Clock-counter of telephone conversations. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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The prefix to the clock (Fig. 1) responds to outgoing calls. It contains a rectifier on a VD2 diode bridge, a voltage regulator on a DA1 chip, a VD4 zener diode and a capacitor C5, comparators on DD1.1-DD1.3 logic elements, a D-trigger on a DD2 chip, electronic switches on an optocoupler U1 and transistor VT1.
Connect the prefix to the electromechanical clock in accordance with the diagram shown in fig. 2. At the same time, an oxide capacitor C1 is added to the clock.
The plugin works like this. After connecting it to the clock and the telephone line, within a few seconds, the capacitors C2 and C3 will charge: C2 - through the resistor R5 to a high logic level, and C3 - through the zener diode VD3 and the resistor R4. The output of the DD1.2 element will be a high logic level, and the output of the trigger will be low. The transistor is closed, the clock is de-energized.
If you pick up the handset, the voltage in the line will decrease, the zener diode VD3 will close. The voltage on the capacitor C3 will decrease to a low level in a few seconds, and on C2 it will practically not change. At the output of the element DD1.2, a low logic level will appear, and after a fraction of a second, a high level will be set at the output of the element DD1.3. This will lead to the appearance of the same level at the output of the trigger, and hence to the opening of the transistor. The clock will start timing.
If you put the handset on the device, the voltage in the line will increase. After a few seconds, the capacitor C3 will be charged, a high level will appear at the output of the element DD1.2. The trigger will reset, the transistor will close, the clock will stop.
When ringing signals are received, an alternating current will flow through the emitting diode of the optocoupler, the phototransistor will start to open, and the capacitor C2 will quickly discharge. During the intervals between ringing signals, it will not have time to charge. Therefore, if you pick up the phone, after a few seconds, a high level will appear at the output of the DD1.3 element, and a low level will appear at the trigger output. The transistor will remain closed, the clock will not start timing.
In addition to those indicated in the diagram, it is permissible to use optocouplers AOT110B-AOT110G, AOT122A-AOT122G, AOT127A-AOT127V in the attachment; transistor - KP302A or KP302B; diode VD1 - any low-power rectifier; diode bridge VD2 - KD906G, KD407A or composed of four diodes KD102A, KD102B or similar; zener diode VD3 - KS515A, VD4 - KS482A, KS510A, KS191Zh. Capacitor C1 - series K73, C2, C3, C5 - K52, K53 or similar with low leakage current, C4 - KM. Resistors - MLT, S2-33.
The parts are mounted on a printed circuit board (Fig. 3) made of one-sided foil fiberglass. The board is placed in a housing made of insulating material.
Establishing the device is reduced to the selection of capacitors C2 and C3. The capacitance of the capacitor C2 should be such that after a series of ringing signals, when the handset is picked up, the clock does not turn on. Perhaps, to obtain such a result, it is necessary to install a larger capacitor C1. Capacitor C3 must be selected with such a capacity that, after 1 ... 2 s after removing the tube, the clock mechanism will receive supply voltage.
It should be added that the device must use such an electromechanical or quartz watch with a mechanical dial that starts timing immediately after the supply voltage is applied.
Author: I.Nechaev
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