This circuit is use for indicates when the input voltage differs from two defined limits, V1 and V2. The limits that can be adjusted and the circuit is designed to trigger the adjustable “window”. Supply voltage, Vcc must be higher than the highest input voltage by at least 2 volts. One application here is to monitor a 12V car battery. V1 can be set to 14V and V2 to 11V thus giving an indication of more than the cost or weak batteries. Op-amp used here is the CA3140 MOSFET. They are used to advantage because they have less output offset voltage and can switch to 0volts close. If any other use op-amps is like the LF351 or CA741 will need to have an offset null control. This is just a 10k preset reached between pins 1 and 5, the wiper connected to the negative supply op-amps or 4 pins. This is the figure of the circuit.
With this circuit the op-amp will turn on the LED if the input voltage out of limits, the two 1N4148 diodes to form an “AND”-gate at the output. Input voltage is to be monitored are fed through a series of 10k resistors on the input of both op-amps. If the input voltage is greater than the limit set by V1 it will CA3140 output swing to almost full supply voltage and LED light. Similarly, if the input voltage is less than the limit set by V2 the op-amp will swing to the Vcc and the LED light.
