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Making Small Radio Transmitter

Written by Paulo Martirez on Sunday, June 10, 2007

The small radio transmitter 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.

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.

Materials Needed:

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

Building Instructions:

1. Start off by scaling PCBPLAN (See PCBPLAN image below) and TRACKS (See Tracks image below) by the same amount so that they measure approximately the correct size (1.75" x 2.5") when printed out.

2. Then make your PCB. As mentioned earlier, PCBPLAN gives the accurate positioning of the holes, whereas TRACKS gives the positions only approximately. So use PCBPLAN when drilling the holes in your PCB board. Then draw on the tracks, using TRACKS 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.

3. 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).

4. 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.

5. Your transmitter is ready for testing.




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  1. 1 comments: Responses to “ Making Small Radio Transmitter ”

  2. By saranga on August 25, 2011 at 4:05 AM

    nice very nice

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