The Johnson Controls A19 Series is a single stage, heating or cooling, SPDT temperature control that uses a liquid-filled sensing element and capillary. The unit has an exposed or concealed set point dial with adjustable differential, and it will switch line voltages.The A28 Series is a two stage, heating or cooling, w/2-SPDT temperature control that uses a liquid-filled sensing element and capillary. The unit has an exposed or concealed set point dial with adjustable differential, and it will switch line voltages.The A319 Series is a single stage, heating or cooling, SPDT temperature control that uses a Type 21 thermistor sensor. The unit uses a set point dial and operates on 120 VAC or 240 VAC with a wide adjustable differential. A separate LED indicates relay operation.The A419 Series is a single stage, heating or cooling, SPDT temperature control that uses a PTC sensor. This unit features a digital display and buttons to adjust the set point differential, setback temperature, anti-short cycle delay, and failsafe control. The unit operates on 120 or 240 VAC and a 3/8in (0.95 cm) display with H/C, F/C and setback indication. A separate LED indicates relay operation.