Weightbearing platform & laser gun

The Laser Gun Game is a weight-bearing system made to have fun while learning correct body posture in standing. If Julian stands correctly the laser gun is wirelessly activated by the platform which allows him to shoot the laser gun at a hanging target. If he is not in a correct weightbearing posture the gun will not shoot until he corrects his posture. The game consists of three components: A platform, a laser gun and the target.


Big brother testing out platform

The platform tracks his weight distribution. That is to say, the amount of pressure through both his legs. In an ideal stance the distribution should be more or less equal through both legs–with slight variance to allow for sway.


In the platform is made of two moveable foot panels. Each foot panel has four large pressure sensors. Two in the front and two at the heel.

The sensors on each foot are compared to the same group on the other foot. If the resulting difference is within the acceptable variation-range the laser gun is activated.


To account for this variation in our natural sway, the platform is set at an initial 10% acceptable weight variation between the groups of sensors. A knob on the platform allows for this sway-allowance variation to be changed. It allows for anywhere between 10% to 50% of the total difference between the pressure on the sensor groups. 10% is the strictest setting and 50% is the most lenient setting. 50% acceptable difference means that the difference with any of the sensor sets can be twice as heavy as the other. heavier than the other. If the total weight distribution difference between both feet is within the accepted level the laser gun is activated wirelessly by the platform.

Communication between the platform and laser gun is done wirelessly with two Series-2 XBee modules. In this version of the Laser Gun Game only the platform talks to the gun. In the next version platform will also talk to the target allowing it too to change target options depending on quality and length of proper posture on platform.

Much to the boys’ dismay…the gun is a new, gutted Nerf Mega. As I haven’t decided what laser type will be used in the next versions of the game, my main concern and challenge in the design and technical aspects of the gun were with safety. The laser gun has three main safety features:

  • The laser is always OFF unless the platform activates it.

  • There is a motion detector in front of the platform which turns the laser off when motion between the target and the platform/laser is detected.

  • On startup, the gun is calibrated to turn off the laser if the gun itself turns away from the target. This is achieved by an ultrasonic proximity sensor in the gun, which measures the distance to the target. Since the size of the target is known, the gun calculates the hypotenuse of the distance to the target. From that point on when the ultrasound sensor measures any distance greater than that initial hypotenuse value the laser is deactivated. However, as he is much shorter in size than the distance to the target there is also an accelerometer in the gun which deactivates the laser if the gun’s angle downwards exceeds

Laser gun

Using the laser gun

  1. The gun is switched on and pointed at centre of target for calibration.

  2. The ultrasonic proximity sensor determines distance (A) to target.

  3. B is calculated using the Pythagorean Theorem

  4. From this point forth, if the ultrasound sensor reads a distance to the wall of greater than B (e.g. C), the laser is deactivated.

  5. At the same time as the ultrasound distance (A) is determined in #2 above. The accelerometer also sets its X axis of alignment to 0 degrees.

  6. The accelerometer continuously checks the angle down and if it exceeds 30 degrees the laser is deactivated.

  7. The laser will be activated ONLY when all four of the following conditions are met:

    • Platform sends okay-to-shoot signal to gun

    • The distance to wall is less or equal to original calculated hypotenuse value

    • The angle of gun is >-30 degrees.

    • There is no motion in front of platform

Laser gun components

  1. 3 X AAsLaserGunParts

  2. On/Off

  3. USB access

  4. Arduino Uno Rev3 / Sparkfun Xbee shield / XBee series 2

  5. Momentary switch for trigger (toggle on off)

  6. LED to indicate okay to shoot

  7. Accelerometer

  8. Mess of wires

  9. Piddly little speaker (to reproduce Arduino’s piddly ability to make sound)

  10. Red laser

  11. Ultrasound sensor


This first prototype of the target is square and is controlled by an Arduino Mega2560. It has 15 LEDs and 15 photo resistor. The players score is displayed on a Blue 4-digit 7 segment display. The blue four digit score counter sets to zero on startup. There are fifteen LEDs and fifteen associated photo resistors. The LEDs randomly light for 3 seconds and if, during that time, the linked photo resistor senses a increase in light the score counter is increased and displayed on the 4 digit display. Next version will include moving parts powered by two servos and a stepper motor to move target objects around. Including a random() function setup to activate specific, individual target components for different score values.

Conclusion and considerations

It took three months to create this project. Julian tried it for about 3 minutes. He hated it! Standing correctly on the platform and simultaneously aiming and shooting the gun was too much to handle.

It taught me another valuable lesson. How much I take for granted!

It wasn't all wasted however because we ended up using the platform as part of his workout sessions. And the platform has gone on to control the next variation of the Laser Gun Game...The Marble Run!