Simplicity. Just love simple schematic.

  3.1.18 Simplest RF transmitter circuit (27MHz, AM) for RC toy.

  This simple and reliable RF transmitter has two charms. In the first of all, all unnecessary parts was removed. Second - it continues to work, & does not require any tuning.

  It's enough to connect all the parts together and the transmitter will work. If you want to introduce it to a specific RC toy (27MHz, of course), then it is necessary to choose a resistor R3. It should be exactly the same as in the radio receiver, close to RX2 chip. Lazy way is to replace the R3 with a variable resistor (300k is good enough) and turn it until the toy starts to respond to the commands.

* Reducing the value of C2 will increase the output power, but in a most cases it is not necessary. But if it is necessary, an increase in the voltage on Q1 & Q2 to 4.5V (leaving 3V, for supply of U1) works more efficiently.

  The next photo shows footprint on the board or how little does it occupy space.

  I like RC toys, but this board (at the moment) is converted into a calibrated frequency source 75MHz. It's not hard to do, if you need it. After replacing XTAL (by 25MHz one) it is enough to change L3 to LC resonant circuit tuned to the third harmonic (25x3=75MHz). The antenna is removed and replaced by attenuator made of several resistors.

  Finally, this board returned to normal life and became a 27 MHz transmitter. Except that the encoder chip (TX2S) has been replaced with MSP430F2011. Mainly in order to compensate for the missing remote controls for toys. A simplified & working schematic diagram - below.


  The rest has (include software) already been described on this page, and there is no point in repeating. Except that you can, with the same success, transmit any telemetry data. And you can get signal at a distance of miles, given that at this time 27MHz range is practically not used, and  deadly quiet.

  *If you want to do something in Radio Control Radio Service (RCRS) frequency range, then you’d better get familiar with FCC rules ( from § 95.701 to § 95.787 )  Just in case :)

  It's all & simple like that.

*** For any inaccuracies to be corrected, my mail is lower.

Apr.14,2019  SK_Lab_Boston



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