SeeedStudio - DTMF(Dual Tone Multi Frequency) Shield - DIY Maker Open Source BOOOLE Operating Voltage: 5v, Current draw: 3mA. Input signal range: 27.5 to 869mVrms (-29 to 1dBm ). Trim pot for easy gain adjustment. 3.5mm stereo audio jack (TRS) and 0.1 standard interface headers. LED to indicate tone decoded. Dimension: 0.79 x 2.16, 20mm x 55mm.