resistance comparator

elektor july/august 1983 L resistance comparator This multimeter accessory is not just a bleeper; in spite of its sim- plicity. The circuit indicates by a bleep whether a resistance being measured with the multimeter is smaller than a predetermined value; it can also be made to do so when the measured resistance is larger than the reference value. The device compares the voltage drop across the resistance under test with a reference voltage. Re- quired components are a quad op-amp, a diode, a crystal buzzer, a capacitor, two electrolytics, two trimmers and four resistors. Power is provided by a 9 V battery. In the circuit diagram shown in figure 1 the voltage drop across Rx, which is in parallel with the multimeter, is taken from sockets A/C and B/D. The high internal resistance of the multimeter is hardly affected by the parallel connection of the comparator be- cause Al is connected as an im- pedance converter. Stage A2 com- pares the voltage at the output of Al with a level pre...