The purpose of this circuit is to connect an appliance (fan, lamp, motor, soldering iron, etc.) or an electronic circuit by simply touching an NA type switch (normally open). With another touch, the device hangs up. Its operation is based on an astable multivibrator with transistors. The source that powers this circuit uses two voltage regulator integrated circuits and must have excellent filtering and decoupling to avoid instabilities. A simple touch of the key: “power” causes one of the multivibrator's transistors to saturate, while the other goes to cut. With the transistor T1 saturated and T2 cut off, a high level appears at the base of T3 that leads, together with T4 causing the relay to dock. The memory circuit is formed by a NiCad battery that keeps the multivibrator fixed in its last state, even when there is a power cut. When the power is normal, the 2.4 V battery remains on a permanent charge, even if the controlled device is switched off. When the battery is supplying the circuit, the current is of the order of 5 to 10 mA, which makes its autonomy reasonable but efficient in cuts up to 5 hours. It is necessary to avoid keeping the “power” key pressed, because if this occurs, the multivibrator starts oscillating and the controlled device starts to turn on and off.
