| 摘要 |
<p>Coin operated telephones are supplied from a power pack connected to the power mains, and have a signalling unit in addition to the speech circuit. When the receiver is lifted, the signalling unit sends a signal informing the exchange that the calling telephone is coin operated. Switching means prevent a manipulated signal being evaluated at the exchange in special conditions, as, for example, mains failure. The filter circuit comprises an auto-transformer with a longitudinal coil (41) and two parallel loops, and containing two coils (4L1) and a single capacitor (C1) and the other two coils (L2) and two capacitors (C2, C3).</p> |
