Introduction: A Reaction Speed Game


This project shows you how to make a game where two persons could compare how quickly they react to a stimulus. There are three participants in total: a ‘referee’ and two contenders; the referee gives a signal, the contenders must react as quickly as possible, the audience will clearly see who the winner is. The ‘Description of circuit’ section explains in details how the game works.


Components to build the circuit

- two 5 volts SPDT (single pole double throw) relays

- three LEDs (I used green, red and orange)

- three normally opened pushbuttons

- three 100 Ohm resistors wires

- a battery holder for three AA batteries

- three AA batteries

Tools to build the circuit

- soldering iron with solder

- wire cutters

Note: this project being oriented to kids, it’s desirable to use a 12 volts soldering iron for safety reasons.

The materials and tools needed to build the enclosure depend on what kind of enclosure you want to make. I tried to keep the technology as simple as possible while using ready made components available at my workshop. I used:

- an exacto knife

- a drill (a 12 volts drill is desirable for safety reasons)

- a screwdriver

- 1.5 mm and 2 mm diameter drill bits

-a ruler

- a well sharpened pencil

- some glue

- colour paper

- small pieces of 8 mm thick plywood

- four round wooden plugs

- small pieces of copper clad textolite

- small pieces of double sided adhesive tape

- four small wood screws

Step 1: Description of Circuit

The circuit is built around two relays: RY1 and RY2. The LEDs 1 and 2 are for the contenders, the LED 3 is for the referee.

The referee presses his/her button S3, the LED3 light up - it’s the signal to the contenders; each of them should press their button quicker than the rival does presses theirs.

Let’s imagine that the ‘green’ (LED2) pressed his/her button S2 first; the coil of the RY2 gets energized, the magnetic field induced in the relay’s core makes the contact change its position. When it happens, the contact closes the line to the LED2 which lights up; at the same time, the contact opens the line to the RY1 coil. Now, even if the ‘red’ presses his/her button, RY1 won’t work, and LED1 won’t light up. The reverse is true when the ‘red’ presses his/her button before the ‘green’. This feature is introduced to clearly define the winner by the principle ‘either - or’.

The power supply is provided by three AA batteries, the overall voltage being 4.5 volts; that’s enough to male the relays work. However, you can’t energize the LEDs directly from this supply to avoid excessive current passing through them.

The LEDs used are rectangular LEDs type HLMP; typical voltage drop values for them (when the current through the LEDs is 20 mA): red 1.8 volts, green 2.2 volts, orange 1.9 volts. The values of the resistors to be connected in series with the LEDs are calculated as:

R = (Vsupply - Vdrop)/ I

Therefore: R(red) = 135 Ohm, R(orange) = 130 Ohm, R(green) = 115 Ohm

I used three resistors of 100 Ohm, because it simplifies the task of finding the needed components; however, the current through the LEDs remains within admissible limits (see reference).

Step 2: Enclosure and Cover

I used a small cardboard box that was available at my shop, and I glued black paper on the sidewalls of the box. The cover is made of thin corrugated cardboard to provide enough rigidity to the cover; after having cut openings in the cover, I glued white paper on it.

There are four small triangles made of plywood glued to the corners of the enclosure; they serve to fix the cover by means of four screws.

Step 3: Battery Holder

I used a holder for three AA batteries that I took from an out-of-function-since-long-ago toy. The holder is fixed to the bottom of the enclosure by means of four small pieces of double sided adhesive tape.

Step 4: Circuit Assembly

The circuit is simple enough to be assembled ‘point-to-point’ as show in picture. The components are placed on the interior side of the cover; the relays are fixed with double sided adhesive tape; the LEDs are inserted into the openings tight enough to be held in position.

The free ends of the button wires are soldered to the pads made of copper clad textolite glued onto the cover.

Black wires are used for the ‘-‘ line, red ones for the ‘+’ line; wires of any other colour could be used for the other connections.

Step 5: Buttons and Button Holders

There are three normally opened pushbuttons installed on button holders that I made of wood plugs 30 mm in diameter and 10 mm thick (such plugs serve to hide screw holes in furniture). A round piece of coloured paper is glued on the top of each plug to identify them according to the LEDs colours.

Two 1.5 mm holes are made in each plug; the wires soldered to the pushbutton are passed through these holes and the pushbutton is glued to the top of the button holder.

After School Challenge

Participated in the
After School Challenge