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1n1c
This episode of Kit Clinic looks at the UHF remote
switch as featured in our March 1988 issue. A lot
of readers have had trouble with this project
which will work only if you follow the instructions.
Dear SILICON CHIP,
I have built the UHF remote
switch to team with my car burglar
alarm. I urgently need to get it going
because my neighbours are starting
to gfve me a hard time. This is
understandable because I've been
fumbling for my car alarm switch at
6.00am in the morning and setting
off the alarm.
I've checked the unit out closely
and so has a technician mate of
mine and we can see nothing wrong
with it. Can you suggest where the
fault would lie?
• One of the big problems about a
circuit operating at 302MHz is that
components are critical. Not only
have you got to have the right components but you must mount them in
the right way. Component lead
lengths must be short.
For capacitors, transistors and
resistors and any other components
for that matter, lead lengths should
be no more than a few millimetres
long.
To demonstrate just how important it is to keep lead lengths short,
consider the inductance of a lead
only 10mm long and 0.5mm in
diameter. This is given by the
formula:
L = .0021(2.303 log 1041/d - l)µH
where 1 = length in cm
d = diameter in cm
For the above figures, this gives a
result of 7 nanohenries. That can
be very significant at 300MHz. It
represents an impedance of 13.20.
If you have a capacitor with
leads 10mm long, the inductance of
the leads can easily stop it functioning as a capacitor at 300MHz.
IT WASN'T HARD TO SEE why the reader's transmitter (at left) wouldn't
work. For starters, the .0022µF capacitor was much too large. But the real
p~ohl_em was the BFR9~A transistor which was not properly connected into
circmt. The photo at right shows how the transmitter should look.
78
SILICON CHIP
Resistors are just as badly affected. If they have leads 10mm
long their inductance and stray
capacitance effects can play havoc
with the circuit.
All that is really just a preamble,
setting the scene so to speak.
The reader's problems
When we had a look at this
reader's transmitter and receiver
units we had cause to wonder
whether he had read the article or
even looked at the accompanying
photos. The transmitter had the
most obvious problems. First, there
was this humongous .0022µF
capacitor which was much larger
than in our prototype unit.
The MC145026 integrated circuit
was installed in a socket. This, combined with the fact that the
capacitor was physically larger
than it should have been, prevented
the capacitor from laying over the
top of the IC, as it did in our prototype. This meant that the reader
had to resort to having quite long
leads on the .0022µF capacitor and
he had to sleeve the leads with cambric to stop them from shorting
together.
Well, IC sockets are more trouble
than they're worth in a lot of circuits. We seldom use them in our
projects because they are often
unreliable. True, they do allow you
to change ICs quickly but unless you
do something silly like reverse the
supply voltages, ICs are very
reliable.
In truth, neither the use of the
socket or the large .0022µF
cpacitor probably stopped the
transmitter from working. We mention them solely because they were
noticeable deviations from the
original circuit and, as such, we
would have expected a "technician" to fix them before giving up
on the circuit.
The real reason why the trans-
A MUCH BETTER JOB had been made of the receiver although 5mm clearance holes had not been drilled to take the
transistor bodies. It wouldn't work because lpF ceramic capacitors had been used where ever .0011,LF (1000pF) types
had been specified. In addition, there were problems with L2. Once these problems had been cleared, the receiver
worked as intended.
mitter did not work was that the
BFR91A transistor was not connected into circuit. Instead of being
mounted on the underside of the
board with its body sitting in a 5mm
hole in the board, it was mounted
on top of the board and with its collector and base leads soldered between the 2-6pF trimmer and 470pF
capacitor, respectively.
The emitter lead was soldered
through to the centre hole which
does not connect anywhere but was
meant to be drilled out to 5mm in
diameter.
We corrected these deviations
easily enough and after that the
transmitter worked as it should.
Another small point, which did
not affect the transmitter operation, was that the LED leads were
not long enough to allow the LED to
protrude from the case.
Receiver PCB
On the receiver side, the problems were not so obvious. In this
case, the two BFR91A transistors
were at least properly connected
into circuit although 5mm holes had
not been drilled in the board to take
the transistor bodies.
A more serious problem was that
12 was not correctly wound and the
coil had been glued to the coil
former. Some of this glue had
penetrated the threads of the slug
and so it was glued solid and
therefore not adjustable.
That was enough to stop the
receiver working properly but the
real killer was the use of lpF
capacitors where .OOlµF (l000pF)
capacitors were specified. The
three capacitors concerned are
associated with the receiver input
stage - there was no way it could
work.
And while we can well understand how easy is to be confused by
capacitor labelling, the capacitors
in question were simply labelled
"1 ". Being very tiny ceramics, they
could only be lpF and not .OOlµF.
Having fixed 12 and replaced the
lpF capacitors with .OOlµF units,
the transmitter and receiver worked as they were supposed to.
Lessons learnt
It is our opinion that projects involving RF circuitry can be tricky
for readers to build but if this project had been assembled correctly,
so that it was identical to our prototype as shown in our photos, it
would have worked. Note that no
components had failed or been
damaged.
Nor had the circuit failed to work
because some of the components
had much longer leads than they
should have had. We often see RF
projects that have failed to work
because of long component leads
but in this case the problems were
much more obvious. A careful
visual inspection revealed all the
faults.
For our part, we are conscious
that our assembly instructions need
to be quite specific but if our instructions are ignored there are
bound to be problems.
~
JANUARY
1989
79
