DCI1 5 CIRCUIT DESCRIPTION Take a look at the DCI1’s Schematic Diagram as we walk through the circuit description. We will start with the DTMF audio input and work our way through to the power supply section. The DTMF audio input uses a standard style RJ-11 modular phone jack (J6 - Phone Line Monitor In) for easy interfacing with your phone line or other audio source (a direct audio pinout for J6 is shown in the wiring section). The input circuitry near the phone jack allows you to continually monitor the audio signal and pull off any DTMF data that comes in so it can be decoded.
The audio sniffer formed by C14, C15, R8, R9, and R10 will not capture the line so it does not interfere with the normal use of your phones. All it does is quietly feed any audio tones present to the decoder IC for processing. Protection diodes D2 and D3 limit the incoming signal to +/- 0.7 V so that potentially damaging voltages like the 90 Vp-p ‘Ring’ signal do not blow up your decoder IC! The DTMF decoder IC (U3) has the task of monitoring all the input audio signals from the phone line (or other audio source) and pulling off any valid audio tones.
When a DTMF audio tone is DTMF 4-bit detected, U3 presents a 4 bit digital code (meaning 0000 Tone Code through 1111, each position is a bit) to the micro-controller 1 0001 (U1) that represents which one of the possible 16 different valid characters it is receiving. 2 0010 3 0011 Why a 4 bit code you may ask? Think back to your High School math class for a moment when you learned about 4 0100 different base numbering systems. Humans like to work in a base 10 system (decimal system, deci = 10) using the digits 5 0101 1 through 10 (or 0 through 9 to be more precise). This is 6 0110 probably due to the 10 fingers and 10 toes we all learned to count on as kids.
Computers on the other hand (pardon the 7 0111 pun) like to work with a base 2 numbering system (binary 8 1000 system, bi = 2) using digits 0 and 1; 0 for off / low and 1 for on / high. In order to represent the 16 different possible 9 1001 tones (base 10) you need to have 4 bits in a binary system 24 = 1111 (binary) = 16 (decimal). Wow… kind of got off on a 0 1010 tangent there! * 1011 Internally, U3 has a set of counters that latch the 4 outputs # 1100 depending on the detected tones. The counters are referenced to a television color-burst crystal operating at A 1101 3.579 MHz (X1) for rock solid measurements.
Notice how C1 C 1110 couples a bit of the X1 crystal frequency over to U1 (the micro-controller) to set its timing as well. C 1111 U1 is the pre-programmed 68HC908JK1 micro-controller. D 0000 DTMF Tone 4-bit Code 1 0001 2 0010 3 0011 4 0100 5 0101 6 0110 7 0111 8 1000 9 1001 0 1010 * 1011 # 1100 A 1101 C 1110 C 1111 D 0000