Hitchhiking for a cassette recorder. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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Unlike reel-to-reel machines, where hitchhiking should respond to a break or end of a magnetic tape, in cassette recorders such a device should work to stop the receiving unit. This will prevent the tape from stuffing into the free space of the cassette and winding it around the main shaft of the tape recorder.
One of the options for hitchhiking is with a photoelectric sensor. The sensor consists of an LED V1 and a photodiode V2, between which the disk of the cassette of the receiving unit with three holes rotates. In the absence of lighting, the resistance of the photodiode V2 is high, so the transistor V3 is open, and the transistors V4, V5 are closed, and the voltage on the collector of the latter is close to the voltage of the power source. When passing through a section of the disk with a hole, the photodiode is illuminated by an LED, transistor V3 closes, and transistors V1, V5 open, so the voltage at the collector of transistor V5 decreases to a few fractions of a volt. Thus, during normal operation of the tape path, the input of the rectifier (V7, V8) receives a sequence of pulses, the amplitude of which is close to the supply voltage. The direct component of the rectified voltage (positive - with respect to the common wire - polarity) keeps transistor V6 closed, so relay K1 is in the released state, and the motor power circuit is closed.
When the receiving node stops, the capacitor C2 is recharged through the resistor R5, and the transistor V6 opens. As a result, relay K1 is activated and its contacts K1.1 breaks the power supply circuit of the engine.
The tape recorder, in which such a hitchhiking is built in, must be supplemented with the S1 button. It is pressed together with the Start key and held down for 2 ... 3 s (until the transistor V7 closes under the influence of the constant component of the pulse voltage rectified by diodes V8, V6).
The device can use any low-frequency germanium transistors from the MP39-MP42 series with a static current transfer coefficient h21e>40. Relay - type RES-5 (passport RS4.591.003). Establishment is reduced to the selection of the resistor R3 to obtain pulses on the collector of the transistor V5 with an amplitude close to the supply voltage.
In standby mode, autostop consumes a current of 25 mA (most of it falls on the LED), at the time of operation - about 40 mA.
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