Wide-range resistance monitoring apparatus and method

An inexpensive circuit monitors the resistance between a body (e.g., a static dissipative work surface, a tool, etc.) and ground potential, and compares that resistance with high and low resistance values defining a preset resistance range. An operational amplifier is connected to the body and causes current to flow from the body and through the op amp feedback network. A switching arrangement coupled to the operational amplifier alternately switches the amplifier feedback network between low and high resistance values defining lower and upper ends, respectively, of the preset resistance range. A logic circuit connected to receive the op amp output (a) lights a green indicator lamp if the resistance under test fails within the preset resistance range, (b) lights red and green lamps alternately and pulses a buzzer if a measured resistance falls below the lower limit of the resistance range, and (c) lights yellow and green lamps alternately and pulses the buzzer if the measured resistance exceeds the upper level of the resistance range. The resistance range can be relatively large, spanning five orders of magnitude of resistance or more. The circuit is particularly useful for preventing static discharges from destroying CMOS and MOSFET electronic devices.