RF004

                                          

Small Radio Transmitter
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Schematic design of Small Radio Transmitter
Part List
Capacitors	Resistors
C1      =      5 pF Ceramic C2      =      6 pF Ceramic C3      =    15 pF Ceramic C4      =  Trimmer Capacitor C5      =    15 pF Ceramic C6      =      1 nF Ceramic C7      =   100 μF / 25V Electrolytic  C8      =    4.7 μF / 25V Electrolytic  C9      =   100 pF Ceramic Semiconductors T1      = BF244A or BF245A FET T2      = 2N3819 FET T3      = BC307 / 8 / 9 or BC557 / 8 / 9	R1      =  100K Ω ¼W R2      =  220K Ω ¼W R3      =      22 Ω ¼W R4      =     1K Ω ¼W Trimmer R5      =     1K Ω ¼W R6      =   56K Ω ¼W R7      =    1M Ω ¼W  R8      =   1K2 Ω ¼W Diodes D1      = Varicap diode (eg. BB119) D2      = 1N4148
Component Layout :	PCB Layout :
This image is an accurate layout, scanned from the instruction sheet. I have used * to mark one corner for reference.	This image shows the track layout on the soldering side of the board. This is NOT a very accurate layout, because I didn't actually have a plan of the track layout. To get this image, I put a bit of OHP film onto the bottom of the PCB, and trace.
Notes
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 1M Ω. 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 "component layout image" and "PCB layout image" 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, "component layout image" gives the accurate positioning of the holes, whereas "PCB layout image" gives the positions only approximately. So use "component layout image" when drilling the holes in your PCB board. Then draw on the tracks, using "PCB layout image" 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. 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 "PCB layout image"). 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 Ω. You can make your own fancy antenna if you like, but I have found that a foot or so of wire is fine.

Source : Electronics Lab