| 摘要 |
A speed threshold value crossing detecting circuit which receives a pulse each time an object moves a unit distance has a time limit circuit activated by each pulse, a detector circuit determining whether the time limit of time limit circuit is over by the time of each successive pulse to detect whether the moving speed of the object has reached a preset speed, and a time limit control circuit for increasing the time limit of the time limit circuit when the detector circuit indicates that the preset speed has been reached. The hysteresis thus introduced in the time limit makes the detector circuit output turn uniformly on or off and suppresses the effect of small fluctuations in the pulse frequency not indicative of speed variations. Preferably the time limit of a retriggerable one-shot multivibrator is increased by selectively disconnecting an auxiliary resistor from the one-shot's RC time-setting network when the preset speed has been reached.
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