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By GREG SWAIN
Here's an easy-to-build UHF transmitter that
you don't have to adjust for frequency. It uses
a SAW filter for high stability and is fully
compatible with the UHF Remote Switch
described in December 1989.
Although designed specifically
for the DSE Remote Control Switch
described last December, this UHF
transmitter can also be used with
previous remote switches described in SILICON CHIP (eg, March 1988
& August 1988). However, this new
transmitter has several improvements compared to earlier designs.
These improvements have been
16
SILICON CHIP
wrought mainly by scrapping the
LC tuned circuit used in previous
designs and replacing it with a Surface Acoustic. Wave (or SAW)
filter. A SAW filter offers several
advantages over an LC circuit:
(1). it locks the transmitter frequency to 304MHz (303.88MHz to be
precise);
(2). it eliminates the need for
transmitter alignment; and
(3). it means that there is very little
frequency drift due to hand capacitance or temperature variations.
Another advantage of this
transmitter is that it automatically
ceases transmission after about 10
seconds if the button is accidentally
(or purposely) held down. This can
easily occur if you put the transmitter in a pocket. As well as extending battery life, this feature is now
also a DOTC (Department of
Transport and Communications)
requirement.
As can be seen from the photos,
the circuit fits in a plastic keyring
style case and is powered by a 12V
lighter battery. A red LED flashes
when the button is pressed to indicate that the unit is operating.
In essence, the new transmitter
duplicates all the features of the
commercial transmitter supplied
with the Dick Smith Electronics
UHF Remote Control Switch
(SILICON CHIP, December 1989).
That design also used a SAW filter
but unfortunately is now becoming
difficult to obtain. The unit described here will be used as a replacement transmitter in future kits or
can be built simply to provide additional transmitters for the remote
switch.
A complete kit of parts for the
project will be available from Dick
Smith Electronics [see panel).
How it works
Fig.1 shows the circuit details of
the UHF Remote Transmitter. It
uses a digital encoder IC to key a
UHF oscillator operating at
304MHz [Ql) on and off.
ICl is a Motorola MC145026 9-bit
trinary encoder and is used to
generate a 9-bit code word. Trinary
code is similar to binary code except that there are three logic
states instead of two [ie, high, low
and open circuit). The 9-bit code
word is selected by the connections
to ICl's nine address pins, Al-A9.
In practice, this means that each
address pin can be tied to the
+ 12V rail, to ground or left open
circuit, thus giving 19,683 combinations. However, because of the particular decoder IC used in the
receiver [MC145028), A9 should only be tied high or low and this
reduces the number of coding options to 13,122.
In fact, in the DSE receiver
(SILICON CHIP, December 1989), A9
is tied to ground which means that
A9 in the transmitter must also be
tied to ground.
The 9-bit serial data is shifted by
an internal oscillator in ICl, the frequency of which is set to 770Hz by
the RC timing components on pins
11, 12 & 13. Pin 14 is used to control
,the transmission. When S1 is initially pressed, this pin is low and
the internal oscillator is enabled to
shift the data out.
The 33µF capacitor on pin 14 of
IC1 now charges via an internal
pullup resistor. After about 10
seconds, the voltage on pin 14
100(!
+
47k
16
-T
1
2
3
Al
14
+
33
16VW
A2
A3
47k
15
1k
!Cl
MC145026
A5
SAW
6
FILTE{1=--f
*
J
5.6pFl
.,.
A6
13
7
*
3.3pF
4 A4
5
.001!
-
* SEE TEXT
56k
A7
12
.01
g AB
c<at>e
A~K
11
120k
VIEWED FROM
BELOW
10 A9
.,.
HIGH ST ABILITY UHF REMOTE TRANSMITTER
Fig.1: the circuit uses an MC145026 trinary encoder IC to key UHF
oscillator Ql on and off. The SAW filter sets the output frequency to
303.8MHz while inputs Al-A9 are connected to give the address code.
reaches logic 1. When this happens, the internal oscillator stops
and data transmission ceases. The
47k0 resistor is there is discharge
the 33µF capacitor when S1 is
released, so that the circuit is ready
for the next transmission.
During transmission, the output
from pin 15 consists of a series of
pulses and these are used to
modulate the external oscillator
(Ql). The frequency of this
oscillator is set to 303.8MHz by the
SAW filter in Ql 's base circuit. The
3.3pF capacitor between collector
and emitter provides the necessary
positive feedback to ensure
oscillation.
Another capacitor (5.6pF) is used
to bypass the 4700 emitter
degeneration resistor to increase
the oscillator output. This output
appears at Ql 's collector and
drives a transmission line antenna
which actually forms part of the
PCB pattern.
The PCB fits neatly into a small keyring case and is powered by a 9V lighter
battery. The SAW filter locks the output frequency to 303.8MHz and
eliminates drift due to hand capacitance.
AUGUST 1990
17
PARTS LIST
auaa
A6A5A4A3A2A1
Fig.2: keep all component leads as
short as possible when assembling
the PCB. The SAW filter is oriented
so that the side closest to its four
pins is adjacent to the lkn resistor.
Inductor L1 is part of the PC
pattern.
Fig.3: to code the transmitter,
each A1-A9 input is connected
to the high rail, the low rail, or
left open circuit. If the unit is
for use with the DSE receiver,
then tie A9 low. Be sure to
match the receiver code.
1 transmitter case
1 PC board, code ZA-1432
(copyright Dick Smith
Electronics)
1 PC-mounting pushbutton
switch
1 SAW filter (303.8MHz)
1 12V lighter battery (GP23 or
equivalent)
Semiconductors
1 MC145026 trinary encoder
(IC1)
1 PN3563 NPN transistor (01)
1 red LED (LED 1 )
Capacitors
Power is derived directly from
the 12V battery and is applied to
the circuit via push button switch S 1
and LED 1. During transmission,
LED 1 flashes due to variations in
the current drawn by ICl as it outputs the serial data stream. At the
end of the transmission, LED 1 will
remain dimly lit due to the circuit's
quiescent current consumption.
