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 Jim's Weird Wired World

Simple Remote Control Unit

 This is a simple wireless remote control unit, which was originallydesigned for a model hovercraft which I was developing with thehelp of my friend Uday Arya. The project has been discontinued fortechnical reasons, but this remote control unit survives. Themethod used seems to be quite popular among many remotecontrol units.

Overview

Block Diagram

  The controller consists of a Dual-tone multifrequency (DTMF)encoder based on the UM91214 Telephone dialler IC. This versatile

chip contains a keyboard matrix scanning section and a dialler. Thedialler can be used in both pulse dialling as well as tone diallingmodes. We are interested only in tone dialling, since we need theDTMF signals for control. The DTMF signals produced consist of twodistinct frequencies summed together. The pair of frequencies isunique for every key on the standard telephone keypad.

 The encoded signal is now within the audio band, and can hencebe sent to the modulator. The modulator is a commerciallyavailable FM microphone kit. Since the microphone kit expectsinput voltage to be in millivolts, the output of the dialler IC isattenuated. The FM transmitter is set to a frequency of about

100MHz. Operation of this section can be checked by using any FMreciever tuned to the same frequency as the FM transmitter.Pressing any of the buttons attached to the encoder should producea clear tone in the reciever. If the tone sounds noisy, try tuning thereciever carefully. In case the tone sounds harsh, the reciever maybe overmodulating, in which case the attenuation must beincreased. If it is too faint, then the attenuation must be reduced. It

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is better to start with a 10k or 47k pot in place of the resistivedivider and adjust this for best reception. Once completed, the potis disconnected and the resistance measured with a multimeter. The closest available fixed values are then used.

Note that even though provision for a standard telephone keypad

is provided, only keys corresponding to 1, 2, 4 and 8 are connected. This is to simplify the decoder circuitry. The dialler IC requires onlya 3V supply, hence a simple Zener diode regulator is used to dropthe supply voltage to the correct value. The 3.57MHz crystal iseasilly available, since it is used in TV colourburst oscillators.

 Transmitter

At the reciever side, a commercially available FM reciever kit isused. This provides a good quality audio output. A mono output issufficient, hence a stereo decoder need not be used. The decodedaudio is fed to a KT3170 DTMF decoder chip from Samsung. Similar(not pin-compatible) chips are available from other companies, suchas Mitel semiconductor's MT8870. The datasheets for these may beobtained from the respective company websites. The decoderproduces a four-bit binary encoded version of the input signal. Tosimplify the decoder circuitry (which often has to sit within a smallspace), the outputs are used directly, and no attempt is made todecode them. These outputs are TTL-compatible, and can be usedto drive relays, LEDs and other logic circuits.

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Reciever

  The CMOS CD4013 dual D-type latches are used in a togglefashion, such that their outputs change state whenever the inputstoggle. The latched outputs can then be used directly to drive alogic circuit, or through a transistor driver to switch on or off a smallload. A relay can be driven through a transistor driver to control anAC load, such as a lightbulb or a TV. Please take extreme care while

constructing circuits dealing with AC voltages. Make sure all high-voltage sections are well insulated. If you have no experience withAC circuits, please seek assistance during construction.

Output Drivers

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Construction

 This project was constructed mostly on Veroboard, except for theFM transmitter and reciever kits, which had their own custom PCBs.Here is a picture of the transmitter.

 Transmitter

From left to right, it's the battery case, parallel-port interface,main PCB (with the DTMF encoder) and FM Transmitter kit. Parallelport interface makes use of a CD4066 quad analog switch toreplace the four switches in the transmitter side. These are thencontrolled by D0-D3 on the parallel port. Hence, the remotelycontrolled device can now be under computer control. My testremote control device was this LEGO cart, which could moveforwards and backwards, as well as turn on and off it's headlights(Orange LEDs) and rooftop light (Red LED)

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"Buggy!"

Improvements

 This system can be extended up to 12 devices by using a 4-to-16decoder driven by the outputs of the DTMF decoder. The outputs of the decoder may then be used to drive a set of latches in themanner shown above. Of course, the transmitter must be modifiedto have the full telephone keypad.

Disclaimer

 These circuits are provided for your own study only. They all deal with electricity andelectronics, a sufficient knowledge of both is assumed. I will not take any

responsibility for any personal injury or damage to property arising from use or abuseof the information provided on these pages.

 This page ©2001 Jim George. Last updated on 08/07/02