This circuit uses a pickup coil to take the signal from a telephone line without contacts. The circuit is from a 90s Popular Electronics.
Offset Adjustment on an Amplifier with the LM139 (CB14245E) In the figure we have the way of...
This circuit compares the voltages of the two inputs giving as output 0 or 1 according to the...
This circuit was suggested by Signetics in 1980s documentation. The circuit makes use of the...