Telephone Line-Feed Detector Meets FCC Regulations.

The article reports on the U.S. Federal Communications Commission (FCC) regulations regarding telephone line in the on-hook state. According to FCC regulations, a telephone line in the on-hook state must be galvanically isolated from the local ground. The figure shows one way to detect the presence of the direct current line-feed in both the off- and on-hook states while meeting the FCC regulations. A transformer-transfer impedance change is used for this purpose. If there's no dc on the line between tip and ring, then the n-channel FET Q1 shorts transformer T1's secondary. This loads the primary so the ac at the primary, generated by U1A, is minimal. Also, Q2 does not conduct and the #DETECT line is high, at +5 V. If there is a dc voltage higher than about 10 V on the line, Q1 does not conduct, the transformer secondary is open, and there are a couple of volts of ac on the primary of the transformer. The frequency of the Wien-bridge tone generator formed by U1A is about 20 kHz. Therefore, the waveform at the output, pin 1, is nearly sinusoidal.