05-05-2011, 11:31 AM
Today, most telephone equipment usea DTMF receiver IC. One commonDTMF receiver IC is the MotorolaMT8870 that is widely used in electroniccommunications circuits. The MT8870 isan 18-pin IC. It is used in telephones anda variety of other applications. When aproper output is not obtained in projectsusing this IC, engineers or technicians needto test this IC separately. A quick testingof this IC could save a lot of time in researchlabs and manufacturing industriesof communication instruments. Here’s asmall and handy tester circuit for the DTMFIC. It can be assembled on a multipurposePCB with an 18-pin IC base. One can alsotest the IC on a simple breadboard.For optimum working of telephoneequipment, the DTMF receiver must bedesigned to recognise a valid tone pairgreater than 40 ms in duration and to acceptsuccessive digit tone-pairs that aregreater than 40 ms apart. However, forother applications like remote controls andradio communications, the tone durationmay differ due to noise considerations.Therefore, by adding an extra resistor andsteering diode the tone duration can beset to different values.The circuit is configured in balancedlinemode. To reject common-mode noisesignals, a balanced differential amplifierinput is used. The circuit also provides anexcellent bridging interface across a properlyterminated telephone line. Transientprotection may be achieved by splittingthe input resistors and inserting zener diodes(ZD1 and ZD2) to achieve voltageclamping. This allows the transient energyto be dissipated in the resistors and diodes,and limits the maximum voltage thatmay appear at the inputs.Whenever you press any key on yourlocal telephone keypad, the delayed steering(Std) output of the IC goes high onreceiving the tone-pair, causing LED5 (connectedto pin 15 of IC via resistor R15) toglow. It will be high for a duration dependingon the values of capacitor andresistors at pins 16 and 17.The optional circuit shown within dottedline is used for guard time adjustment.The LEDs connected via resistors R11to R14 at pins 11through 14, respectively,indicate theoutput of the IC.The tone-pair DTMF(dual-tone multi-frequency)generatedby pressing the telephonebutton isconverted into binaryvalues internallyin the IC. Thebinary values areindicated by glowingof LEDs at theoutput pins of theIC. LED1 representsthe lowest significantbit (LSB) andLED4 represents themost significant bit(MSB).
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http://electronicsforuefylinux/circuit/jun2003/ci-2-dtmf.pdf
