| 摘要 |
<p>The push-contact keyboard allows the partial short-circuiting of the telephone earpiece transformer primary before the switching of the multi-frequency tone in order to attenuate audible tones in the earpiece. The switch pad consists of a stack of flexible elastic insulating and conducting surfaces. When the key (21) is depressed, two elastic conducting surfaces (9,12) separated by an insulating layer (13) with circular recesses are pressed into contact then two lower conducting pads (3,6). The upper conducting surface pads may be of rectangular tab form or have a spiral grooves cut in them. The configuration of contacting layers is based on a rectangular printed circuit array such that each button selects a unique row-column crosspoint. The correct signal generator connections are automatically made a short interval of time after the first contacts short the transformer winding.</p> |
