This circuit monitors an old-style analog telephone line. We found it in the September 1991 issue of Radio Electronics magazine.
This circuit uses a voltage comparator and can produce signals up to 20 MHz. The resistors R are...
It can be obtained from Mouser Electronics the new LM290xAQ operational amplifiers from Diodes...
This circuit was obtained from a Texas Instruments 1985 document. The circuit is powered by 1.5 V...