DTMF Generator/Decoder

The tone generator (top) uses the 5589 chip and a DIP switch. You can actually hear the tones through the speaker. The bottom circuit uses the 8870 to decode a tone and display its associated number on the 7-segment LED.

Touch-tone is familiar to many (telephone), it is a mature technology, and readily available with off-the-shelf, single-chip, low-cost components. For these reasons DTMF is often used in remote control applications that typically use telephones (e.g. accessing your messages from an answering machine, retrieving your account balance info from your bank's database).

This tutorial will not discuss telephone interfacing. Rather it will give you a basic working foundation which you can build upon. The generator/decoder above are tethered together by a single wire. But you can expand upon this foundation for wireless remote control using a microphone. For longer distances maybe you can add a pair of walkie-talkies, generating audible tones into one, and decoding with the other.

Another possibility is to use infrared (IR). Since tones are just electrical pulses, you can replace the speaker with an IR emitter and add an IR detector to the decoder.

Yet another experiment is to interface either the generator or the emitter or both to a PC or embedded microprocessor (e.g. 8051, PIC or Stamp). In this scenario, the PC or a peripheral, through touch-tones, can respond and control.

If you are familiar with how telephones work, the basic circuit might also help you to build devices the respond to your call. For example, you can build upon the decoder and add relays to control household devices that respond when you call your home.

Well hopefully I got you motivated. The bottom line is that DTMF was designed for optimal performance with each tone being very distinct. This makes decoding the tone very easy even in surrouding noise. It is this performance that makes DTMF ideal for clear transmission and reception in remote control (wireless or through phone lines) applications.

• Parts List with Potential Vendor Source
• Theory of Operation
• Construction
• Final Words
• References

  Parts List and Potential Vendor Source

  Below are parts I used for constructing my DTMF generator/decoding pair. Additionally, I list the source from which I bought it from, along with the vendor part number and cost (in 1998). Note: I have no association with these vendors. I tend to buy parts from these US-based electronic mail-order companies. They usually have items in stock. When Jameco (reasonable prices) doesn't have a part, I then look at Digikey and JDR and Radio Shack. Additionally, I used a combination of soldering and wirewrapping on a prototyping board (Radio Shack). Part placement is not critical.

  TABLE 1: DTMF Generator

  PART DESCRIPTION	VENDOR PART	PRICE (1998)	QUANTITY
  TCM5589N TONE GENERATOR 16-PIN DIP	JAMECO #32803	3.95	1
  3.579545 MHZ CRYSTAL	JAMECO #14533	1.05	1
  8 OHM SPEAKER	JAMECO #88410	1.95	1
  16-PIN WIREWRAP SOCKET	JAMECO #37479	1.35	2
  8-POSITION DIP SWITCH	JAMECO #38842	0.79	3
  TIP31 NPN TRANSISTOR	JAMECO #33048	0.49	1

  TABLE 2: DTMF Decoder

  PART DESCRIPTION	VENDOR PART	PRICE (1998)	QUANTITY
  8870 DTMF DECODER 18-PIN DIP	see below		1
  3.579545 MHZ CRYSTAL	JAMECO #14533	1.05	1
  7-SEG DISPLAY COMMON CATHODE	JAMECO #24782	0.99	1
  74LS48 7-SEG DRIVER 16-PIN DIP	JAMECO #47811	0.89	1
  16-PIN WIREWRAP SOCKET	JAMECO #37479	1.35	1
  18-PIN WIREWRAP SOCKET	JAMECO #38148	1.35	1
  0.1 uF CAPACITOR	JAMECO #151116	0.15	2
  100 KOHM RESISTOR			2
  300 KOHM RESISTOR			1

  Teltone offers the 8870 chip. This part can be ordered from Component Distributors, Inc. They sell Teltone's M-8870-01 (18-pin DIP package) unit for about $1.50 in single quantities and can ship internationally. They have local offices around the US. Check out their webpage for locations. (Added the following 08/13/99) One visitor alerted me to B.G. Micro. This mail-order house sells the 8870 for $2.25 and the 5089 for $1.25. They are based in Dallas, Texas: Telephone: 1-800-276-2206.

