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.