This doorbell uses quality components. It's not rubbish. The circuit is quite unbelievable. You can not obtain some of the components individually and the effectiveness is magic. We have so much to learn!
| Transistor Type: | Collector current: | Max frequency |
| 8050 NPN | 1.5A | 100MHz |
| 8550 PNP | 1.5A | 200MHz |
| 9014 NPN | 150mA | 150MHz |
| 9018 NPN | 50mA | 700MHz |
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."
Normally, this stray capacitance is a persons hand or body, touching or coming near the transmitting (output) stage and altering the frequency.
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 into the case.
When the project was tested inside the authors house, the range was increased to double.
When the transmitter was taken outside, the range was over 60 metres (200ft) and the full range could not be tested as the sound from the doorbell was too faint to be heard!
We can learn so much from a product that is already on the market.
Firstly, you know it works, it is reliable and you know what can be built for $10.00.
Secondly, you know what type of components can be purchased cheaply and what to expect from them.
In this case the transmitting transistor has the highest gain - so they have taken a special effort to get a good quality transistor.
Now, let's look at the transmitter circuit:
THE TRANSMITTER CIRCUIT
The transmitter circuit is made up of two building blocks - the 303MHz RF oscillator and the 32kHz crystal controlled oscillator.
The 303MHz oscillator consists of a self-oscillating circuit made up of the coil on the PC board and a 9p (9 puff) capacitor (actually 4p and 5p in parallel).
The circuit starts-up by the transistor producing noise. This rising-and-falling signal on the collector is passed to the parallel tuned circuit (the tank circuit) and the base sees a very smooth sinewave at a frequency of 303MHz.
This sinewave is then amplified by the transistor and this is how the 303MHz frequency is generated.
Now we come to the purpose of the 15microhenry choke on the tank circuit.
When the circuit oscillates, it takes a larger and small amount of current.
This current passes through the choke and the turns produce a back-emf or back voltage that fights against the flow (change) in current. The end effect is a voltage created at the point where the choke is connected to the track-work on the board. This effectively allows the track work to produce a waveform and since the frequency of this wave is very high, a percentage of the energy is radiated into the air as electromagnetic energy.
The choke allows the track-work to effectively rise and fall while providing a very low resistance path for the flow of current during certain parts of the cycle.
The second building block is the crystal oscillator.
This is made up of a two-stage DC coupled amplifier with feedback via the 2n2 and crystal.
If the crystal is removed, the oscillator is seen as producing very narrow spikes with a frequency determined by the 2n2, as shown in the diagram below: