Optical Interface Circuit for Lap Counter/Timers
Jan. 20, 2005

This page displays a circuit board used to interface optical sensors (phototransistors)
to a lap counter which was not designed for use with them. The circuit is described in
detail in an accompanying PDF file(*). The novel circuit conditions the signals such that
even older lap counters can be made to work (extremely) reliably when used any type
slot car from the slowest classic slot car to the very fastest modern slot car.

The board is designed to work with just about any kind of lap counter.
Counters that were specifically targeted were:

AFX LED counter the AFX digital LED counter/timer,

DS-300 counter the DS-300 digital LED counter/timer, and

Aurora electro-mechanical counter the Aurora electro-mechanical counter.

The board provides voltage regulation (can be optionally omitted), LED power (with a series
resistor), adaptive sensitivity (to adjust to ambient light) and pulse "stretching" such
that slow counters get a solid signal even from ultra-fast events (like magnet cars).

The output circuit is an open collector circuit which can be configured for negative
or positive polarity(*). This provides universal compatibility with lap counters (e.g. DS-300
requires positive polarity, AFX LED counter requires negative polarity).

(Click on any image to see in full size)
Lap counter6

PC Boards
The first printed circuit boards arrived today. They were made by ExpressPCB.
They give you free software to lay out the PC board. You send the file
to them over the Internet and, in a few days, you have your boards.
Three prototype boards were $51. That's an incredible price for such a
small lot with a one day turnaround. However, higher quantities would allow
for a more inexpensive board and a silkscreen overlay.
By the way, each board accommodates two lanes.

Lap counter3

Board going together
I'm starting to populate the board here. I had to cut it down as ExpressPCB only
makes prototype boards in one size: 2.5" x 3.8". This keeps down the cost. I needed the
board to be smaller to fit in the base of an AFX LED lap counter/timer.

Lap counter5

Replacing original circuit
I had built the original circuit on a Radio Shack perf board. It worked but it was challenging
to assemble and debug. The new PC board made assembly a breeze. Just drop in the components
and solder it.

Lap counter4

Backside of boards
You can see the difference between a printed circuit board and the hand-made prototype.

Lap counter2

Board going into counter
The board fits into the base of the AFX LED counter/timer.

Lap counter

Another view of the base
Again, you can see how the board resides within the counter.

Computer screen

Button it up
The board is in. We can now close (what used to be) the battery cover.

Computer screen

Test it out
I hooked up the power and, so far, so good.

Computer screen

Test Drive
I put the AFX lap counter into my layout to give it a try. It worked the very first time.
No debugging was necessary. The counters shown have LEDs mounted on the bridge railings.
Six inch straights underneath contain a phototransistor for each lane.

Build your own interface board!
I realize that not everyone can build these interface boards. But, for those of you
interested in doing it or getting ideas for a similar project, I've provided the
circuit board information here.


Schematic for Optical Sensor Interface
This is the circuit I used to interface optical sensors to the AFX Digital Lap Counter.
You could get away with less but this design is rock solid.
(Click on image for larger view.)

Circuit Board Layout

Layout for Optical Sensor Interface Board
The circuit board design was created with ExpressPCB. Here is the layout file: InterfaceBoardRev1.3a.pcb

The software to view/modify the board can be downloaded for free from: expresspcb
They will build three of the boards for you for $51.

More information on the circuit can be found in the PDF file here:
Optical Interface for AFX lap counter/timer

gtw 01/01/2007