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an old 27 Mhz transmitter - help!

A

andrew_h

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
andrew_h said:
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

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?
 
R

Ron Hubbard

Jan 1, 1970
0
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
 
D

Dorian McIntire

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
Dorian said:
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.

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).
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.

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

Sounds like you are agreeing with me.
 
A

Andrew Holme

Jan 1, 1970
0
andrew_h said:
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

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.
 
T

Tom Biasi

Jan 1, 1970
0
andrew_h said:
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

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:)
 
A

andrew_h

Jan 1, 1970
0
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.
 
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