27MHz remote control

Posted on Dec 17, 2012

The transmitter is a very simple crystal oscillator. The heart of the circuit is the tuned circuit consisting of the primary of the transformer and a 10p capacitor. These two components oscillate when a voltage is applied to them. The frequency is adjusted by a ferrite slug in the centre of the coil until it is exactly the same as the crystal. The crystal will then maintain the frequency over a wide range of temperature and supply voltage fluctuations. The transistor is configured as a common emitter amplifier. It has a resistor on the emitter for biasing purposes but the 82p across the 390R effectively takes the emitter to the negative rail as far as the signal is concerned.

27MHz remote control
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27MHz remote control - image 1
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The crystal has a fundamental of about 9MHz and it will oscillate at this frequency unless assisted to oscillate at a higher frequency. This is done by the tuned circuit oscillating at 27MHz. Now we will look at the impedance-matching feature of the transformer. The impedance of the output of the transistor is about 1k to 5k and this means it is the impedance (resistance) "it works at." In other words, it is the characteristic impedance of the transistor in this type of stage. The impedance of a whip antenna is about 50 ohms and the transformer matches these two by having a TURNS RATIO. The primary has about 12 turns and the secondary about 3 turns. This provides part of the matching requirement. The `pi' network, made up of the 150p, 15 turn air-cored coil and 100p capacitor assists further in matching the output of the transformer to the antenna. When the power is applied, the transistor turns on fairly hard due to the 82p in the emitter being uncharged. This puts a pulse of energy through the 10p and as the transistor turns off slightly due to the 82p charging, the energy in the 10p capacitor is passed to the primary of the transformer to start the 27MHz cycle. The action of the emitter rising and falling during start-up, allows the base to rise and fall and this puts a pulse on the crystal to start it oscillating. The frequency of oscillation OF THE CIRCUIT is generated by the tuned circuit in the primary of...

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