Figure 12 - The completed DTMF decoder accepts both a phone cord and an audio input
Figure 12 - The completed DTMF decoder accepts both a phone cord and an audio input

To make a versatile DTMF decoding unit that will display digits in real-time as they are captured, a circuit that includes both an RJ-11 female telephone jack and some kind of audio input jack as shown in Figure 12 will allow decoding from a variety of formats. For live telephone use, the RJ-11 cord is plugged into any wall jack and will show digits pressed from any phone in the house or from any remotely located phone on the current call. The 1/16 audio jack allows a small digital audio recorder or computer sound card to be plugged in for decoding of DTMF signals already captured and stored for analysis.

The 8870 DTMF decoder is so robust that I was able to feed the audio from my computer into my decoder unit and pull telephone numbers out of YouTube clips that contained the sound of telephone dialing! It was also possible to feed the audio from the headphone jack on an RF scanner into the device and see people dial telephone numbers on older cordless phones that did not include a scrambling system to make them secure. So, the next time you enter your password or banking information over a phone system, ask yourself if your line is secure from eavesdropping. You just never know who may be listening to your call. With a simple DTMF decoder, your passwords and credit card numbers are extremely easy to decode.
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