Hi Art, If you just use another thermostat in parallel with your existing one, it w= ould work the way you want, right? Except you can't set a standard thermost= at below 55 degrees. All the home thermostats I've seen use a thermistor. Y= ou could get a second thermostat for the remote area, replace its thermisto= r with a different thermistor having a lower base resistance. It would thin= k everything was hotter than it really was. Then experiment until you find = out the temperature setting that gets you where you need to be. Best regards, Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of Art Sent: Wednesday, December 20, 2017 6:15 PM To: Microcontroller Public Subject: [EE] simple low voltage switch for ac 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 --=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 .