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AMATEUR RADIO By GARRY CRATT, VK2YBX Build this simple DTMF for custom control circuits Dual Tone Multi-Frequency signalling, or DTMF as it is commonly known, is being adopted by amateurs to control a variety of station operations. You can build your own DTMF encoder using just one integrated circuit and a numeric keypad. Dual tone multi-frequency signalling has been with us since the development of the "Touch Tone" dialling system, introduced in the late 1940s by Bell Laboratories in the USA, to replace decadic dialling in telephone systems. Since that time amateurs have adopted the signalling system on an ever increasing basis to control a variety of station operations. While this system is used by many dedicated groups to control amateur repeater operations, remote antenna switching and beacon control, the DTMF system is now gaining momentum in the PSTN (Public Switched Telephone Network), as older decadic telephone ·e xchanges are being withdrawn from service and replaced with " tone" exchanges. Even the latest telephone answering machines use DTMF signalling for remote operation. As an aside, the chances are that your local telephone exchange has tone dialling available. If so, and you have a phone which is capable of tone dialling, then you can have this convenience. It's just a matter of phoning your local Telecom C1 7 i 47 ~16VW voo osc + + ..:c.. 4.5V ..:C.. IN R1 1M C2 33pfl IC1 5089 OUTPUT 8 CJ 33pfl osc OUT vss 6 COL COL 3 2 4 5 ROW ROW ROW ROW 4 3 2 1 11 12 13 14 -:- • 0 C4 10 + - 16 + Table 1 R2 1k ., # DTMF SENDER Fig.1: the circuit uses a standard TCM5089 DTMF encoder chip (IC1), together with a numeric keypad. When a key is pressed, IC1 synthesises and mixes two sinusoidal tones to produce a single dual tone output. 72 SILICON CHIP business office and asking for "tone dialling". They can change it over very quickly and the resulting dialling is much quicker. Decadic dial phones still work so you can have the best of both worlds. Anyway, back to the subject in hand. Many amateur transceivers available on the commercial market incorporate a DTMF "pad" on the microphone, allowing the transmission of these tones for control purposes. However, there are many older transceivers in use that do not have this facility. Would you like it for your transceiver? You can have it. This article describes the construction of a self contained DTMF generator which can be acoustically coupled to any microphone on an amateur transceiver or telephone handset. A standard 12-key DTMF encoder selects one tone from a high group of sinusoidal tones and another from a group of low sinusoidal tones and mixes the two Tone Frequency Row 1 Row 2 Row 3 Row 4 Column Column Column Column 697Hz 77GHz 852Hz 941Hz 1209Hz 1336Hz 1477Hz 1633Hz 1 2 3 4 PARTS LIST 1 TCM5089 DTMF encoder (IC1) 1 3.579545MHz crystal, Jaycar Cat. RQ-5272 1 plastic zippy box, 130 x 67 x 43mm, DSE Cat. H-2853 1 4 AA or AAA cell holder 1 numeric keypad, Altronics Cat. S-5380 or equivalent 1 miniature 80 loudspeaker Capacitors 1 4 7 µF 1 6VW electrolytic 1 1 0µF 16VW electrolytic 2 33pF ceramic Resistors (¼W, 5%) 1 1MO 1 1 kO We simply lashed the circuit up to prove the concept but it could just as easily be built onto a small piece of Veroboard and housed in a plastic zippy box. Fig.2: when a DTMF key is pressed, the selected row and column inputs are pulled low. together to produce a single dual tone output. Table 1 lists the standard tones for 12-key and 16-key DTMF keyboards. How it works Our circuit (Fig.1) uses a standard DTMF encoder chip from Texas Instruments, the TCM5089. This is a CMOS chip designed specifically to operate from a standard NTSC colour burst crystal (3.579545 MHz]. The crystal ensures good frequency stability and highly accurate tones. It also eliminates the need for any frequency adjustment. The TCM5089 synthesises the sine waves and their resulting harmonic distortion is less than - 30dB (3%). When generating a dual tone signal, the encoder generates one column tone and one row tone and adds them for its output. For example, if you press button 8, the two tones