Detector Building

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Detector Building is a Division C event which has been run officially since 2020. Previously, it was a featured trial event at the 2019 National Tournament and was also run at various 2019 state tournaments including North Carolina, Texas, and Florida. The event has a device and exam component, both of which are tested on event day. Teams are expected to build a device that can accurately measure the ppm of NaCl in a water sample, while also displaying a related LED color. For testing, teams are also expected to complete an exam related to constructing a ORP or Redox Probe. Teams are also expected to create and submit a design log showcasing how their device was built.

The event was first run at the 2019 National Tournament and was first officially ran as a Division C event in 2020. In the 2020 season, many community members claimed the rules were too vague, limited design freedom, and were ignored by some event supervisors. Changes were made throughout the 2021 and 2022 season rules to address these issues, including adding support for a virtual format of the tournament.

Event Summary

For the 2024 season, students will build a durable ORP or Redox Probe that accurately measures and displays the concentrations of NaCl in parts per million from 0 to 5000 ppm of different water samples as well as voltage. Devices must be built with a microcontroller, a display, LED lights, and a sensor/probe.

Teams are required to prepare a Design Log with a diagram of their device, raw sensor reading/ppm, scatterplot of ppm/voltage data, mathematical model(s) used for ppm-voltage conversions, and printout of code.

At the competition, water samples with various concentrations of NaCl will be provided, and students will determine these concentrations with their device. Each range of concentrations will correspond to a unique LED color combination. The event also includes a written test covering topics including voltage dividers/other electrical components, ORPs, environmental significance of chloride concentrations, half reactions, temperature vs concentration, standard cell potentials, and determining real-world relationships from raw data.

The Device


Microcontrollers are small computers that run a specified program on an integrated circuit. Manufacturers have the option of mounting microcontrollers onto a circuit board connecting to an ordinary computer, which can read data from the microcontroller and write a program to the microcontroller. This lets users control the microcontroller from their laptop or desktop computer, allowing ease of coding. Some examples of microcontroller boards are TI Innovator, Raspberry Pi, and Arduino.


See also: Amazing Mechatronics § Arduino

Arduino is a family of microcontroller boards that run open-source Arduino software and are approved by the Arduino S.r.l. company. Arduino code is written in syntax based on the C language.

The Arduino can transmit data to an output on your computer known as the serial monitor. You'll be able to use the serial monitor to read data from the Arduino, and this may help at debugging code in the early stages of the process.

The Arduino has both digital pins and analog pins, as well as pins that are neither. Analog and digital pins may be input (to read data) or output (acting as a source of current). Digital pins are all-or-nothing pins: they may be HIGH, where they have an output of 5 volts (3.3 volts on some Arduino models), or LOW, where they serve as ground (0 volts). However, they cannot be set to intermediate voltage values between HIGH and LOW. Analog pins can also be used to source current, but unlike digital pins, analog pins can acquire a range of voltage values between 0 V and 5 V.

An Arduino connected to a breadboard for Water Quality. Teams who do not use laptops may choose to show their temperature reading on a liquid crystal display instead (depicted).

Pins that are neither digital nor analog include the 5 V and ground pins. The Arduino references its voltage measurements relative to the ground pin, which acts as 0 V.

Electricity can break an Arduino. If more than 40 milliamps of current travel through a pin, the Arduino may reboot or the pin may "burnt out," leaving it permanently disabled. A current this large would also blow an LED. Make sure to have enough resistance in your circuits!

Useful commands

Serial.begin(9600) - initializes the serial monitor, with data transferred from the Arduino to your laptop at a rate of 9600 bits per second

Serial.println("Hello world!") - prints Hello world! onto the serial monitor

Raspberry Pi

Raspberry Pi is a microcomputer that runs the Raspbian operating system, which is derived from Linux. In this way, it is more versatile than many other microcontroller systems. For instance, Raspberry Pi can link directly to an HDMI-enabled monitor as well as a keyboard and mouse.

Raspberry Pi requires socket-type wires in order for you to connect it to other electrical components.

Design Log

The Design Log is a report on the force sensing device that contains a photograph and description of the device, ppc of NaCl in water samples/voltage raw data, plotted ppc/voltage data, mathematical model(s) used for ppc-voltage conversions, and annotated printouts of code.

The Design Log should not be overlooked for it contributes a fifth of the total possible points. The following suggestions are included to maximum points earned by teams on their Design Logs. In general, making the Design Log as self-explanatory as possible is recommended.

Suggestion Good Example Bad Example
  • Use a photograph (not drawing) of your device with a clear light-colored background. For example, photograph your device on a sheet of white paper.
  • Position your device components in the photograph such that all important components are clearly visible and not overlapping.
  • Include labels for all important components. For wires, specify what it is used for, e.g. ground, power, analogue inputs, etc.
  • Use descriptive labels that provide as much detail as possible. Specify values (e.g. resistances) and part numbers (e.g. "NTCLE100E3") where applicable.
Detector-building photograph good.png Detector-building photograph bad.png
  • Provide a descriptive title and label all columns/rows with units.
  • Use consistent and appropriate significant figures for values. For example, 3.61827319923 V from an Arduino probably is not reasonable.
  • Check that values are actually ppm and voltage (and not analogue input values). For example, a value of 700 probably is not a voltage measurement.
Detector-building table good.png Detector-building table bad.png
  • Use a descriptive title for your plot.
  • Label all axes including units.
  • Set axis ranges to maximize usage of the plotting area.
  • Provide an equation (rather than an expression) for the model. An equation has an 'equals' sign.
  • Write the model equation such that ppm is the output, i.e. ppm = _____, and voltage is the input.
  • Describe and provide units for variables in the model equation.
  • Display plots on a full page so they can be viewed comfortably.
Detector-building graph good.png Detector-building graph bad.png
  • Organize code into sections to facilitate reading. For example, group pin declarations together.
  • Annotate code with comments that describe the purpose of each line/section of code.
  • Highlight (not underline or circle) the sections of code as required by the rules.
Detector-building code good.png Detector-building code bad.png

