Dimmer switch as thermostat

[Jules]

Has anyone tried to convert a basic light dimmer switch into a thermostat using a thermistor as the sensor. I have 15Watt seed trays which are mains powered, and don't have a thermostat.
The dimmer contains almost all the parts needed (Diac, Triac, pot, caps.)
If so, is there a schematic which you used successfully? Thanks

 

[3v0]

Are you talking about trays to grow/start seeds in.

 

[Jules]

Yes, I have several heated propagators, and there is no way of controlling the heat - they are always on, and seedlings can get fried. When the sun shines into the greenhouse, the temp under the lid goes too high. I want a thermostat to switch on if the temp falls below 21C (70F) which it does at night. By day, the sun supplies all the heat.
A thermostatically controlled propagator is very expensive, and I have seen a few ideas to convert a dimmer switch to a thermostat, keeping most of the components. I want a remote thermistor as the temp sensor. Any ideas?

 

[Nigel Goodwin]

I suggest you look up "burst fire control", a dimmer isn't a good way of controlling heating.

 

[3v0]

Yes, I have several heated propagators, and there is no way of controlling the heat - they are always on, and seedlings can get fried. When the sun shines into the greenhouse, the temp under the lid goes too high.... Any ideas?

Yes. I put some controls into my wife's greenhouse. A PIC talkes to a string of 1 wire temperature sensors and toggles relays as needed to control the heat. I do not have any heated propagators but we have heating and cooling for the air and a pond heater. The pic talks to a PC in the house that has a display for each monitored temperature and sound an alarm via speech if things go out of bounds.

You could build a simple version.
Just a PIC with a LCD and 1 wire interface code and a sensor for each propagator. Relays. Depends on how fancy you want to get.

If you can program it would not be difficult. Most of the code is stock routines that can be found for most compilers..

 

[Jules]

OK, I've read the burst fire control postings, and it's an easier way of getting what I want. I like the analogy with the microwave oven - heating held on, then off. I don't need the diac control, in fact the zero crossing becomes redundant, no need for it. I haven't found a burst fire schematic to do this. Has anyone got a link? Thanks