  It has also come to my attention that there are similar DTMF decoding chips. Namely Harris Semiconductor's CD22202. JDR Microdevices offers this chip. It is not pin-compatiable with the 8870 but functions in the same way. The JDR part number is CD22202E and costs $3.99. Please note that I have not used the CD22202E. I imagine that if you wire it up taking into account that the pin-numbers differ with those of the 8870, your decoder should still work.

  Theory of Operation

  So what are these tones?

  In DTMF there are 16 distinct tones. Each tone is the sum of two frequencies: one from a low and one from a high frequency group. There are four different frequencies in each group.

  Your phone only uses 12 of the possible 16 tones. If you look at your phone, there are only 4 rows (R1, R2, R3 and R4) and 3 columns (C1, C2 and C3). The rows and columns select frequencies from the low and high frequency group respectively. The exact value of the frequencies are listed in Table 3 below:

  TABLE 3: DTMF Row/Column Frequencies

  LOW-FREQUENCIES
  ROW #	FREQUENCY (HZ)
  R1: ROW 0	697
  R2: ROW 1	770
  R3: ROW 2	852
  R4: ROW 3	941
  HIGH-FREQUENCIES
  COL #	FREQUENCY (HZ)
  C1: COL 0	1209
  C2: COL 1	1336
  C3: COL 2	1477
  C4: COL 3	1633
  C4 not used in phones

  Thus to decipher what tone frequency is associated with a particular key, look at your phone again. Each key is specified by its row and column locations. For example the "2" key is row 0 (R1) and column 1 (C2). Thus using the above table, "2" has a frequency of 770 + 1336 = 2106 Hz The "9" is row 2 (R3) and column 2 (C3) and has a frequency of 852 + 1477 = 2329 Hz.

  The following graph is a captured screen from an oscilloscope. It is a plot of the tone frequency for the "1" key:

  

  You can see that the DTMF generated signal is very distinct and clear. The horizontal axis is in samples. The frequency of the tone is about 1900 Hz - close to the 1906 Hz predicted by Table 3 (697+1209).

  Construction

  This section is organized as follows:
  • Schematics
  • Construction Hightlights

    Schematics

    The schematic in the figure below is relatively straightforward. I recommend that you use a combination of soldering and wirewrapping using sockets for all IC component placement.

    Schematic 1: Tone Generator

    

    You can click below to download the Adobe Acrobat format of the schematic. If you don't know what Acrobat is, visit Adobe to download a copy of its FREE PDF-file viewer.

    Download the Acrobat (PDF) file of the schematic. Recommended! 6399 Bytes

    Schematic 2: Tone Decoder

    The schematic for the DTMF decoder in the figure below. Again you can use a combination of wirewrapping and soldering. Part placement is not critical.

    

    Download the Acrobat (PDF) file of the schematic. Recommended! 7728 Bytes

    Construction Highlights

    Tone Generator

    The DTMF generator circuit is straight forward to construct. Only 3 of the 5089's 4 column pins (3,4,5) and all 4 row pins (11 to 14) were used. Thus it uses only 12 of the 16 touch tones (just like your phone). In this schematic you'll note the "/" in front of column and row pin labels (e.g. /C1). This means that these pins are active low. In other words, a pin is enabled when it is grounded. When the circuit is powered on, these pins normally high (+5V). C1-C3 and R1-R4 are wired to an 8-position DIP switch. In a single-package this DIP contains 8 single-pole-single-throw (SPST) switches. It is much cheaper to use than 8 real SPST switches. You slide a DIP position to open or close its switch. When closed that particular switch connects its associated column or row pin to ground and makes it active.

