1 DTMF Remote control – A software DTMF decoder

This project is a simple DTMF Remote control. 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 microcontroller chip, only few external components are used. and like that will be a Single Chip DTMF Remote Control.

The Remote control has 4 Outputs capable to drive 10 Amps at 220VAC and accepts three simple commands – Set/Reset Output, Check Output Status and Change Password via DTMF digits. The commands are:

#Pass#1-4#0-1## - Ser output 1-4 to 0-ON, 1-OFF. The result is Beep on success.

#Pass#1-4## - Verify output 1-4, the result is Beep for Output on and Buzz for output off.

#Pass#NewPass#NewPass## - Change Password, where password is a 10 DIGIT string. The result is Beep on success. The default password is 1234567890.

The Output Values and the Password are backed up in EEPROM.

Compactiable with all gsm modem and phones .

Hardware:

The hardware consists in the phone line interface and the relay drivers.

All the logical functions are integrated in the 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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An external comparator is used in order to detect the passes through zero of the audio input. The square signal obtained from the comparator is applied to the PORTC0 Trigger Schmidt input of the PIC micro controller.

The audio out pin is PORTC1. This pin stays in high impedance mode (input) when no sound is transmitted. When a beep sound is transmitted the pin is programmed like output. A Piezo Buzzer is connected to the Audio Out pin to generate a local Beep sound.

The Hook On pin PORTC3 drives a Reed Relay. The contact of these Relay keep the Phone Loop closed while the DTMF Remote is active. The relay is optional, if the Answering Machine used with the DTMF Remote does not disconnect the phone line in a short time the relay and the resistor are not necessary.

The relay driver consists in 4 transistors and four relays. The relays are connected to 4 electrical plugs.

The power supply is a 12V wall wart adaptor that powers directly the relays and a 7805 linear regulator in order to obtain 5v for the PIC micro controller.

The schematic diagrams for the micro controller and the relay drivers are attached at the end of this document.

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2 Software DTMF Decoder with password

DTMF Decoder IC like CM8870 is easily available in market so I have used it ,but no

problem if you get any other like KT3170 only pin name will be different.  (Refer to

datasheet for standard circuit)

The decoder uses digital counting techniques to detect and decode all 16 DTMF tone pairs

into a 4-bit code.

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l l- d3

l DTMF l- d2

signal in -l Decoder l- d1 4-bit binary out

l chip l- d0

l l- strobe

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CIRCUIT DESCRIPTION

After detecting presence of a valid DTMF signal StD goes high for Short duration (Refer

Data sheet) and outputs Q1-Q4 gets latched according to received tone-pair  (Refer above

table)

ALGORITHM

Poll for Ring  Detection.

Activate of relay to place 220 Ohm (1/2 watt.) resistor in line so as to lift phone .

(The 220-ohm loop on telephone line disconnects the  ringer from the telephone

line in the exchange.)

 Poll for StD and when it goes high Read port 0  to find out received tone-pair from

Q1-Q4.

Take control action according to received tone & Poll for the strobe again if you

wish and  deactivate relay when you finish.

More relays can be connected

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