Long ago in a life Far, Far Away... I took an electronics course... but that was High School... and now it's more than 20 years later... I'm trying to build a Differential Temperature Controller for a solar water heater. The idea is that the controller measures the temperature at the solar panel and at the storage tank. When the temperature at the panel is higher than in the tank, a relay closes and turns on a circulation pump. After the temperature is balanced the pump turns off. I found a schematic here... http://www.redrok.com/images/diftemp.gif and used the one on the right (the one with thermisters). From that I made the circuit diagram. I built it up on a board and it looks amaturish, but all the connections are good. Here's my problem... I'm not getting sufficient voltage from the op amp output to switch the relay. I measured with a Voltmeter and the output is showing the voltage thru the relay coil. When I disconnected the relay from the output I can measure the voltage and see it change with the temperature difference so it is working... Now... my question... How can I get the output high enough to switch the relay without being so high that the relay won't turn off when the temp calls for it to be off...? Thanks for all suggestions!!