The circuit presented, from 2006, serves to control loads at a distance, robots, etc. It can be easily adapted to become a shield for microcontrollers. The principle of operation of this device is the removal of the control signal from the parallel port, activating a DTMF tone generator IC (Dual-Tone Multiple-Frequency) and then applying the tone to a 1 W short wave transmitter, which guarantees a range of a few kilometers. Eventually, this integrated circuit can be found in phones that have the phone function, as well as the 3.58 MHz crystal. The receiver is made up of two parts: a commercial receiver with a short wave range and a DTMF tone recognizer. connected to a 4028 decoder in which the signal is removed at two outputs. The fact that DTMF tones are used in this application makes the circuit more reliable than equivalents that use a PLL like LM567. The signal for activation is obtained from the receiver's own headphone output. Programming knowledge is not necessary to operate the circuit from the computer, since there are programs ready for the command of the parallel port.
