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an old 27 Mhz transmitter - help!

Hi,

Back in the late 70's or so, my late father made a remote which, when the antenna was pulled out and button pressed, would switch on two mains lights in the house. The idea was to give it to mum, so when she came back in the night she could switch the lights on.

I am new to electronics and am learning more each day (did Physics but didnt pay as much attention as I should have).

Opening the remote, I see a 27.445 Mhz crystal - along with basically a heap of resistors, 4 transistors, an inductor and some ceramic caps. (powered from 9V battery).

My question is - what would the most-likely role be of the 4 transistors?? Would they amplify the signal that the crystal outputs??? Also, I've noticed that with at least two of the transistors, one collector connects to another on the second transistor. Would that be to amplify the current greatly?? (sort of like a darlington pair ?)??.

If more info is needed pleast let me know - would appreciate any help on the subject. Thanks! Andrew

Reply to
andrew_h
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Crystals don't output a signal, they filter a signal. That is, they are very narrow band pass filters. When you connect one in a feedback loop around enough gain, the loop has enough gain to sustain an echo (oscillation) only in the narrow band of frequency that passes through the crystal. One or more transistors may provide the gain for this loop. The others may be additional power gain between the oscillator and the antenna.

The two connected collectors may be part of a push pull complementary output stage. Are you sure it is two collectors that are connected?

Reply to
John Popelish

I took Electronics way back in high school although I was interested in it ever since I read Heinlein's book, Have Space Suit Will Travel, way back in the 6th grade. :-)

Ron

Reply to
Ron Hubbard

Crystals do "output a signal". The crystal, in conjunction with an active component such as a transistor in this remote is used in an oscillator circuit to create the 27.445 MHz signal used to communicate with the receiver. Crystals in receivers are typically used to create a reference frequency which is used to down-convert the received signal for amplification by an IF (intermediate frequency) amplifier stage or stages.

Types of "crystals" known as resonators (typically ceramic devices) are used to filter signals.

Transistors in transmitters are used in oscillators to create transmitted frequencies, boost this level of the transmitted frequency, modulate the transmitted frequency, create the modulating signal and other tasks possibly needed in such a device.

Dorian

Reply to
Dorian McIntire

That is what I said in different words. A crystal, by itself, has no output, but in conjunction with a gain element and feedback, can become an oscillator, with the crystal being the frequency determining element (filter).

All crystals act as resonators, whether or not they are called resonators.

Sounds like you are agreeing with me. >

>
Reply to
John Popelish

The purpose of the transistors could be as follows: one as a 27 MHz crystal oscillator; two could be an audio frequency multivibrator; and the fourth would then be a modulator - switching the oscillator on and off at audio frequency. If there is only one inductor in the whole circuit, this configuration seems likely. Designs like this were published in 1970s hobby magazines.

Reply to
Andrew Holme

Hard to say what your father did. Back then we often took a CB unit and transmitted a tone to a PLL receiver to activate something. Sometimes to control lights and sometimes to control something that went BOOM. But I'll say no more, I don't know abot statue of limitations:-)

Reply to
Tom Biasi

I will take a picture of the circuit board of the remote and post it.

I appreciate people's help and time on this - if someone could even go through it and attempt to work out exactly how it worked, I would be greatly in debt!

For my own sake, I'd love to know exactly how it worked.

Reply to
andrew_h

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