Written Test

Light-emitting Diodes (LEDs)

An LED is a semiconductor light source that emits light when current flows through it. When a current flows through the diode, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons.The color of the light (corresponding to the energy of the photons) is determined by the energy band gap of the semiconductor. White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.


Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with high light output.

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Recent developments have produced white-light LEDs suitable for room lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology.

Advantages and Modern Use

LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper and medical devices.

Unlike a laser, the color of light emitted from an LED is neither coherent nor monochromatic, but the spectrum is narrow with respect to human vision, and functionally monochromatic.

Mass Sensing (2023 Build)

These topics were part of the 2023 rules, and are no longer relevant to this year's event

For the 2023 season, students are required to build a force/mass-sensing device that accurately measures and displays mass in grams of different objects using a microcontroller board such as the TI Innovator, Arduino, or Raspberry Pi. The device must contain red, green, and blue LEDs to indicate mass ranges. Teams are also required to prepare a Design Log that contains a photograph and description of the device, mass in grams/voltage raw data, plotted mass in grams/voltage data, mathematical model(s) used for grams-voltage conversions, and annotated printouts of code.

At the competition, students will adjust the mass ranges that correspond to each LED color combination on their device before measuring and displaying mass in grams and LED color combinations for four different objects provided by Event Supervisors.

The event includes a written test that covers topics including force/stress/strain/resistance relationships, theory and operation of electrical components, relationships between force/mass/pressure, and determining real-world relationships from raw data.

Conductivity Sensing (2022 Build)

These topics were part of the 2022 rules, and are no longer relevant to this year's event

Aqueous Solutions Chemistry

Most of the basic questions have info that Chem Lab/Aqueous Solutions would provide.

Van't Hoff Factor

The van't Hoff factor is the number of particles each solute formula unit breaks apart into when it dissolves. This is usually used in colligative properties which include freezing point depression, boiling point elevation, osmotic pressure, and vapor pressure.

The equation for solutes that don't fully dissociate can be represented as:

[math]\displaystyle{ i = αn + (1 - α) = 1 + α(n - 1) }[/math]

where i is the van't hoff factor, α is the fraction of solute particles, and n is the number of ions. The general equation is

[math]\displaystyle{ i = \frac{\textbf{moles of particles in solution}}{\textbf{moles of dissolved solute}} }[/math]

Temperature Dependent Resistors (2021 Build)

These topics were part of the 2019-2021 rules, and are no longer relevant to this year's event


These little resistors made of metal oxides and are cheap and fairly accurate. At their best, these resistors can achieve an accuracy of ±0.1 °C. There are two different types of thermistors - PTC and NTC. PTC stands for "positive temperature coefficient" and increases in resistance as temperature increases. NTC stands for "negative temperature coefficient" and decreases in resistance and temperature increases.

Steinhart-Hart Equation

Steinhart-Hart equation can help calculate the temperature based on the resistance that the thermistor outputs.

[math]\displaystyle{ \frac{1}{T} = a+b\ln R+c(\ln R)^3 }[/math]

T = Temperature (in Kelvin) a, b, c = Steinhart-Hart parameters

Stainhart-Hart parameters need to be calculated for individual thermistors. This can be done using this website: Thermistor calculator

Beta ([math]\displaystyle{ \beta }[/math]) Parameter equation

This equation utilizes one variable, unlike the Steinhart-Hart equation's three variables.


Each team's Total Score is the sum of their Build Score, Design Log score, and Written Test score. The team with the highest Total Score wins.

Second number after the slash represents the points that regionals will score by.

Component Description Points Possible
Build Score 3 unknown masses at Regionals and 4 unknown masses at States/Nationals.

i. Accuracy Score for each mass = 15 pts - (relative error of the mass measurement x multiplier) but will not go below 0 pts.

(1) Regional Multiplier = 20

(2) State Multiplier = 30

(3) National Multiplier = 40

(4) Teams not able to produce a reading will receive an accuracy score of 0 for that mass.

ii. LED Score for each mass = 4 pts awarded for the correct LED color (as determined by the mass displayed by the Device)

More on violations are in the rules manual

76 points / 57 points
Design Log A report on the device assessed on 8 criteria, each scored out of 4 points. 32 points
Written Test A written test covering topics listed in the rules. Up to 50 points
Total Score The sum of the Build Score, Design Log Score, and Written Test Score. Up to 158 points / 139 points

Tiebreakers: (a) Highest Build Score, (b) Highest Written Test Score, (c) Selected questions on the Written Test.

Teams with any construction or competition violations will be ranked behind teams without violations.

Other Resources