September 12, 1997

Remote control turns battery on and off

Allen Harstine, Integrated Measurement Systems Inc, Beaverton, OR

The circuit in Figure 1 acts as a latching solid-state relay. A short application of a magnetic field in the vicinity of S₁ toggles the relay on and off. The relay switches power from a 3V battery at load currents as high as 10 mA. This remote switch is ideal for battery-powered applications. In the off state, the circuit draws no current; in the on state, the circuit draws only 100 µA. All the parts are inexpensive and readily available.

A magnetic field near S₁'s reed switch closes the switch. The closing of S₁ causes emitter-base current to flow in Q₁ through the path of R₃ and D₂. This current flow forces Q₁ into saturation, and the battery voltage switches to the output. The presence of an output voltage switches Q₂ to the on state; Q₂ then latches Q₁ on.

The unique feature of the relay is its ability to toggle. A short, temporary application of the magnetic field causes the relay to latch to the on state; a greater-than-1-sec application of the field causes the relay to latch to the off state. The off switching occurs because of C₁'s voltage-doubling action. When S₁ closes, R₂ slowly charges C₁. When S₁ opens, the voltage at the anode of D₁ is equal to the sum of the voltage across C₁ and the battery voltage (through R₁). D₁ conducts the resulting current to the base of Q₁, which overdrives the latching current from Q₂ and turns off Q₁. Once Q₁ is off, Q₂ releases its latching signal. (DI #2074)

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