This circuit can be interface to a computer, allowing caller to computer interaction. This project article focus on a simple DTMF (dual tone multi-frequency) decoder circuit. The frequency use in the dial tone system is of audible range suitable for transmission over the telephone cable. Pressing key '9' will generate the tone form by 852hz & 1477hz. For example, if the key '1' is being press on the phone, the tone you hear is actually consist of a 697hz & 1209hz sine signal. When a key is being pressed on the matrix keypad, it generate a unique tone consisting of two audible tone frequency. Basically on the caller side, it is a dial tone generator. It uses electronics and computer to assist in the phone line connection. As more and more people find phone technology a useful communication tools, line connection use human operator has become a tedious task.Īs technology matures, pulse/dial tone method was inverted for telephony communication. The caller will pick up the phone, giving instruction to the operator to connect their line to the destination over the other end of the telephone. In the early days, our phone system used to be operated by human operator in a telephone exchange room. Positive supply voltage, which is usually between 3 and 15 V depending on the variation. Open collector output which may discharge a capacitor between intervals. The timing (OUT high) interval ends when the voltage at THR is greater than that at CTRL (2/3 V CC if CTRL is open). Provides "control" access to the internal voltage divider (by default, 2/3 V CC). This output is driven to approximately 1.7 V below + V CC or GND.Ī timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts.
The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (hence TRIG is typically 1/3 V CC, CTRL being 2/3 V CC by default, if CTRL is left open). Ground reference voltage, low level (0 V) It is now made by many companies in the original bipolar and also in low-power CMOS types. Introduced in 1971 by American company Signetics, the 555 is still in widespread use due to its ease of use, low price, and stability.