Electronic bell switch with melodic chime. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Calls and audio simulators
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During the operation of the ZM-2U4 electric bell (manufacturer - LEZ, Lobnya, Moscow Region), its shortcomings were revealed associated with the mercury switch installed in it. Each time the call button is pressed, an arc discharge occurs between the contacts and mercury inside the switch bulb, as a result of which the contacts oxidize and burn. Initially, this leads to a decrease in the reliability of the call, and in the future - to its failure.
To restore the operation of the bell, I installed an automatic machine instead of a mercury switch, made according to the scheme shown in fig. 1.
On the elements DD1.1 and DD1.2, a pulse generator is assembled, the repetition rate of which is determined by the values of the capacitor C1 and resistor R2, and the duration is determined by the value of the resistor R1. The pulses are fed to the amplifying stage, made on a field-effect transistor VT1. It controls the bell electromagnet Y1, shunted to suppress voltage surges by the VD2, R3 chain.
The machine is powered by a rectifier (Fig. 2), made on a regular transformer T1 of the bell, a diode bridge VD1 and a smoothing capacitor C1. Voltage is supplied to the machine when the bell button SB1 is pressed. To power the microcircuit, a parametric stabilizer R4VD3 was used.
The field effect transistor can be replaced with a composite bipolar one, for example, the KT829 or KT972 series. The base of the transistor is turned on instead of the gate, the collector instead of the drain, the emitter instead of the source. A limiting resistor with a resistance of approximately 3,3 kOhm must be installed in the base circuit. Capacitor C1 - ceramic series KM-6 or metal film (K73-16, K73-17, etc.), C2 - K50-35. Diode bridge - any other series indicated on the diagram.
The details of the machine are mounted on a printed circuit board (Fig. 3) made of one-sided foil fiberglass.
Insulating tracks are obtained by cutting the foil with a sharp knife or a special cutter. The board and capacitor C2 are strengthened inside the bell housing, and the bridge is in the recess at the back.
Author: A. Slepko
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