Minty 2600 |
Overview
This is a "beginner" project to construct a simple 2600 Hz tone generator using the DTMF generator chip inside an old 2500-type telephone. Touch-Tone telephones back in the 1980s included a common 8-pin tone generator chip and a 3.58 MHz "colorburst" crystal to create the dual-frequency audio tones used in Touch-Tone dialing. While there were several manufactuers of these DTMF generator ICs, they all pretty much followed the same basic pinout. The 3.58 MHz crystal oscillator input is divided down to produced eight different ("high" and a "low") audio tones via the chip's digital-to-analog converter. These eight tones are what make up the "Touch-Tones" you hear when dialing a telephone. Since the DTMF generator IC has fixed internal clock dividers, the only way to change the chip's audio frequency output is to change the frequency of the crystal driving it. For example, the ROW4 key input is (usually) sent to a fixed "divide-by-3828" counter. If we change the 3.58 MHz crystal to a 10 MHz crystal, the "new" output audio tone will be 2612 Hz, instead of the usual 935 Hz tone. This new tone is within tolerance of the normal 2600 Hz signalling tone used in old telephone long distance signalling networks, or even for "over-the-air" signalling in some radio systems. Don't confuse this device with a "blue box." Those generate the additional multi-frequency tones which were used in analog telephone signalling systems. This device will only generate a single 2600 Hz, and is mostly for fun and experimentation.
Construction Notes & Pictures

Good to your local thrift store and buy up every telephone you can find. Also pick up some Altoids.

Keep taking apart the telephones until you find one with a 8-pin IC that looks something like in the above picture. This particular telephone set (ITT 2500-type) used a Texas Instruments TCM5094 Tone Encoder. This was a good find, as the datasheet is readily available on the Internet. You'll quickly discover that finding datasheets to old, obsolete chips can be nearly impossible. A useful feature of the TCM5094 is that it has a "Signal Tone Enable" pin for generating only single tone output frequencies. Since not all DTMF generator ICs will have this feature, we'll be using the "press two keys at once" method to generate a single output audio tone.

Here is an example of a "generic" DTMF generator IC. This is a HMC HM9187 Tone Dialer, and has the same basic pinout and internal clock dividers as the TCM5094.

Make the circuit up as shown. A LM386 can replace the NJM2113 with minor circuit revisions. You'll also need to track down a 10 MHz crystal and its associated loading capacitors. You'll also have to track down a low-profile 8 or 16 ohm speaker. The speaker from an old Motorola bag-style cellular phone will work perfectly.
A JRC NJM2113 (or Motorola MC34119) audio power amplifer was chosen for it ease of setting the amplifier gain, its low-noise, and the fact that a parallel capacitor across the gain-setting resistor can be used to filter the output tone of any digital stepping noise.

Build and test the circuit with an audio frequency counter or oscilloscope. The (speaker) output sine wave should have very little distortion. The audio frequency counter is reading "2.612 kHz."

Punch or drill out holes in the Altoids tin for the speaker and the switches. Step drill bits work perfectly for drilling the thin metal.

Mount the circuit board and speaker as shown above. Pieces of art foam secure the battery. Two little metal clips hold the speaker to the lid of the Altoids tin.

Completed view. It's a little ragged, but still a fun beginner project.
