Designed for a model hovercraft, 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 dialer. The dialer can be used in both pulse dialing as well as tone dialing modes. We are interested only in tone dialling, since we need the DTMF signals for control. The DTMF signals produced consist of two distinct frequencies summed together. The pair of frequencies is unique for every key on the standard telephone keypad
The encoded signal is now within the audio band, and can hence be sent to the modulator. The modulator is a commercially available FM microphone kit. Since the microphone kit expects input voltage to be in millivolts, the output of the dialer IC is attenuated. The FM transmitter is set to a frequency of about 100MHz. Operation of this section can be checked by using any FM receiver tuned to the same frequency as the FM transmitter. Pressing any of the buttons attached to the encoder should produce a clear tone in the receiver. If the tone sounds noisy, try tuning the reciever carefully. In case the tone sounds harsh, the receiver may be overmodulating, in which case the attenuation must be increased. If it is too faint, then the attenuation must be reduced. It is better to start with a 10k or 47k pot in place of the resistive divider and adjust this for best reception. Once completed, the pot is 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 dialer IC requires only a 3V supply, hence a simple Zener diode regulator is used to drop the supply voltage to the correct value. The 3.57MHz crystal is easily available, since it is used in TV colourburst oscillators.
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