Mains Remote-Alerts

Posted on Feb 5, 2014

Pressing the pushbutton of the transmitter, a sound and/or light alert is activated in the receiver. The system uses no wiring or radio frequencies: the transmitted signal is conveyed into the mains supply line. It can be used at home, in any room from attic to cellar, simply plugging transmitter and receiver in the wall mains sockets. Transmission range can be very good,

Mains Remote-Alerts
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provided both units are connected to the mains supply within the control of the same light-meter. Q1 and Q2 are wired as a Darlington pair to obtain the highest possible output from a Hartley type oscillator running at about 135KHz frequency. The 220Vac mains is reduced to 30Vdc without the use of a transformer by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. The 135KHz sinewave generated by the transmitter is picked-up from mains wiring by C1 and selected by the tuned circuit L1-C3. Q1 greatly amplifies the incoming sinewave and converts it in a 12V-peak squarewave. D4 & D5 limit the input voltage at Q1 base to less than 1V-peak to avoid damaging of the transistor due to the high voltage transients frequently occurring on the mains line. D6 eliminates any negative component of the signal and Q2 drives the load. C7 is necessary to smooth the signal residues appearing across the load. The tuning of the coils at 135KHz frequency should be obtained with the ferrite core almost totally inserted in its slot, if 455KHz IF transformers are used for both L1s. Using IF transformers different from those specified, a change in both C3s value can be needed. The value of these capacitors may be varied from 1 to 3. 3nF but must be the same in transmitter and receiver. The load can be a beeper, a LED or both. Omitting the beeper and choosing the LED as the only load, its limiting resistor R7 should be...

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