A Multi-Purpose Voltage Controlled Switch Alan Yelvington All the parts needed to build this all purpose controller are available at Radio Shack. It will switch loads or power sources either on or off at user set voltage limits. It measures the voltage of the battery and operates a small relay which in turn can operate a larger relay connected to an appliance or a power source (like a PV array). Control Design Here's the basic scoop. The controller is made up of four sections. 1) the voltage reference (LM317), the voltage threshold detectors (1458 Op Amps), the control logic section (4011 NAND Gate), and the relay driver section. The reference voltage section not only provides a "benchmark" voltage level (10.0 VDC ±0.1 VDC) that is used to compare the input voltage; it also provides a regulated voltage to power the rest of the device's logic. This keeps the logic stage in the face of battery voltage changes. The input regulator (LM317) allows this circuit to function from 11.5 to 36 VDC. The threshold detectors (1458 Op Amps) are used to compare the reference voltage to the input voltage form the battery. The input voltage is divided down through the 10k½ input potentiometers to make voltage switch point user adjustable. The outputs of the detectors are apparent by the lit LEDs. The control logic section is an RS Flip Flop made of the two NAND gates in the 4011. This serves as a toggle so that the opposite threshold must be met before the switch will toggle back. This prevents the relay from "hunting" and chattering at a threshold point. The NAND gate also permits a very wide range between the two voltages setpoints determined by the detectors. INSERT SCHEMATIC The simplicity of this device is what gives it its flexibility. For example, Load divert when the batteries are at 105% SOC with return to charging when the cells are at 90% SOC. Generator start at 50% SOC and generator stop at 95% SOC. The option of either a normally open or normally closed circuit (through the relay) makes interfacing the control easy in all applications. If you would be interested in buying an etched and drilled circuit board for this project, I would be interested in knowing. If I receive enough responses, then I'll make some PC boards, instructions, and part lists. I expect I can do this for 10 to 15 bucks. Send me a self-addressed stamped envelope and I'll get back to you with the verdict. DON'T SEND MONEY!! Access Alan Yelvington, 219 Blanche St., Houghton, MI 49931.