Abstract: Taking advantage of the IC's very low quiescent current, this circuit enables a small signal of ±1mV or more to switch relatively large load currents.
Figure 1's circuit draws only 6µA, but it enables a small signal of ±1mV or more to switch relatively large load currents. It takes advantage of the IC's very low quiescent current—1.2µA (max) per amplifier (less than a typical battery's self-discharge)—which is able to flow through R1 without turning on Q1. When operated with a sensing coil (as shown) and stimulated by a magnet, the circuit performs the function of a reed switch, but with greater sensitivity. Other applications include alarm systems, bipolar threshold sensing, and audio volume switching.
Figure 1. This load switcher enables a small signal to turn on a much larger load current.
Inducing a signal of either polarity in the coil (by passing a magnet near it, for example) causes the dual op amp to draw more current from its VCC terminal. The increase produces a voltage across R1 that exceeds Q1's VBE threshold, activating the complementary monostable multivibrator consisting of Q1, Q2, and associated components. As a result, Q1 connects battery voltage to the load. For many applications, you can replace the monostable with a simple pnp output stage.
A similar idea appeared in the July 4, 1996 issue of EDN.
Automatic Updates
Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™.
Download, PDF Format
