Jagpreet Singh

What is DTMF? Dual-tone multi-frequency signaling   (DTMF) is used for   telecommunication signaling   over analog telephone lines in the voice-frequency band between telephone   handsets and other communications devices and the   switching center . The version of DTMF that is used in   push-button telephones   for tone dialing is known as   Touch-Tone KEYPAD The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as '1' ) will send a  sinusoidal  tone for each of the two frequencies (697 and 1209  hertz  (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multifrequency. These tones are then decoded by the switching center to determine which key was pressed. DTMF keypad frequencies (with sound clips) 1209 Hz 1336 Hz 1477 Hz 1633 Hz 697 Hz 1 2

7490 Pinout Let's look at the 7490 briefly to see how it works. Here is the pinout: The 7490 is a  decade counter , meaning it is able to count from 0 to 9 cyclically, and that is its natural mode. That is, QA, QB, QC and QD are 4 bits in a binary number, and these pins cycle through 0 to 9,  QD QC  QB QA 0      0     0     0 0      0     0     1 0      0     1     0 0      0     1     1 0      1     0     0 0      1     0     1 0      1     1     0 0      1     1     1 1      0     0     0 1      0     0     1 You can also set the chip up to count up to other maximum numbers and then return to zero. You "set it up" by changing the wiring of the R01, R02, R91 and R92 lines. If both R01  and  R02 are 1 (5 volts) and either R91  or  R92 are 0 (ground), then the chip will reset QA, QB, QC and QD to 0. If both R91 and R92 are 1 (5 volts), then the count on QA, QB, QC and QD goes to 1001 (5). So: To create a  divide