The DTMF detector is integrated in software. All the logic requested to receive and decode DTMF Commands over the phone line is integrated in the 16F877 chip.
only few external components are used. As soon as possible I plan to switch to a 16F629-20 Pic and like that will be a Single Chip DTMF Remote Control. The debug possibilities of 16F877 made him a good choice in order to develop the application. Once the development is finished the downsize to a smaller Pic will reduce the cost and integrate the Comparator to.The Line interface is designed to share the Phone Line with an Answering Machine; no supplementary Phone line is requested.
The Remote control has 4 Outputs capable to drive 10 Amps at 120VAC and accepts three simple commands – Set/Reset Output, Check Output Status and Change Password via DTMF digits.
The hardware consists in the phone line interface and the relay drivers.
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All the logical functions are integrated in the 16F877 micro controller.
The line interface was built as simple as possible. The purpose of this interface is to permanently monitor the Phone Line for incoming DTMF signals, output confirmation Beep’s to the line and keep the phone loop closed while the DTMF Remote is in use.
The phone line is connected at the JP1 jumper. R3, C1, line trafo and D2 are protecting the rest of the DTMF detector from the ringing signal and other power spikes coming from the phone line side. C1 should be a good quality capacitor designed for 250VAC at least. D2 should be a 1W – 5.1V zener diode in order to absorb all the power spikes.
The ground of the DTMF Remote must be floating in order to work as expected and cannot be connected to earth ground.
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