💾 Archived View for gemini.spam.works › mirrors › textfiles › hamradio › fm-trans.txt captured on 2020-10-31 at 18:32:00.
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This is a simple FM wireless microphone. It will transmit about a mile.
___ ______________________________________________________________
\|/ ___ ) | | | | | |
| --- ) R ___ R R | -----
|_||__| C10 ) L1 4 --- C3 5 7 | ---
|| | ) | | | | ___ C6 ----- B1
C1 ------- | | | | --- --- 9VDC
|_____ | | C9 __R6__ ---- | |
| \ Q1 | | | | | | | |
| \|______|_______|__|(__| Q2 | R | | |
___ /| | | |( \ | C5 8 | | |
C2 --- / BF199 | | \|___|__|(___| _+_ | |
|____V R | /| |( | --- | o
| 2 ___ / BC183C | | C7 | \
| | C4 --- V | | | \ S1
R | | |_______ MIC / \ | | \
1 | | | | \_/ | | O
| | | R _+_ | | | |
| | | 3 --- C8 | | | |
| | | | | | | | |
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R1 = 100 ohms C1 = 1.5 pf NPO ONLY Q1 = BF199, ECG 229,
R2, R4 = 10K ohms C2 = 100 pF, NPO ONLY 2N5179 or eq.
R3 = 1K ohms C3, C4 = 330 pF NPO Q2 = BC183C, ECG123AP
R5, R7 = 47K ohms C5, C9 = .1 uF 2N4401 or eq.
R8 = 4.7K ohms C6 = .001 uF NPO ONLY L1 = approx. 1 uH
M1 = electret mike C7 = 22 uF electrolytic S1 = SPST switch
B1 = 9V battery C8 = 6.8 uF electrolytic
C10 = 10 - 40 pF trimmer capacitor.
The coil L1 is so small that in the PC board version of this, it's just a coil
etched on the board with traces. I think I breadboarded it with about four or
five turns of bell wire around a pencil and that made about the right value.
You tune the sucker with the adjustable cap C10. If I remember, the
transistors are garden variety ones, though I think Q1 was reasonably high
frequency. I was feeding in a signal other than an electret microphone.
This is neat to use with a cheapo FM walkman-type radio for cheap telemetry.
(We were actually going to build a device to transmit strain gauge signals
from someone's teeth over a whole day's time, and it was to be small enough
to be relatively unnoticed INSIDE THEIR MOUTH the whole time. Never did
get the O.K. to proceed with that one. Working for university research
gets pretty strange sometimes.)