generated will be 852Hz (row 3) and 1336Hz (column 2). Table 2 shows the expected row and column frequencies when using a 3.579545MHz crystal, and the variations from the "standard" set of tones. For all practical purposes, these tones are sufficiently ac- 1 680 Parts availability: The TCM5089 is available from VSI Electronics (Aust) Pty Ltd. Phone (02) 439 4655. VSI has offices in every state. DTMF keypads are available from Altronics and Jaycar, as are the 3.579545MHz crystals. Readers may also be interested to know that Motorola have available a DTMF receiver/decoder, the MC145436, which can be used for DTMF control. Details from VSI. curate for our control applications. The tolerance of the crystal should be better than 0.02 % . Unlike the dynamic or scanned inputs commonly encountered in computers, the static row and column inputs of the TCM5089 ensure that no noise is generated. Fig.2 shows how a single key selects two tones. Several of the other inputs to the Table 2 Tone Frequency Deviation from Standard Row 1 Row 2 Row 3 Row 4 Column Column Column Column 701.3Hz 771.4Hz 857.2Hz 935.1 Hz 1215.9Hz 1331.?Hz 1471.9Hz 1645.0Hz +0.62% +0.18% +0.61 % -0.63% +0.57% -0.32% -0.35% +0.73% 1 2 3 4 KEYBOARD REAR VIEW 0 0 Fig.3: this diagram shows the pinouts for the Altronics Cat. S-5380 numeric keypad. SEPTEMBER1989 73 TCM5089 need explanation, as follows: • Single tone enable input: this inhibits the generation of single tones when taken low or left open. If this pin is held high, single tone operation is enabled. • Tone enable output: when held low, this pin causes the output of the encoder to be dis a bled. • Keyboard active output: this output provides for switching of an external receiver, transmitter or other functions. The output is low whenever one or more column inputs are active, and at a high impedance when all column inputs are inactive. The supply voltage may be anywhere between 3V and 10V. As the current drawn during signalling is around 2mA, the device lends itself to battery power and portable operation. Construction Our photo shows the circuit in an experimental lash-up but it would be easy to build it onto a THE VHF FM MONITOR RECEIVER described in the March 1989 issue is now available in kit form from Dick Smith Electronics. The kit is priced at $59.50, comes with full instructions and features a re-designed PCB for improved parts layout. It's available from your nearest Dick Smith Electronics store. piece of Veroboard or matrix board. This could then be housed in a zippy box or mounted inside existing amateur equipment. Our prototype used the standard 12-keypad layout which is most commonly available. Depending on IREECON '89 is The Convention ■ ■ where the present - the future - and beyond come together. IREECON is for EVERYONE within the profession of electronics. the chosen supply voltage, the on/off switch is optional. At 10V DC, the standby current from the unit is less than 200µA. But at 6V or under, the standby current is less than 10µA so no on/off switch is necessary. ~ fflELBOURNE SEPTEMBER 11-15, 1989 An electronics exhibition SHOWCASE not to be surpassed anywhere • • see the very latest equipment from electronic companies in the know, showing a VARIETY of technologies not available at one time under one roof at any other exhibition in Australia. You've seen the rest NOW see the BEST IAEEC®n • CONVERGENCE HIGHLIGHTED • IREECON '89 incorporates a WEALTH OF FIRSTS for both exhibition trade visitors and lecture program delegates. Those FIRSTS listed below -are just the beginning - just take into account the countless FIRSTS on the exhibition floor. A significant AWARD will be given for the best paper presented by a young engineer • • exhibitor product promotion presentations are opeq to everyone FREE of charge • • a technical lecture program which will open a window on the world of the 1990's and beyond . : • 74 • The Institution of Radio and Electronics Engineers Australia \ Commercial Unit 3, 2 New McLean St (PO Box 79) EdgeclitfNSW 2027 (02)327.4822 - Fax (02)327.6770 - Publx AA21822 (Quote User No. SYl35) I/ f • MONDAY, SEPTEMBER 11 SILICON CHIP FRIDAY, SEPTEMBER 15, 1989 •