Description
AFX slot car sets are very enjoyable but you can increase the
fun with a lap counter. This circuit will count from 00 to 99, with independent
counters for each track.
The sensing device used is a Hall effect sensor (UGN3503;
available from Dick Smith Electronics). One of these sensors is glued under a
section of each track (printed side up); between the slot and one of the track
rails is the best spot. In this position, it will allow the ground effects
magnets on the cars to pass over them.
The sensor will provide a voltage of about 3V when a car passes
over it and about 2V without a magnetic field.
Circuit Diagram
Both counter circuits are identical, with dual op amp IC5
handling the signals from both sensors. IC5a and IC5b are wired as comparators,
with a 2.5V reference derived from zener diode ZD1 via the 10kΩ and 12kΩ resistors.
Each time the output of IC5a goes high it clocks IC1a, a 4518
BCD counter. NAND gates IC2a & IC2b provide a carry out to the other half of
IC1 for a 2-digit display. More counters may be cascaded this way to provide
extra digits. The BCD outputs of IC1 drive 7-segment decoders IC3 & IC4
which drive common cathode LED displays.
Pushbutton S1 resets the counters to 00 for both tracks for the
start of a new race.
Placid Talia,
Oakleigh, Vic.
Oakleigh, Vic.
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