Although it draws only 6 µA, the circuit in Figure 1 enables a small ac signal as low as ±1 mV to switch relatively large load currents. The circuit takes advantage of the IC's very low quiescent current, which, at 1.2 µA maximum per amplifier, is less than the self-discharge of a typical battery. This current can flow through R₁ without turning on Q₁. When you operate the circuit with a sensing coil (Figure 1) and stimulate it with 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.

Inducing a signal of either polarity in the coil by passing a magnet near the coil, for example, causes the dual op amp to draw more current from its V_CC terminal. This increase produces a voltage across R₁ that exceeds Q₁'s VBE threshold, which activates the complementary monostable multivibrator that consists 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. Load current is independent of the applied signal, provided that the signal is sufficient to turn on Q₁. (DI #1887)