Circuit Project: Wireless Doorbell

  
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It is a wireless doorbell with a cost of about $10. 00. So, we have to change our attitude to building projects and use items such as this to learn how things work and modify them to suit our own requirements. This doorbell uses quality components. It`s not rubbish. The circuit is quite unbelievable. You can not obtain some of the components individually
Circuit Project: Wireless Doorbell - schematic

and the effectiveness is magic. We have so much to learn! The first thing you will notice is the clever circuitry. Some of the design goes against everything we have learnt in electronics. That`s why we have to study other people`s designs and realise the more you know, the more you realise you don`t know.  The oscillator circuit is very interesting, but first we will look at the RF oscillator. The doorbell operates on the 303MHz band and the 30 metre range (100ft) is obtained without the use of an antenna! The circuit is actually radiating from the printed track of the tank circuit. The Tank Circuit is a single-turn coil and a small capacitor (5p & 4p in parallel). In this project we show how to add a small antenna to the circuit to get double the range plus two other improvements to increase the range. Some of the improvements will load the circuit and alter the frequency at which it operates. Others can be done without any effect on the circuit. Fortunately, the transmitting stage is what we call tight  and is not affected by surrounding stray capacitance.  The circuit has been kept near the power rails by the use of a choke in the positive rail. The positive rail is then reflected to the negative rail via the battery. This feature helps us when we want to add an antenna. A 7cm length of tinned copper wire is connected to the collector of the transistor and bent around the board so that everything can be put back...



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