Small Radio Transmitter  [Updated 27 May 1996]
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DESCRIPTION
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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 50 metres 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. 

This ZIP file contains the following:

- CIRCUIT.GIF is the circuit diagram.

- PCBPLAN.GIF shows the PCB layout from above (components shown). 
PCBPLAN.GIF is an accurate layout, scanned from the instruction sheet. I
have used * to mark one corner for reference. 

- TRACKS.GIF shows the track layout. You will need this to make the PCB. 
One corner is again marked * for reference. The white holes are obviously
the component holes. The orentation of the layout is as viewed from the
components side. Ie. it is not as viewed from the track side. If you print
this file out, it will be ready to be "ironed on" to your copper clad
board (see below). 

MAKING THE PCB
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Print out the TRACKS.GIF file onto OHP film, using a laser printer. The
size of the image should be correct when printed out - but to be sure,
check that the outermost black edges are of size 44mm by 71mm. If you
don't have a laser printer then print it out on paper, and photocopy it
onto OHP film. Place the OHP film onto the copper clad board, with the
printed side toward the copper. Anchor the OHP film to the baord using
some sellotape. Using an iron on high-heat setting ("cotton"), iron the
sheet for a few minutes, so that the toner is transferred to the copper. 

Then peel off the OHP film. "Mend" any tracks that have not transferred 
properly, using an etch-resist pen. Drill the holes. Fill in any gaps 
round the holes using the etch-resist pen. Then etch it.

This method is the "toner transfer method". I think that it is the 
easiest way for making PCBs, but you can do it whatever way you like!

CONSTRUCTION
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It is important that you use a PCB for this circuit. Do not attempt to
make the circuit on stripboard, as this will not then have a "printed
coil". It is important that the 'printed coil' has been transferred
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  22pF trimmer cap
C5  15pF ceramic
C6  1nF ceramic
C7  100uF electrolytic
C8  4.7uF electrolytic
C9  100pF ceramic
C10 1nF 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. 

Good luck with the transmitter. If you have any improvements to the
circuit, I would be glad to hear from you. Also if you have any probelms,
feel free to email me. 

--Dan Evans.
dan.evans@sjc.ox.ac.uk
http://users.ox.ac.uk/~sjoh0014

Original version: 5 April 1995. Updated: 27 May 1996.
