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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Outgoing call time counter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony To record the time of telephone conversations, special counters are used. The proposed device differs from those previously described in that it takes into account the time of conversations more accurately, as it analyzes the signals in the telephone line. The principle of time-based payment for intracity telephone calls involves paying for the time of only those calls that originate from the subscriber. At the same time, a certain limit of conversation time is provided at the expense of a monthly fixed fee. Naturally, it is interesting and useful for users of the telephone network to know their "spoken" time in order, for example, to switch to the "talk economy" mode in time or to estimate additional costs at the end of the month. The device operation algorithm is explained by the functional diagram shown in fig. one.
In the presence of a ringing signal (bell), block 5 generates and outputs a control voltage to the time counting prohibition unit (block 7), which in this case breaks the circuit of the counting pulses. The time indicator (block 9) displays the previous readings without changing them. In this case, there is no time count, regardless of whether the subscriber picked up his phone or not. When the subscriber picks up the handset, the device for analyzing the presence of "his" call (block 2) is triggered and prepares the circuit for passing counting pulses through the time count resolution node (block 3). The necessary conditions for the passage of counting pulses from the generator (block 8) through the prohibition node (block 7) are the absence of a ringing signal before picking up the handset and the end of the “no answer” signals after dialing the number by the subscriber. Thus, the calculation occurs only when the outgoing connection has taken place. In the event of a busy signal, the count will also not start. The current talk time is displayed on the time indicator (block 9). Block 4 operates in the accumulation mode. To reset it, the "Reset" button is provided. All elements of the device are powered by a power source (unit 10), which, in turn, is powered by a telephone line. The schematic diagram of the device is shown in fig. 2. The counting pulse generator is assembled on a DD2 chip. A feature of its construction is the absence, for simplicity, of a quartz resonator used in a typical switching circuit. At output 9 of the DD2 microcircuit (pin 1) there are counting pulses with a frequency of 1 Hz, and at the output K there are pulses with a frequency of 512 Hz, which are used to operate the HG1 liquid crystal indicator and supply to counters DD5-DD10.
The device for analyzing the presence of a call works as follows. The call signal (alternating voltage with a frequency of 25 Hz and an amplitude of up to 110 V) from the line through the capacitor C3 is fed to the diode limiter VD7-VD10. The applied inclusion of diodes allows you to convert the sinusoidal voltage of the ringer into almost rectangular pulses with an amplitude of 2,1 V, which switch the trigger DD1.1 at input C. Since at the input D of the trigger DD1.1 at this moment there is a high level from the output of the element DD4.1 , a high level will also appear at the direct output of DD1.1. This signal prohibits the passage of counting pulses through the element DD3.1, which come from the DD2 chip. If at the same time the subscriber picks up the phone to talk, time counting will not occur. When the subscriber picks up the handset, the Schmitt trigger on the DD4.1 element is switched. Its input receives a signal from the divider R2R3. In this case, the level will change from high to low, but with some delay due to the presence of capacitor C2. Capacitor C2 is necessary to filter out dialing, ringing, and interference signals that could lead to false triggering of the DD4.1 trigger. The low level from the output DD4.1 goes to the input D of the trigger DD1.1 (which will now not change the initial zero state, regardless of any signals at the input C, if there was no call before) and the input of the element DD3.2, which allows the account time. From the output of DD3.2, the counting pulses are fed to the element DD3.3, the second input of which (pin 8) is supplied with a control level from the output of the device for analyzing the presence of a "not responding" signal (sending sinusoidal pulses with a filling frequency of 425 Hz, a duration of 1 s and a pause 4 s) or "busy" (similar messages with a pause of 1 s). Pulses are emitted on diodes VD7-VD10, amplified by transistors VT3, VT4 and charge capacitor C5 almost to the power supply voltage. During a pause, the capacitor C5 is discharged through the tuning resistor R9. Transistor VT2 is closed at this time. Resistor R9 sets such a time constant that within 4 s (pause) there is no switching of the Schmitt trigger DD4.3, which converts exponential fronts at its input into steep ones at the output. Thus, while the "not responding" signal is emitted on the capacitor C5, the trigger DD4.3 is in a single state and its output signal prohibits the passage of count pulses through the element DD3.3 - its output is low. After the subscribers are connected, the "no answer" signal disappears, the DD4.3 trigger switches and the low level allows the passage of count pulses through the DD3.3 element to the X3.1 connector and then to the input of the time counter of "own" conversations. At the same time, these pulses are fed to the input C of the DD1.2 trigger and the input of the account indication pulse shaper (element DD4.2). The trigger DD1.2 switches and opens the transistor VT2, which shunts the input of the signal analysis device. This is necessary to prevent the charging of the capacitor C5 by speech signals and noise signals in the line. Resetting triggers DD1.1 and DD1.2 occurs by applying a pulse generated by the differentiating circuit C4R13, which occurs when the handset is lowered onto the device and, as a result, the trigger DD4.1 switches to a single state. In the event of a call signal in the absence of the subscriber (the handset will not be picked up), triggers DD1.1, DD1.2 are switched in the same way as in the operating mode described above, and they are reset to zero by an impulse from the differentiating circuit C9R13 that occurs at the time of switching the Schmitt trigger DD4.3 to the zero state after the end of the ringing signals. Element DD4.4 prevents resetting trigger DD1.1 in the event of a call and picking up the handset, since in this case the resetting pulse from the output of DD4.4, which occurs when the handset is picked up, is divided in half on capacitors C4, C9 and its amplitude becomes insufficient for switching trigger DD1.1. A low level at the input D of the trigger DD1.1 (when lifting the handset) allows the trigger to remain in the zero state when the dialing pulses and noise pulses in the line act on its input C through the C3R12 circuit. The time counter of "their" conversations is assembled on DD5-DD10 microcircuits. The counting pulse frequency is 1 Hz. Counters DD5, DD6 count units and tens of seconds, DD7, DD8 - units and tens of minutes, and DD9, DD10 - units and tens of hours. The total time that the counter can count before overflowing is 99 hours 59 minutes. Minutes and hours are displayed on the HG1 liquid crystal display. The operation of the device in the counting mode is determined by the mid-decimal point flashing at a frequency of 1 Hz (pin 9 HG1). Resetting the counter and indicator readings is carried out by applying a pulse to the inputs R, generated by the differentiating circuit C10R17 when the SB1 button is pressed. The device is powered from a telephone line using a micropower zener diode VD6, the operating current of which is set by the current stabilizer VT1. Diode bridge VD1-VD4 provides independence from polarity when connecting the device to the line. The current consumed by the device in standby mode does not exceed 100 μA. Setting up the device is easy and comes down to adjusting three trimmers. The trimmer resistor R3 sets the voltage across the capacitor C2 (with the tube lowered), equal to the supply voltage (3 V). The purpose of the resistor R9 was mentioned above. Resistor R14 sets the counting pulse frequency, which should be equal to 1 Hz (you can control the signal with a frequency of 512 Hz at the output K of the DD2 microcircuit). Connection of non-certified devices to public telephone networks is prohibited, therefore the described device can only be connected to PBXs and similar networks. Author: I.Zabelin, Moscow
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