PT Version 11/23/2012

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PCB-Mounted Slotted Photosensor (as used for Finish-Line Sensing on Timestopper model TS200)


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IAC's PCB-Mounted Slotted Photosensor (as used on the patented Timestopper TS200 for sensing at the finish line of each car lane; see U.S. Patent No. 7,285,035) is comprised of a high-power infrared LED emitter and a high-sensitivity photodiode, each set opposite to one-another across an open 0.043 inch wide by 0.110" deep slot.   Included on the printed circuit board (PCB) is a 0.01uF decoupling capacitor to reduce switching noise.   The photodetector includes an amplifier, a Schmidt trigger circuit, and an output transistor.

The printed circuit board provides a 0.125" Dia hole for using a tie-wrap to clamp wiring to the PCB for strain relief.

Four 0.042" Dia holes are provided, two to each side of the coupling capacitor, for soldering your own #22 gauge (or smaller) wires to the PCB.   As viewed looking down on the component-side of the PCB oriented with the slotted photosensor device defining the top of the board, these four wire holes are arranged clockwise starting with the upper left as: a) circuitry ground, b) +5V Vcc input to the photologic circuitry, c) +5V to power the LED, and d) signal output from the PCB.   On Timestopper TS200 units, the color coded wiring to this sensor is a) green, b) red, c) black, and d) yellow (or white) respectively.

Installation drawings with the TS200 documentation show examples of how these slotted photo-sensors are used with IAC's Timestopper TS200 for finish-line sensing (see Figures 5 through 12).

The logic of the signal output is such that the signal will be a low voltage (less than 0.4 volts) when the slot is blocked.   The signal output will be that of Vcc when the slot is open, i.e. free of any obstructing object.

Do not use a supply voltage Vcc greater than 7 volts absolute maximum.

It is best to use separate wires for 1) a wire from your +5V voltage supply to the +5V input on the PCB to power the LED and 2) a wire to the +5V Vcc input on the PCB to power the photologic circuitry.   Having these as separate wires both prevents voltage drop along the wire to the LED, as caused by the roughly 10ma current drawn by the LED, from disturbing the Vcc voltage at the photologic circuitry and prevents switching signals from influencing the output of the LED.

Rise times resulting from a fast-enough switching event can be less than 100ns.

You should provide a 470 ohm series resistor to limit the current into the LED. And you should provide a 1.2k-ohm pull-up resistor between your Vcc supply and the signal received from the PCB.   An example hookup is shown in the figure below.

The following electronic schematic drawing shows where to connect a 4-wire cable to the sensor PCB from your electronics, where the outlines of the sensor PCB, the slotted photosensor device, and the user's electronics are not drawn to scale:

schematic of a sensor connected to user's electronics

The following drawing shows the actual placement of the four cable connections on the sensor PCB:

drawing of a sensor showing labled wiring holes for a 4-conductor cable

A picture of the PCB-mounted sensor as delivered is shown below with its mounting tube.   The tube has an outside-diameter that is 7/16", and an inside-diameter that is 1/4":

image of sensor on PCB along with a mounting tube

A picture of a 4-conductor telephone cable, as used to connect a sensor PCB to a Timestopper TS200 for sensing at the finish line by means of a collision block, is shown below spliced suitably for soldering to the cabling holes of the sensor PCB.   The cable should be spliced to expose 3/8" of each of the outer two conductors, and to expose 3/16" of each of the inner two conductors, with insulation on each conductor still in tact.   Of the 3/8" and 3/16" dimensions, approximately 1/8" should then be spliced to expose the bare copper wire of each conductor.   Lengthening each splice length to allow exposing more of the bare copper may make insertion and soldering into the PCB easier to accomplish, after which the extra length at the ends can be clipped away from the finished assembly:

image of a spliced cable

A picture of the same spliced cable alongside a PCB-mounted sensor:

image of a spliced cable alongside of a PCB-mounted sensor

A component-side picture of a PCB-mounted sensor connected (soldered) to a 4-conductor telephone cable with a tie-wrap securing the cable to the PCB via a hole provided for the tie-wrap:

component-side image of PCB-mounted sensor connected (soldered) to a 4-conductor telephone cable

A side-view picture of the above assembly:

side-view image of the above assembly

A picture of the above assembly is shown below, but with the PCB inserted within the mounting tube.   The tube length is cut with scissors to 5/8" such that both the slotted photosensor device and the tie-wrap lie outside the tube.   It is easier to place the tube over the PCB from the cable end (if done before installing the tie-wrap to secure the cable to the PCB) than it is to slip it on from the end of the PCB that has the slotted photosensor.   The position of the "nut" of the tie-wrap is important as in the position shown it can fit into the same 7/16" diameter mounting hole as the rest of the tubed assembly:

picture of the above assembly including its mounting within a 5/8

Additional pictures of the PCB-mounted sensor as used with a collision block at the finish line of a Pinewood Derby track lane are shown below.

This picture shows the sensor inserted into its mounting-tube, and shows the assembly (i.e. the PCB and the tube) pushed up out of the trackfrom below to illustrate that the tube does not extend over the actual body of the slotted sensor device.   The tube should only extend up the PCB to the bottom surface of the slotted sensor device, i.e. only to surround the portion of the PCB that lies below the slotted sensor device.   A collision block waits to be placed into position such that the plastic flag that extends below the block will block the gap of the slotted sensor:

image of sensor on PCB

This next picture shows the sensor and tube assembly pushed back down into the track into its desired position for operation, but the collision block pictured still awaits placement over the sensor.   Note that there is groove placed in the track's top surface that facilitates easy clearance of the flag of the collision block when the flag is knocked out of the slot by a moving Pinewood Derby car striking the block (see additional pictures below of such a car).   In this view, a car would travel from the lower-left toward the upper-right:

image of sensor on PCB

This next picture shows a different perspective view of what is illustrated in the above picture:

image of sensor on PCB

This next picture shows yet another perspective view of what is illustrated in the above pictures:

image of sensor on PCB

This next picture shows a similar perspective to the last, but the collision block is now located over the sensor with the plastic flag of the block blocking the slot of the photosensor.   This is the ready position waiting for an object such as a Pinewood Derby car to come along and trip the sensor by knocking the block and its attached flag away from the sensor:

image of sensor on PCB

This next picture shows another perspective view relative to the last view:

image of sensor on PCB

This next picture is a top view looking down on a lane of a track showing the sensor and tube mounted in a hole at the finish line of the track, and a collision block sitting downstream of the finish line and awaiting placement over the sensor:

image of sensor on PCB

This next picture is also a top view, but the collision block is now in place over the sensor and with its flag inserted within the slot of the slotted photosensor and waiting for a car to travel from the bottom of the view to collide with and knock the block toward the top of the view:

image of sensor on PCB

This next picture is similar to the last view:

image of sensor on PCB

This next picture shows a car approaching a collision with a collision block:

image of sensor on PCB

This next picture shows a car reaching the finish line where it first touches a collision block:

image of sensor on PCB

This next picture shows how the flag on a collision block fits into the slot of the slotted sensor:

image of sensor on PCB

This next picture shows the collision block (with its flag) lifted away from the sensor:

image of sensor on PCB

This next picture shows a collision block as it might look to an approaching car.   Note that the solder-side of the sensor's PCB would be facing toward an approaching car, that the sensor and its PCB would be protected beneath the top surface of the track, and that the sensor and its PCB would be mounted within a protective rubber tubing held by friction within a 7/16" diameter vertical hole at the finish line:

image of sensor on PCB


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