    You could use a 12-key keypad available from many surplus or electronics mail-order companies. But you must be aware of what you buy. Not all keypads can be used with the 5089. I think the proper keypad will have 9 pins: 8 (for 4 rows plus 4 columns) plus 1 for a common which you'd connect to ground. Often surplus keypads do not come with techsheets, and you will have to manually figure out which pin is associated with which row or column. I found to my surprise that my particular surplus 12-key keypad (from Electronic Goldmine) did not have this common pin and so I resorted to using a DIP.

    In this photo DIP positions 1 and 4 (C1 and R1 respectively) are in their ON positions. C1 and R1 is "1" on your phone's keypad. The speaker will emit the touch-tone associated with the "1" key (see Table 4)

    The speaker is driven through the TIP31 transistor. Note: the labels 1, 2 and 3 that refer to the base, collector and emitter pins respectively in the schematic are not standard. Be sure to check your spec sheet for your TIP31.

    TABLE 4: DIP SWITCH POSITIONS

    (1) DIP: 1+4	(2) DIP: 2+4	(3) DIP: 3+4
    (4) DIP: 1+5	(5) DIP: 2+5	(6) DIP: 3+5
    (7) DIP: 1+6	(8) DIP: 2+6	(9) DIP: 3+6
    (*) DIP: 1+7	(0) DIP: 2+7	(#) DIP: 3+7

    Table 4 shows the DIP positions that will activate the tone associated with the key. The numbers in bold and parenthesis are your desired key tone (like your phone). Thus if you wanted to dial a "0", you would slide only positions 2 and 7 on the DIP switch.

    Tone Decoder

    The decoder circuit is also easy to construct. You will have to physically wire (using alligator clips for instance) the TONE OUT pinout from the generator to the TONE IN pinout of the decoder.

    Once physically wired together, the 7-segment display will light up the number associated with the touch-tone you activate with the DIP switch.

    Note: A "0" tone lights up as "[" and not zero. This is because, "0" key's tone is actually a ten in binary (1010). Because the 7-segment displays only a single digit, ten is displayed as a "[". Similarly, the "*" (binary 1011) and "#" (binary 1100) light up as "]" and "U" respectively.

    Final Words

    Well hopefully you got your DTMF generator and emitter working in an afternoon or two. You might have some fun trying to put the tone generator's speaker next to the mouthpiece of your phone, and try dialing a phone number! This might actually work since the emitted tone is loud enough. I haven't tried it yet. Setting the DIP switches to dial a phone number will a bit of a pain though.

    There are some obvious "build-upons" you might wish to attempt, which I haven't tried. The first would be to replace the DIP switch with a 12-key keypad. I think Jameco has one (Jameco # 153488 $6.95 each) that might work. The second would be to add a microphone to the TONE IN line of the tone decoder. This would eliminate the need to physically wire the two circuits. You might have to put the speaker rather close to the microphone though.

    You could also make the two circuits portable by using batterys (say a 9V) and a +5V regulator (e.g. 7805). Perhaps you can also amplify the speaker. This would be a neat/cheap pocket dialer.

    Like I mentioned earlier, tones are just electrical pulses. Thus instead of driving a speaker, you can drive an infrared (IR) emitter. You would place a IR detector in the TONE IN line of the decoder. The tutorial on Long Range IR might help you with this endeavor.

    Interfacing a PC with the tone generator is relatively simple. Here, the DIP switch lines would be replaced by digital output lines from the computer. Decoding is also easy. You would just replace the 74LS48 and 7-segment display with the digital input lines of your computer. The parallel port or a dedicated PC interface card tutorials might help you with such an experiment.

    As for me, I hope to build a RF circuit and use my tone generator/decoder to remotely control devices. Wish me luck. I want to write up a tutorial on simple/cheap/long-range RF circuits. I am thinking of using Mitel's chips. If you have any leads, please email me.

    Let me know (email) if you succeed in any of these attempts! Happy building.

    References

    • DTMF IR Remote Control System Nuts & Volts Vol. 15, No. 6, June 1995 by John Iovine Note: I think his tone decoding schematic is wrong.

      Click here to go Paul's Main Page

      Click here to go to Columbia University's Robotics Lab

      Click here to email me