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Figure 2 - The CM8870 DTMF decoder datasheet showing a recommended circuit
Figure 2 - The CM8870 DTMF decoder datasheet showing a recommended circuit

The 8870 DTMF decoder sheet shown in Figure 2 includes an example circuit as well as a table showing how the binary output represents 1 of 16 possible DTMF tones. There are actually five output pins used on the 8870 decoder - four of them represent the binary data and a 5th pin toggles from low to high and then back to low every time a valid DTMF signal has been decoded. This allows your receiving circuit or device to know when a number has been repeated. Without this data receive pin, you would have no way to know if the same key has been pressed multiple times as the last data on the 4 bit output will simply remain the same. So every time a new DTMF tone has been decoded and sent to the 4 bit binary port, the data ready pin will toggle to high for a short time. On my 8870 version, the binary data output pins are marked Q1, Q2, Q3, Q4 and the data ready pin is marked as STD.

To get DTMF data into the 8870 decoder chip, pins 1, 2, 3, and 4 are set up as an analog input as shown in the datasheet example. The audio signal is processed by a robust signal processor inside the 8870 and any DTMF tones heard will be registered on the data output port. The input is very versatile and can be directly connected to the phone line or to any audio playback device such a computer sound card or digital recording unit. Basically, and device capable of sending an audio signal can be fed into the 8870 DTMF decoder, and as long as the signal quality is decent, the DTMF tones can be decoded. Cordless phones and analog cell phones that are not scrambled are particularly vulnerable as a cheap RF scanner connected to the DTMF decoder will show the listener every button pressed in real-time.



Figure 3 - Breadboarding the DTMF decoder circuit using the CM8870 chip
Figure 3 - Breadboarding the DTMF decoder circuit using the CM8870 chip

There are several ways you can build your DTMF decoder. You can feed a line from the telephone jack to the 8870 to decode tones pressed on any phone on the same line, or you can feed the output from some audio source such as an RF scanner, digital recorder or computer sound card directly into the DTMF decoder. You could build a DTMF decoder that uses both methods, including an RJ-11 telephone jack and an audio input jack. For testing purposes, it is much easier to feed the signal from your telephone system into the DTMF decoder so you can use the phone keypad to verify the operation of your circuit.

Using the 8870 DTMF decoder IC and a breadboard, build up the circuit as recommended in the data sheet, which will include a 3.579MHz crystal resonator and a few passive components. For the 300K resistor, a more common value of 270K or 220K will work just fine. The .1uF ceramic capacitor and 100K resistor in the input removes all DC offset, leaving only the AC audio signal for the DTMF detector. This setup allows a wide range of audio devices to be fed into the input without having to worry about impedance matching or the level of the output. You could even run an electret microphone into the input and still get a valid DTMF decode by simply placing the speaker from an audio recorder next to the microphone. The 8870 DTMF decoder really works well on a wide range of devices.

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