| 摘要 |
<p>A push button (T) telephone has a circuit (WG) stores the digits and transmits them to the line (a,b) via a dialling relay (USI). The number can be re-dialled by pressing a single button. Usually the first digit is a local code and one waits for tone before dialling the remainder. The duration of this pause is stored by the circuit (WG) and reproduced when re-dialling. Power (U,O) for the circuit (WG) is drawn from a capacitor (C2) which is charged from the line, via a transistor (T2), only during dialling. The transistor (T2) is driven by a second transistor (T3) which is driven by a contact (t), common to all the digit buttons. A relay contact (USU) then changes over and maintains the supply while each digit is dialled, plus 800ms. During the pause, when re-dialling, the circuit (WG) turns on (P) a third transistor (T1) which keeps the capacitor (C2) charged, via a resistor (W1), until the next digit. Rectifier bridges (G1,2) make the circuit independent of line polarity.</p> |
