AJP's Circuits

27MHz Radio Controlled Transmitter
and Receiver
The range of this transmitter using 50cm aerials is about 40mtrs.
If you extend the aerials to 1.25m long you'll acheive over 100mtrs.
This Radio Controlled circuit uses standard available RC Crystals. 
26.995, 26.540 - Brown    27.045, 26.590 - Red
   27.095, 26.640 - Orange   27.145, 26.690 - Yellow
 27.195, 26.740 - Green    27.245, 27.790 - Blue
The transmitter is in three sections, the modulator, the oscillator and the output buffer amplifier. The oscillator is of colpits style which sends its' signal into the O/P amplifier in turn to the 50cm wire aerial. The transmitter's modulation is generated by a 555 timer IC producing an audio tone at roughly 580Hz and is the pulsed DC sourse of voltage for the oscillator and O/P stages. It is Ideal to tune up this circuit using a wave detector meter placed a few inches away from the transmitter. I have a circuit on this site. The formulae being Frequency in Hz =1/((R1+(2xR2))xCx0.7 R1=47K, R2=100K, C=10nF.

The Receiver is made up of three sections. An FET mixer oscillator stage which combines the incoming signal from the aerial with a local oscillator running 455KHz lower than that of the signal being received. The IF is filtered of using a 470KHz IFT (This is actually tunable from 450 to 480KHz) then amplified via a A.M. detector IC ZN414 (Now known as MK484) and the audio O/P fed through a high gain AF amplifier consisting of a BC108 transistor. The audio signal is then fed into a NE567 tone decoder where the Transmitter's 580Hz tone is detected and in turn energizes the relay via NPN transistor BC307.
The formulae being Frequency in Hz =1/1.1xC1xR1. Components R1=16K, C1=0.1uF, C2=1uF.
I have made R1 0 to 22K variable to account for component tollerance.

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This page was updated on 22nd February 2001.