In the operation of an electric bell ZM-U (the manufacturer - The Les, Lobnya, Moscow region) revealed its shortcomings associated with installed in it is a mercury switch. With each press of the call button between the contacts and the mercury within the bulb switch arc discharge occurs, in causing the contacts to oxidize and burn. Initially, this leads to a decrease secure the call, and further - to its failure.
To restore the call, I have installed instead of the mercury switch machine made according to the scheme shown in Fig. 1.
On the elements DD1.1 and DD1.2 assembled pulse generator, the frequency of which is determined by the values of capacitor C1 and resistor R2, and the length of the value of the resistor R1. The pulses arrive at the amplifying cascade, performed a field effect transistor VT1. He controls the electromagnet Y1 call, shunted to suppress emissions voltage VD2 chain, R3.
Powered machine from the rectifier (Fig. 2), performed on a regular transformer T1 call, the diode bridge VD1 and a smoothing capacitor C1. The voltage at the machine enters when you press zvonkova button SB1. To power the chip used parametric stabilizer R4VD3.
Field-effect transistor can be replaced composite bipolar, for example, series CT or CT. The base of transistor include instead of the shutter, the collector is the drain, the emitter is the source. In the circuit database, you need to install a current limiting resistor resistance of about 3.3 ohms. The capacitor C1 is ceramic series km-6 or metal film (K73-16, K73-17, etc.), C2 - C50-35. Diode bridge - any another indicated on the scheme of the series.
Parts of the machine are mounted on a printed circuit Board (Fig. 3) of one-sided foil fiberglass.
Insulating track is produced by cutting foil sharp knife or a special cutter. Charge and the capacitor C2 strengthen inside the case of the call, and the bridge is in a recess behind.
Author: A. Slepko