SCALECTRIX LAP TIMER
Circuit and PCB designed by Phil Townshend c. 2002

The function of this circuit is very simple. As a car passes or an LDR sensor, it starts a timer in 1/100ths of a second. The next time it passes the sensor, the display freezesdisplaying thelap time. the following lap the timer resets and the process is repeated so the timer times alternate laps.
In this model, there were two identical timers, one for each track.

There are 2 versions avaiable, a 3-digit and 4-digit version. Since the displays are not multiplexed and are on all the time, a considerable amount of current is drawn. If all displays were on, displaying 88.88seconds, the following current would be drawn:

4 displays x 7 segments @ 20mA per segment = 4 x 7 x 0.02 = 0.56Amps.

So with the 2 displays combined, this model draws a maximum current of over 1.1Amps!

PCB for the 3 digit display.

 CIRCUIT OPTIONS:
There are 2 circuit options avaiable, a 3 digit and a 4 digit version. Both have the 7 segment displays on a separate board.

The 3 digit version (shown left) counts from 0.00 to 9.99 and uses 3x 0.56" CC 7-segment displays.

The 4 digit version counts from 00.00 to 99.99 seconds and uses 2 double-digit CC 7-segment displays. The project shown right, is a dual version showing how the two 4-digit display boards were made as one and two identical boards used.
Simulation of Lap Timer circuit,
(CrocClips - 15K)
The case was made from 3mm acrylic, bent using a strip heater and wooden formers. Transparent red acreylic was used as a lens for the LED displays. The top was cut at an angle and sanded upside down on a flat surface ubtil smooth. The roof was made from black 3mm acrylic and the whole lot was mounted on a 6mm piece of MDF. One piece of track was also secured to the MDF base with the sensors mounted underneath. The LDR's were soldered onto strip board and bolted under the track.
3 DIGIT VERSION

Download circuit diagram, (PDF - 14K)

Downlaod PCB component and track layouts, (PDF - 71K)

 4 DIGIT VERSION

Download circuit diagram, (PDF - 16K)

Downlaod PCB component and track layouts, (PDF - 79K)
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