The family of adapters, recently appeared a lot of new, specially designed for automatic recording of signals from the telephone line to the household VCR. Part of this apparatus includes, generally, a node of coordination of polarity and level of the input signal, and threshold-switching device.
My proposed versions of the adapter are different from the existing analogue circuitry simplicity, cheapness of manufacture and high reliability. A node of coordination of polarity and level of the input signal is made on the basis of the diode bridge and a resistive voltage divider. Achieved a real opportunity to connect the adapter to the telephone line without observance of polarity.
The threshold-switching device (PKU) does provide load management at a fairly wide variation of input level (line voltage). The load can be used in the motor of the tape recorder, relay, opto-couples the led, incandescent, etc. This device works in binary mode. So, if at the input a logical “1” (handset put, the voltage in the telephone line 60 In), the load remains de-energized. Once at the entrance of PKU appears a logical “0” (the hook, the line voltage about 12 V), the load is activated, and automatically record the information.
In the first circuit of PKU (Fig. a) uses a compound transistor VT1— VT2. At the level of logical “1” input voltage taken from a resistive divider R1, R2, through the diode VD5 is supplied to the power bus. The controlling transition of the transistor VT1 is set to the locking voltage of +0.7 V and the load is off. When the input schema is a logical “0”, the base of transistor VT1 is supplied gate trigger voltage load is switched. The load current is determined by the type of transistor VT2 (30-100 mA).
Schematics and corresponding printed circuit boards for options homemade adapter in the Assembly of the threshold-switching device based on the composite transistor (a), integrated circuit comparator (b) and CMOS switch (in)
Another variant of PKU (Fig. b) is made on the basis of the comparator (chip DA1 К554САЗ), the threshold of which is set by the resistor R4. The current load device can reach 150— 200 mA.
Another circuit of PKU (Fig. C) is assembled on the CMOS switch (chip DA1 К176КТ1). Unused keys switch connected in parallel involved. Load current the output transistor can be hundreds of milliamps.
The PCB for all three variants of the adapter is made of a 1.5 mm foil Micarta or textolite on any of the existing technologies with the wiring of circuit elements according to bring my topology. The signal “write” is connected to the input of the tape recorder, in which it is desirable to use audioapparaat with automatic adjustment of recording level and reliable tape drive mechanism, eliminating the jam of magnetic tape. Naturally, the tape recorder this must given to Mr. case, to work in the recording mode. To protect the input circuits of the adapters from a random impulse overload type lightning discharges at the input it is advisable to install arresters designed for a voltage of about 200 V.
M. SHUSTOV, Tomsk
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