Race Car

PCB-Mounted Slotted Photosensor (as used for start-gate sensing on Timestopper models TS100, TS200, TS300, and TS400)


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[ALERT!!!: Beginning with shipments of TS400 units in late November, 2012, a new PCB layout design has reversed the functions of the inner two wire-holes!! The new boards can be recognized by the fact that their solder-side has only one short trace, and that being to the wire hole for ground. In this regard, the instructions and illustrations on this page are obsolete.]


Pinewood Timestoppers' PCB-Mounted Slotted Photosensor is comprised of a high-power infrared LED emitter and a high-sensitivity photodiode, each set opposite to one-another across an open 0.117 inch wide by 0.345" deep slot.   Included alongside the sensor 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 two 0.125" Dia mounting holes, set 1.200" apart, for No. 4 size screws with which to mount the assembly to a support of your choosing.   A third 0.125" Dia hole is provided for using a tie-wrap to clamp your wires to the PCB for strain relief.


Four 0.042" Dia holes are provided in a row for soldering your own #22 gauge (or smaller) wires to the PCB.   From a perspective view looking down on the sensor's slot and with the four wire holes near what will be the top edge of the PCB, i.e. with the tie-down hole on your left, the four wire-holes are arranged from left to right as: a) +5V to power the LED, b) +5V Vcc input to the photologic circuitry, c) circuitry ground, and d) signal output from the PCB.


Installation drawings with the TS100 documentation show examples of how these slotted photo-sensors are used with Pinewood Timestoppers' Timestopper TS100 for start-gate sensing (in particular, see Figures 2B, 4A, and 4B). These sensors are the type also used for start-gate sensing on Timestopper models TS200 and TS300. But note that the TS100 and TS200 are no longer sold.


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 a) a wire from your +5V voltage supply to the +5V input on the PCB to power the LED and b) 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.


hookup schematic

A picture of the PCB-mounted sensor as delivered is shown below.

image of sensor on PCB