thermostat the relay is strong enough to switch 10 amps of ac. so my little current will be fine. the rcd on the circuit should trip if any water gets where it shouldnt. An extra layer of plastic should ensure the heater element stays dry. it will probably get built into a box Tomorrow. I have a plc knocking about so if i wanted i could use that for control but i think its not needed at this stage. (a suitable plc can be had from 25 euro up used). Maplin have a little project board for a temperature probe and relay but its 12volt and about 15 euro.
I wouldn't recommend doing this if you are not qualified, if you don't know what you are doing you could manage to kill yourself. It'd probably be easy to adapt this to 12 volt dc which would make it portable and safer.
It looks like the thermostat is going to be over kill as it has one built in the temperatures climbing but still only about 23 degrees so far
overnight it reached 28 degrees 5 more tomato plants were born using an average of 27 watts an hour, when on it draws 57 watts. Today I will build a box for it which hopefully will reduce the energy use further (room temp was around 8-10 degrees)