This tutorial introduces you to DTMF technology and the DTMF decoder IC MT It also helps you to understand how to interface the IC to based micro . This board decodes DTMF signal either from an audio source or phone line to 4 bit binary TTL(5V) level output. It also indicates outputs with LED. Can use. Introduction. DTMF (Dual Tone Multi-Frequency) signaling is used for telecommunication signaling over telephone line in the voice frequency band between.

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The MT is an pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately.

Troubleshooting CM8870 DTMF Decoder Circuit

A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments. One can also test the IC on a simple breadboard.

For optimum working of telephone equipment, the DTMF receiver must be designed to recognise a valid tone pair greater than 40 ms in duration and to accept successive digit tone-pairs that are greater than 40 ms apart. However, for other applications like remote controls and radio communications, the tone duration may differ due to noise considerations.


Therefore, by adding an extra resistor and steering diode the tone duration can be set to different values. The circuit is configured in balanced-line mode.

To reject common-mode noise signals, a balanced differential amplifier input is used. The circuit also provides an excellent bridging interface across a properly terminated telephone line. Transient protection may be achieved by splitting the input resistors and inserting zener diodes ZD1 and ZD2 to achieve voltage clamping.

This allows decoser transient energy to be dissipated in the resistors and diodes, and limits the maximum voltage that may appear at the inputs. Whenever you press any key on your local telephone keypad, the delayed decode Std output of the IC goes high on receiving the tone-pair, causing LED5 connected to pin 15 of IC via resistor R15 to glow.

It will be high for a duration depending on the values of capacitor and resistors at pins decoderr and The optional circuit shown within dotted line is used for guard time adjustment. The tone-pair DTMF dual-tone multi-frequency generated by pressing the telephone button is converted into binary values internally in the IC. Dttmf, for every other number dialled on your telephone, the corresponding LEDs will glow. Thus, a non-defective IC should indicate proper binary values corresponding to the decimal number pressed on your telephone keypad.

Connect local telephone and the circuit in parallel to the same telephone line. Keys A, B, C, and D on the telephone keypad are used for special signalling and are not available on standard dtmc telephone keypads.


Pin 5 of the IC is pulled down to ground through resistor R8. Switch on auxiliary switch S2. Now the high logic at pin 5 enables the detection of tones representing characters A, B, C, and D.

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