Timing system for Alpine skiing based on Arduino. Photo Finish

As far as I understand, a modern photo finish is no longer just a light bulb and a photocell, but a pulse coding of a light signal with the corresponding operation of a receiving circuit. That is more like a receiver and transmitter, and even more like the TV remote control 🙂

Of course, there are ready-made solutions used in other areas, so you can simply use them.

The unit “Photo finish” can be built on a “through beam”, that is, on the one side emitter, on the opposite side a photo cell. This is the simplest and most reliable solution. In addition to it, manufacturers usually offer an option: emitter and receiver on the one side, the reflector on the other. Further in the note I will consider only such solutions, since it is closer to the topic “timing for training”, simply because it is convenient.

As a reflector a retroreflective element is used (also known corner reflector).

The retroreflective element by two reflections sends the beam exactly back, “where it came from”. If you just use a mirror as a reflector, then in our case there may be problems with the adjustment (initial adjustment), and even then the reflector on the snow can “float” 🙂

Turnkey solutions

A few pictures of the photo finish module from “real” timekeeping systems for alpine skiing.

TAG Heuer, Alge-Timing, Omega, Microgate

It is difficult to judge whether these manufacturers use self-developed sensors or integrate ready-made solutions. In any case, searching the Internet for a combination of “Retro reflective photocell” gives very similar pictures. For example, the sensor at a distance of 15 meters.

EMX 50ft 12/240V Retro Reflective Photocell Photoelectric Sensor Beam Reflector. $106.24

FIS requirements

For our workouts, follow the rules FIS (FIS Timing Booklet, ver 2.55, Nov 2017) is not obligatory, but it is not harmful to bear it in mind 🙂 In relation to the photo finish, FIS regulates the following parameters.

  1. The height of the beam above the ground. Strictly not regulated, only that the skier could not accidentally “jump over” or “dive”. It is recommended that the beam be at the height of the knees.
  2. The width of the finishing gate is 20 meters (i.e. working distance of sensor).
  3. The delay of the electronic circuit after the photo finish trigger is no more than 5 milliseconds (0.005 sec).
  4. The accuracy of operation (repeatability of the overall response delay) is less than 0.5 milliseconds.
  5. Sensitivity. The photo finish should pass without triggering small objects smaller than 8 mm, moving at a speed of 10 km/h at two meters away from the photodetector. The photo finish should always work on objects larger than 10 cm, moving at a speed of 200 km/h.
  6. The maximum size of the retroreflective element is 10 cm (if the element is round, then this means diameter).
  7. Safety Instructions. The posts on which the sensor and the reflector are installed  should be exclusively wooden, not more than 6 cm wide. The posts should be sawed in the direction of the approaching skier.

What is on Ali-Express

The Ali-Express showcase oddly differs from what the Internet provides on request as mentioned above (“Retro reflective photocell”). For a working distance of more than 7 meters there are no sensors at all. For such distances on Ali, soilutions with separated emitters and photodetectors are recommended. Most sensors with reflectors, it seemed to me, are designed for distances of 2 meters. And sometimes for the similar devices it is indicated that working distance is 4 meters.

It should be noted that in addition to the type of sensor we need, there are similar ones, in which the emitter and receiver are also in the same package. This is a diffusion reflection sensor (without retroreflective element). And a sensor with a retroreflective element, but with two polarization filters to capture mirror objects. Such sensors are not suitable. Diffuse reflection is generally not what is needed, while polarization filters “eaten off” the emitter intensity, which significantly reduces the detection distance.

Stopped at the store LPSecurity, which specializes in security systems. In the store was quite expensive for Ali-Express sensor with 7 meters of working distance ( $48.89, including delivery). I’ll leave a picture here, may be use in future.

I ordered a sensor for 4 meters. Other sellers also had sensors in the same package, but only LPSecurity had a sensor with a round reflector. This is of course not important, but in the above “real” photo finish round reflectors are more common 🙂 The price is $16.15.

