DESCRIPTION
This ZIP file contains information about building a small radio transmitter,
which has a PCB 1.75" x 2.5" (45mm x 68 mm) and has a range of about 30 yards or
so. The documentation with the circuit says the freq range is 100-108 MHz, but I
have found it to be more like 85-100 MHz.
The circuit is (of course) only mono, and accepts an audio input from either
a microphone or other source. The input impedance is 1Mohm. The input
sensitivity is 5mV and the max input signal is 10mV. The transmitted signal can
be picked up on a FM radio. The circuit can be used for short-range
transmission, eg. for wireless microphones.
The actual circuit comes from a 'Kit', available from Veleman electronics
(USA distributor is Tapto Corp., PO Box 1339, CLAREMONT NH-03743-US. UK
distributor is High-Q Electronics, 382 Edgware Road, London, W2 1EB). The kit
number is K1771. It is a very good transmitter.
I bought the kit, and made the circuit, which worked very well. I wanted two
transmitters, so I made my own 'copy' PCB and built the circuit, and in fact my
home-made version seems to work better than the original!! So there is no need
to buy the kit really, as it is quite a simple circuit, and is the best
'home-made' transmitter I have seen.
CONSTRUCTION
Start off by scaling PCBPLAN.GIF and TRACKS.GIF by the same
amount so that they measure approximately the correct size (1.75" x 2.5") when
printed out.
Then make your PCB. As mentioned earlier, PCBPLAN.GIF gives the accurate
positioning of the holes, whereas TRACKS.GIF gives the positions only
approximately. So use PCBPLAN when drilling the holes in your PCB board. Then
draw on the tracks, using TRACKS.GIF as a guide. The important thing is to make
sure you draw the 'printed coil' correctly on the PCB - those lines are there
for a reason!
Then solder in all the parts. Here is the parts list:
D1 Varicap diode (eg. BB119)
D2 1N4148
R1 100K
R2 220K
R3 22R
R4 1K trimmer
R5 1K
R6 56K
R7 1M
R8 1K2
C1 5pF ceramic
C2 6pF ceramic
C3 15pF ceramic
C4 trimmer cap
C5 15pF ceramic
C6 1nF ceramic
C7 100uF electrolytic
C8 4.7uF electrolytic
C9 100pF ceramic
T1 BF244A or BF245A FET
T2 2N3819 FET
T3 BC307/8/9 or BC557/8/9 PNP
Bear in mind that in addition to the components, there is a jumper wire
which needs to be fitted (marked with a dashed line in PCBPLAN.GIF).
The power supply to use is 9-14 V DC, one of the little rectangular 9V
batteries is fine. Connect this to the + and - points on the PCB. The sound
input goes to the points marked "MIKE". The antenna should be connected to the
point marked "ANT". The emitter's output impedance is 50 ohms. You can make your
own fancy antenna if you like, but I have found that a foot or so of wire is
fine.
