| 摘要 |
The circuit operates a main switch SCR to pass an a.c. supply V through a load L when a metallic object is in proximity to coil L1. Coil L1 is driven by an oscillator comprising transistor T2 and associated components. SCR is gated on simultaneous occurrence of an output from level detector T3 and a signal from T5, these being ANDed by T4. The timing of the signal from T5 in the supply cycle is governed by the RC time constant of C4,Z1,R14 and is such that the power supplied to the oscillator and level detector is the minimum necessary, for a wide range of supply voltage and load current.
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