Sensor E3JK-R4M1 (or 2)

On the sensor that was delivered, there is no mention of LPSecurity, the manufacturer indicated Omron, and the sensor name indicated E3JK-R4M2. Fast punching via the Internet showed that Omron removed these sensors from the line in 2015.

Without regard to Omron, the E3JK sensors are now the most common on Ali. Manufacturers pointed on the sensors may be very different. E3JK is like the name of a constructive in which sensors of different types can be assembled. Further, R – designates the sensor with a reflector (Reflector), 4М – means four meters maximum distance of operation. The last digit 1 or 2 indicates the type of Light On or Dark On, that is, the exit state depending on the captured beam. In our case, this, as far as I understand, is completely irrelevant, especially since the output of the relay is from which it is possible to take both on and off from different contacts. I note that the “red price” of such sensors on Ali is $10, albeit with a square reflectors 🙂 By the way, the diameter of the delivered round reflector is 8 cm.

From the data sheet from the site Omron you can find out the characteristics. Here are some of them. Sensing distance 4 or 5 meters depending on the reflector. More precisely, 4 meters – with a square reflector E39-R1, which is included in default the scope of delivery, and 5 meters with the E39-R2, which is purchased separately, which is simply made up of these two. Type of exit – Dark On. At the exit – the relay. The light source is a red LED (wavelength 660 nm). Relay response time 30 ms. Temperature from minus 25 to plus 55 C.

Need to dwell on a couple of moments. First, I did not see a red LED as a light source. There is a red control LED with which it is convenient to adjust the system. As soon as the photodetector caught the reflected beam, the control LED lights up and does not go out until the beam is crossed. The main LED is visible only through a smartphone, as is usually the case with IR LEDs, but it doesn’t shine brightly and locate “deep in the depths”, in general it’s difficult to see it through a smartphone. It looks like this:

Up down. Control LED, photo cell, emitter

The second point is the response time of 30 milliseconds, or three hundredths of a second. It seems like a lot compared to the required accuracy of amateur timekeeping in the hundredth of a second. But the point is not in the time of the relay, but in the variation of the response time. It is not specified in the specifications. If the relay triggers strictly 30 milliseconds each time, that is, adding this time equally to all participants, oddly enough, it even meets the requirements of the FIS 🙂

Photo finish. Result

The measurements showed a distance of 5.3 meters, and the receive ends abruptly, just a couple of centimeters and it’s all over. And before that, everything was fine, it was possible to drive a reflector (feeling the beam) and the connection was not lost. At 4.5 – 5 meters no problems with the adjustment. Thanks to the control LED, the beam is easily caught.

The 8 mm object is perfectly caught even with more than 10 km/h. This does not comply with the FIS regulations. It is possible that in the snowfall will not work properly.

Made from scrap materials covers on the reflector and sensor. This should be done so that the devices are less “blind” in the sun. At home, the receive range had no effect.

Since at output is relay, the output can be simply paralleled with the “Finish” button. That is, you can press the button to test, and you can wait for the signal from the photodetector. Programmatically banned the re-finish within a second (when I processed the button press), so the relay didn’t recognize whether the relay was “bouncing”, the system works reliably.

For the “mechanics” I bought on Ali two mounts for the projector ($2.89). However, when I cut the thread, I found that the ball holder seems to be made of silumin. It will not last long: (And the bracket itself is made of surprisingly soft and thin aluminum, like a tin can. From a safety point of view, this is of course good 🙂

Still, for the supports, a pine bar 40×40 (4 cm transverse size, length 1 meter, price $2.54) found in hypermarket.
Timing system for Alpine skiing based on Arduino Timing system for Alpine skiing based on Arduino Timing system for Alpine skiing based on Arduino Timing system for Alpine skiing based on Arduino Timing system for Alpine skiing based on Arduino

It remains an open question whether 4-5 meters is too small for the finishing gates? Our course is not the only at slope, the width of the “strip” at the finish line is approximately the same. But it will definitely be known only next winter. If not suitable, the sensor is easy to replace.

Here are collected all the notes on the theme “Timing on Arduino for alpine skiing “.


Vadim Nikitin DigInfo.ruVadim Nikitin




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