Building it
All the parts for the UHF
transmitter are mounted on a small
PC board. This board is coded
ZA-1432 and fits into a plastic
keyring-style case. Fig.2 shows the
parts placement diagram.
You can install the parts in any
order but we suggest that you leave
the IC and the switch until last.
Keep all resistor and capacitor
leads as short as possible and use
your multimeter to check the
resistor values before installing
them on the board.
Note carefully the orientation of
the SAW filter - the edge closest
to the four pins goes towards the IC.
Transistor Ql is mounted with its
leads bent at right angles and with
the flat side of its plastic body sitting flush against the PCB.
The leads of the 33µ,F tantalum
capacitor are also bent at right
angles so that the device can lie flat
on the board. Install this capacitor
so that its positive ( +) lead is
closest to the IC. This done, install
the switch with the flat side of its
body towards the battery, then install the two PC-mounting battery
terminals.
The LED should be installed so
that its top sits about 13mm above
the board. That way, it will just pro-
1
1
1
1
1
33µ,F 16VW tantalum
.01 µ,F ceramic
.001 µ,F ceramic
5.6pF ceramic
3.3pF ceramic
Resistors (0 .25W, 5%)
1 120k0
1 56kQ
2 47k0
1 1 kO
1 4700
1 1000
trude through the hole in the lid
when the case is assembled. Check
that the LED has been correctly
oriented (the anode lead is the
longer of the two) before soldering
its leads.
Coding
Before testing, the A1-A9 address pins must be connected to
match the receiver code. This simply involves tying each pin high, low
TABLE 1: RESISTOR CODES
□
□
□
□
□
□
□
Value
120k0
56k0
47k0
1k0
470Q
1000
No
1
1
2
1
1
1
4-Band Code (5%)
brown red yellow gold
green blue orange gold
yellow violet orange gold
brown black red gold
yellow violet brown gold
brown black brown gold
TABLE 2: CAPACITOR CODES
□
□
□
□
□
18
Value
.01 µ,F
.001µ,F
5.6pF
3.3pF
SILICON CHIP
All Value
10nF
1nF
IEC Code
10n
1n
5p6
3p3
EIA Code
103
102
5.6
3.3
Before mounting Ql, bend its leads at right angles then
install it so that the flat of its body rests against the PCB.
The 33µF capacitor is mounted in similar fashion.
Why Kill
your
Computers
Memoryl
COMPUTER
PROTECTOR
The transmitter output drives inductor Ll which is a transmission line antenna
etched into the PCB pattern. This transmitter has been coded by tying A1-A4,
A6 & A9 low and by leaving A5, A7 & AB open circuit.
or open circuit. For example, you
could connect A1 to the high rail,
A2 to the low rail, leave A3 open
circuit, bridge A4 low and so on.
Note that if the transmitter is for
use with the DSE receiver, then A9
(pin 10) must be tied low (ie, connected to ground).
Fig.3 shows the A1-A9 address
pins on the copper side of the PC
board, together with the locations
of the high and low rails. Use solder
bridges or short lengths of tinned
copper wire to connect the selected
address pins to the high or low
rails.
Once coding has been completed,
the PCB and battery can be installed in the case and the unit tested
with its receiver. Note that it may
be necessary to retune the receiver
so that it matches the transmitter
frequency. The range achieved will
depend on whether or not an antenna is fitted to the receiver but
distances of up to 25 metres in open
space should be possible.
Note that the value of the feedback capacitor between collector
and emitter of Ql is critical. If the
circuit does not oscillate with
3.3pF, increase the value to 3.9pF.
How can you tell? Simple - the
LED will flash when the button is
pressed but the transmitter just
won't work.
Finally, you will have to slightly
modify the receiver circuits of
March & August 1988 for use with
this transmitter. In each case,
change the 39kQ resistor on pin 6 of
the MC145028 decoder IC to 56k0
and the 180k0 resistor on pin 1O to
390k0.
1§:;l
Where to buy the kit
This project was developed by Dick Smith Electronics and is available
from all DSE stores or by mail order from PO Box 321 , North Ryde,
NSW 2113. You can also order by phone on (02) 888 2105 or, from
outside the Sydney area, on (008) 22 6610.
The kit consists of all parts including the case and the battery. The price
is as follows :
UHF Remote Transmitter (Cat. K-3259) ... .... ... ....... ..... ..... ... $29.95
Postal orders should include another $3.00 for packaging & postage.
Please quote the catalog number when ordering .
Note: copyright of the PCB artwork associated with this project is retained by Dick Smith Electronics.
~~~
Or Risk the Loss of
Expensive Data?
Available in the following
Current Ratings:
l Amp
3 Amp
6 Amp
10 Amp
Install o
COMPUTER
PROTECTOR
A definite must for all
electronic office equipment
..-----■ cmi:mi ■
~
~Ls!,Lng~se
I
SYSTEMS
WESTINGHOUSE BRAKE & SIGNAL
COMPANY (AUSTRALIA) LIMITED
SIGNAL DIVISION
80-86 Douglas Pde. Williamstown. Victoria.
(P.O. Box 267, Williamstown. Victoria. 3016)
Phone (03) 3971033. Fax (03) 397 1861
Telex : 37 477 Wessys
Vic(03) 3971033. NSW (02)5451322, Old (07) 275 3188
SA: (08) 212 3161 . WA:(09) 446 8844
COM/2
AUGUST 1990
19
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