| 摘要 |
PURPOSE:To turn on/off forcibly a switch when no reference level detection circuit is operated even after a prescribed time is elapsed by providing a timer circuit switching a switch circuit forcibly at a prescribed time depending on a switching control signal. CONSTITUTION:When a DC input is inputted to an input terminal A, the level of the input signal does not cross a reference voltage level and when a low frequency AC input signal is inputted to the input terminal A, since the period where a reference level detection circuit 2 changes is long, the reference level detection circuit 2 might not send a clock to a T/FFs 7, 8 or a lock frequency of the T/FFs 7, 8 might be low. In such a case, a time constant comprising a resistor 13 and a capacitor 14 is supplied to a change in the signal level at a control terminal D of the switch, and when the potential across the resistor 13 is lost, since the output of an exclusive NOR circuit 15 gives an H level, a D/FF 12 is driven to switch a signal changeover switch 1.
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