27 Mhz Projects
Early 80s, 27 Mhz band CB was quite popular. I built a number transmitters/receivers/walkie talkie projects as I gained interests on electronics when I was teenager. I don't remember why I choose 27 Mhz instead of other band like 80 m, perhaps because it doesn't require a lot of space for antenna. Now, after more than 30 years, I would like to 'preserve' some of my old 27 Mhz useful projects, with few component modifications such as I'm now using ceramic filter instead of using IF transformer 455 Khz in the receiver, ceramic filter is easier to get (and cheaper) rather vintage 455 Khz transformer these days !
27 Mhz Oscillator/RC Transmitter/Beacon
This circuit is just a basic HF Crystal oscillator circuit, combined with an astable multivibrator circuit which gives +/- 1 Khz tone modulation. Transistor type are not critical, I took what i have in my components box (BC547, BC109, 2N2222 will work well). I use this equipment a lot for receiver tuning, very convenient as we can listen audible tone on receiver's speaker. Other application, use your imagination : RC servo toys, CW transmitter. Believe me, you need to built one !
Note : I use 8 mm coil former with tunable ferite core, but there is no reason you can not use other coil former. As a guidance, built +/- 2 uH inductor which will resonate on 27 Mhz together with given C4 (22pF). Oscilator will be switched on-off-on-... by multivibrator circuit with freq +/=1.5 khz. Check the signal envelope on the oscilloscope screen. If you don't want to have this tone modulation, then discard the multivibrator circuit and connect R7 to GND directly.
