A License Plate Recognition and Speed Detection System - Index of

Fig. 35 Comparator Operation
The signal from the Schmitt trigger is fed into a NAND gate, which incorporates the
signal from the calibration circuit. The signal then goes to the enable pin of a dual Binary Coded
Decimal (BCD) decade counter. The counter drives two separate LED readouts, the tens place
and the ones place. When the ones value reaches 9, the counter rolls over and places a high value
on the enable pin of the second half of the counter. This half operates the tens place value on the
LED display. Combined, these two LEDs form the visual display of the vehicle’s speed. This
completes the brief description of the circuits in the radar gun.
The unit, according to Ramsey Electronic, is capable of measuring speeds up to and
above 99 mph. We did not test the accuracy of this claim. The User’s Manual suggests one way
of testing the radar gun at higher speeds is to point it at aircraft on landing approach. Since the
September 11 th 2001, attack on the World Trade Center and the tightened airport security that
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