Sounding keychain on one chip. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sounding keychain on one chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Calls and audio simulators

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This version of the "responding" key fob is the result of a creative processing of a similar design, published in the magazine "Radio" N1 / 1991. The key fob described earlier is good only in that. if it uses K564 series microcircuits. However, working with these microcircuits requires certain skills, and it is much more difficult to acquire them than other microcircuits of a similar CMOS series.

The new key fob is much simpler than the previous one, since it can use not two, but one microcircuit and, of course, almost without changing the dimensions of the device, choose it from the K176, K561 series. True, the key fob instead of an intermittent one gives a continuous signal, nevertheless, it copes with its "duties" quite well.

(click to enlarge)

The circuit diagram of the key fob consists of a single vibrator trigger (DD1.1, DD1.2), a sound generator (DD1.3, DD1.4), a transistor amplifier (VT1, VT2) and a sound signal receiver-emitter (BA1). The scheme works like this. In the "waiting" state, a low-level signal is present at pin 4 of the DD1.1 element, and a high level signal is present at pin 3 of the DD1.2 element. When an audio signal is received from the amplifier, the trigger switches. A high-level signal appears at pin 4 of the DD1.1 element, allowing the operation of the sound generator. At the same time, capacitor C7 is charged through resistor R2. At the end of time t - 1 / 2R7C2, the voltage at input 1 of the DD1.2 element drops to the trigger switching level, and the key fob goes silent.

Establishing the circuit is reduced to setting the acceptable sensitivity of the key fob. To do this, for the time of adjustment, instead of R4, a tuning resistor with a resistance of 500 k is connected. By reducing R4, they find such a critical value of its resistance at which the key fob sounds non-stop. After that, slightly increase R4. The closer R4 is to critical, the more sensitive the key fob. After tuning, the tuning resistor is replaced by a constant.
Resistors and capacitors of the circuit are selected for reasons of small size. Diode VD1 - with the lowest direct resistance.

Transistors VT1, VT2 - with the highest gain. The ZP-3 piezoceramic transducer can be replaced by ZP-1, but this will slightly increase the dimensions of the device and the current it consumes in the sound mode. As a power source, batteries from three miniature disk batteries or three batteries from a wrist watch can be used. The printed circuit board and the layout of the elements in the device may be different, depending on the dimensions and design of the case used for the key fob.

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