Small Radio Transmitter [Updated 27 May 1996] ======================= 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 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 -------------- 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 ------------ 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.