Circuit Description

Fig.1 shows the block diagram of the caller ID unit. The power supply for the whole circuit is provided by the 7805 voltage regulator IC. Fig. 2 shows the schematic of the circuit.

DTMF Decoder

The calling party number, which is conveyed through the telephone lines are encoded DTMF, which stands for Dual Tone Multiple Frequency. We cannot directly use these signals in our circuit. So we use IC MT8870 chip to decode these DTMF signals. The MT8870 is a complete DTMF receiver integrating both the bandsplit filter and digital decoder functions. The filter section uses switched capacitor techniques for high and low group filters; the decoder uses digital counting techniques to detect and decode all 16 DTMF tone pairs into a 4-bit code. External component count is minimized by on chip provision of a differential input amplifier, clock oscillator and latched three-state bus interface. Table.1 shows the received digits and its equalent output from this chip. It requires an external crystal of frequency 3.579Mhz for its proper operation. The chip is configured in single ended mode with R1, R2, R3, R4, C1 and C2. The zener diodes were used to reduce the input voltage to the IC8870 to ~5V. Whenever the valid DTMF tone is detected, the equalent binary code is placed at the OP pins Q1 through Q4. at that instance, the STD pin goes

Table 1.1