I'm coming to this thread late but do have a couple of opinions. I am assuming that you can float the entire circuit. It will be=20 battery-powered, so it doesn't need any other outside connections. 1) Use a pair of series-connected N-channel MOSFETs for control. Tie=20 the source and gates of both MOSFETs together. Sources go to PIC Vss=20 rail. Gates go to an output pin. I'd also include a 1M resistor or=20 so between gates and sources to ensure that the FETs stay OFF should=20 the circuit lose power (should happen automatically but you never know). The drains connect across the thermostat wires. For bonus points, a pair of diodes connected so that the anodes go to=20 the FET drains and cathodes tied together can be used for as the=20 power supply for a very-low current charger for the batteries. Keep=20 the current to less than 1 mA. 2) Use an outside-mounted thermistor if you want to make this fully=20 automatic. Again: the thermistor must be completely floating and=20 can't electrically-connect to ground (or anywhere else). Use another=20 port pin to supply power to the thermistor pull-up resistor. Read=20 the outside temperature at a rate that is suitable- a few times per=20 hour is probably good. 3) Power the entire circuit from a decent-sized LiPo or Li-Ion cell. More later - gotta run. dwayne At 07:15 PM 12/20/2017, Art wrote: >I recently discovered a vulnerability in my house heating system during >the latest cold snap. It has to do with hot water lines that go to 2 >kick space heaters in the 'breezeway'. The kickspace heaters were added >by the previous owners, who also enclosed the breezeway to make a mud >room like entrance. And, custom built cabinets, drawers, raised floor, >built in coat racks and bench seats with hinged lids for storage were >added. But, they built the pipes for the kickspace heaters underneath >all the built in niceties, it's horribly expensive to tear all the add >ons out just to get to the pipes and the kick space heaters to insulate >the pipes. > >We discovered the frozen pipes during the recent cold snap, there's no >telling how many times its frozen without our knowledge. It was just >luck that the frozen pipes were discovered last week. > >I need a small low power timer circuit that will turn the oil burner for >that zone on when the outdoor temp gets below 10 or 15 degrees F. The >timer would run for a few minutes every hour or so, just enough to make >sure warm water circulates though the pipes and both kick space heaters >so they don't freeze. > >The timer is not an issue. I want to make a parallel connection to the >existing zone thermostat for the house and the breezeway. When the timer >expires, it would short the thermostat terminals which would start the >oil burner every once in awhile. > >The heating system is a modern 2 wire system, so I need a non >relay/triac method for shorting the thermostat wires together, which is >how the oil burner/tsat combo operates normally. The tstat is fed by a >small 24 volt ac transformer in the oil burner. When the tsat calls for >heat, it closes a relay which draws current from the 24 volt ac source. >But, all the relays I can find draw big coil currents, which is not >conducive to a simple low power timer circuit that runs on battery >power. Latching relays might work, but they're expensive and need extra >driving electronics. > >I need some other means to short the 2 thermostat wires together, >without using much power-which will enable the use of a small battery >powered circuit. > >If transistors or mosfets can be used, it would be simple and cheap. >Bipolar transistors as a switch would be ideal, since driving the base >above .7volts makes battery power practical. > >I have not managed to find much technical information about how the >current is sensed inside the oil burner. Several web resources explain >that the power transformer is shorted out by the relay closure in the >tsat, but say nothing about the current limiting/current flow sensor >that actually makes the oil burner turn on. > >What type of switching circuit do I need? > >Any ideas? > >TY > >Art > > >-- >http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA 780-489-3199 voice 780-487-6397 fax 888-489-3199 Toll Free www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .