digital band-pass filter - for alarm extension

for alarm extension The "alarm extension" described in the September 1983 issue of Elektor, when used with a telephone cannot make a clear distinction between the call tone and the dialling tone. If this distinction is important, you may find the present filter circuit of interest. The circuit consists basically of ac- tive filter stages A2 . . A4, trigger stages A5 . . . A7, and digital filter stages MMV1 and MMV2. The level at the output of the circuit is CMOS compatible and is logic 1 when a call signal has been detected by L4. The output signal may be connected direct to the transmitter (pin 4 of IC1 in figure 2, page 9-53, Elektor September 1983). The supply voltage, Ub, is halved by Al and the operating voltage for the circuit is then taken from across C12. The signal provided by pick-up coil L4 is rich in harmonics: after amplification in A2 it is applied to active low-pass filters A3 and A4 which remove virtually all harmonics. The cut-off frequency of A3 is 10 Hz and that...