Check out this video on the result:
The object of the game is to cut the correct wires to stop the clock in time. During the day they got several coded messages on some of the colour codes used on the wires (but not all and some confusing). Cutting one of the four correct wires results in one of the red LEDS switching off and one off the green switching on. Cutting a wrong wire switches on an orange LED. Reaching four green LEDs switches of the clock and switches on a voice recorder to play. For some scare the recorded message warns an explosion to be imminent, followed by a counting down fading as the bomb switches of completely with only seconds to spare ;-)
When four orange LEDs would be switched on ................ well nothing happens actually. After all the kids were supposed to win. Actually they cut only one wrong wire and the orange LED happened to malfunction (loose connection), but that did not diminish the fun.
I made this for a one time use and kept things as simple as I could. I also kept this Instructable very simple, just the parts list, an explanation on the circuit diagram on the building. If you have some basic knowledge of electronics, it should set you on your way if you wish to make your own version. If you have some serious knowledge of electronics, you can probably improve it significantly.
This also an entry in Kiteman's "International-Go-Pro-Challenge". Check it out!
Step 1: The Parts
A battery operated quartz clock, a fitting battery and a separate battery holder for the battery
A 12V battery
4 red LEDs
4 green LEDs
4 orange LEDs
Two 1 kOhm resistorsA 2.5 kOhm trimpot
A 12V relay with single or double SPDT switches (double makes adding an optional blinking indicator LED easy)
A 2N2222A transistor or any NPN general purpose switching transistor capable of driving your relay
Some colour coded single stranded wire, at least 8 pieces of 20 cm length (to be cut). A piece of ethernet cable is a good source.
A breadboard or some other way to build the circuit and some more wires to make connections.
Some wires with alligator clips to connect the clock, its battery and the optional voice recorder
Some tape to attach or mask parts
A voice recorder that plays back when switched on. I used a small sound recording module used for greeting cards product ID 191184 at Conrad
A blue blinking LED And a another 1kOhm resistor
Something to put everything on or in. I used a case with a board cut to fit and to hold the 12 V battery
Step 2: How the Circuit Works.
The red and green LEDS are paired. As long as the corresponding wire is not cut, the red L>ED determines the voltage over the pair as about 1.7V. This is not enough to light up the green LED in series with a diode (require 2.1V +0.7V), just as we want it. Actually I first tried without the diode, but the green LED lit up faintly, together with the red one. Adding a diode was necessary to increase the difference in voltage.
Obviously, when the corresponding wire is cut, the red LED is no longer in the game and the paired green LED gets its required voltage in series with the rest of the array.
Important is that with each correct wire cut and green LED lighting up, the voltage over the array increases. This in turn decreases the voltage over trimpot P1 and correspondingly the voltage at its wiper terminal connected to the base of the transistor. The trimpot is trimmed to give more than 0.7V at its wiper terminal, as long as at least one red LED is on, keeping the transistor driving the relay “on”.
With al green LEDS on, the voltage should be lower than 0.6 V, switching the transistor (and the relay it is driving) off.
Trimming is done by trial and error, simply by checking the switching of the relay when attaching and removing the wire at only one of the red LEDs.
The relay in turn switches on either the clock and an optional indicator LED or the optional voice recorder.
The array of orange LEDs is set up separately to add four more “incorrect” wires to the game. Obviously each wire bridges a LED, keeping it from lighting up until cut. The orange LED array has no further function.
For easy viewing printing, the circuit diagram is added in pdf below.
Step 3: The Building.
I made the circuit on a breadboard, for an easy build and easy reuse of the components. It also allows easy replacement of the wires, if one does decide to use it again. The clock was attached with alligator clips.It turned out to be quite fragile, but the warning to be very careful was eagerly followed by the kids. After al it is supposed to represent a bomb. Still, the loose connection on the orange LED could very well have happened during the game, So it is important to fasten the wires to be cut in some way, so they do not pull on the connections.
Important: make sure you make the switching contacts for the clock and the voice recorder at same polarity (both at the + or both at the - side of the batteries). The difference in battery voltage does not matter as the other side is not connected.The voice recorder often has a switching contact at the + side, suitable to "bridge" with the relay contact, determining the choice.