Collin Schulz of the Twin Cities wrote in with a fun project he’s working on: a DTMF shield.
This one is for the phreakers of the past. A simple Arduino shield to decode DTMF tones using an audio input. It can easily be hooked up to a cell phone or radio. The shield uses an MT8870 to decode the audio and outputs BCD to a 74LS145 BCD to Decimal chip. It also uses a CD4543 BCD to 7 segment to display the number. The 74LS145 gives a logic low on one of the ten output pins depending on what number is decoded. The board also gives output pins for BCD that can be wired direct to the arduino.
Download Collin’s Eagle files and data sheets from his website.