Race Car

Phototransistor Replacement


(To print this document with proper pagination and without clipping, set the Page Setup from your Browser to Portrait mode and 1/2" margins.)


Phototransitors are used one-per-lane on T-IV, TS300, and TS400 Timestoppers for sensing cars at the finish line of your track.


Schematically a phototransistor can be represented as shown in Figure 1 below, where it is shown to have two electrical leads consisting of a collector and an emitter.   A third conductor, leading from what is called a base, is not generally present in a phototransitor.   It is the base in a phototransistor which receives incident illumination to cause the phototransistor to conduct electricity or not.   Figure 2 shows a bottom view of the phototransistor, where the lead to the base connection is cut off and not available.   Figure 3 shows how multiple phototransistors are hooked together in a daisy-chained-arrangement for a multi-lane configuration, to achieve sensing with one phototransistor per lane.   The logic of the signal output from each phototransistor is such that the collector will be at relatively high (+Vcc) voltage when all light to the phototransistor is blocked, or low (less than 0.4 volts) when sufficient light is directed into the window/lens of the respective phototransistor.   The emitters of the phototransistors are connected in common and held to a low reference potential.   Note that the emitter of each phototransistor, except the last in the daisy chain, has not one but two conductors leading away from it.

schematic of phototransistor

Physically, a phototransistor is encased within a container having a window/lens at one end, and collector and emitter leads extend from the end opposite to the window/lens.   A side-view picture of a Phototransistor is shown below.

image of sensor on PCB

Wires are soldered to the respective leads of the phototransistor as pictured below.   When using shrink-tube insulation, and before heat-shrinking it, be sure to slip the shrink tubing over the wires before soldering the wires to the phototransistor.

image of sensor attached to wires

Using either shrink tubing or electrical-insulation tape, insulate each connection of wire(s) to each phototransistor lead from contacting the adjacent lead.   Otherwise a collector and emitter could become accidentally shorted to one-another preventing proper operation of the respective phototransistor.

image of sensor with shrink tubing

When wires to both collector and emitter are attached and insulated from one-another, the phototransistor can be manually inserted into the protective tubing that was provided with your Timestopper for each respective lane.   Proper orientation will locate the tab, that extends from the case of the phototransistor, in the slit that is pre-cut into the protective tubing.   This is illustrated below.   The final sensor assembly of a phototransistor into its protective tubing can then be inserted upward into the track through a hole you have provided in the center of a lane at the finish-line of your track.

image of sensor inside protective tube