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Alarm system

These instructions outline how to make the alarm system (clock) for the Steal the Nerds escape room. This type of clock is general purpose and can be used in many different types of escape room.

Key details

  • Uses radio group 1 and responds to the following messages:
    • alarm - triggers the alarm
    • reset - resets the alarm, disabling the countdown and setting the time to 20 minutes.
    • diagnostic - places the alarm in diagnostic mode, starting the countdown.
  • Button A - starts and stops the countdown.
  • Button B - adds an additional minute to the timer.
  • Buttons A+B - resets the alarm, disabling the countdown and setting the time to 20 minutes.

Bill of materials

  • BBC micro:bit. These instructions assume a speaker is connected (by default on a V2) but it can be used with a V1 micro:bit with no speaker.
  • BBC micro:bit battery power box.

Either:

  • Card or paper to make a stand.
  • M3/M4 nuts and bolts to secure the BBC micro:bit.

Or:

  • 3D printed stand (here).

Click here for the completed code.

Here is an example of the alarm system using just paper and screws for the stand.

example

Here is an example of the alarm system using a 3D printed stand (here) sat on the box it is stored in.

example

Explanation

The first challenge for a clock on a 5 x 5 LED display is how do we display numbers? The technique used in this clock is to use a vertical stack of 5 LEDs to represent a single number. So how can 5 LEDs be used to represent the 10 digits zero through to 9? Lets start with the first 6 numbers, zero through to 5:

  • zero - no LEDs
  • one - the lowest LED
  • two - the 2 lowest LEDs
  • three - the 3 lowest LEDs
  • four - the 4 lowest LEDs
  • five - all 5 LEDs

Then we are stuck as we have no more LEDs to switch on. However, what we could do is to start turning LEDs off. If we do that in the same order we turned them on we get.

  • six - the 4 highest LEDs
  • seven - the 3 highest LEDs
  • eight - the 2 highest LEDs
  • nine - the highest LEDs

This can be illustrated as follows:

example

We now have a way to represent up to 5 numbers on a 5 x 5 LED screen. Using 4 columns we can represent 4 digits which is enough for 99 minutes and 59 seconds!

example

Another examples is 13 minutes and 47 seconds:

example

Instructions

Setup the following variables:

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Add the following code:

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Extensions

The alarm clock system is easy to extend. Try these ideas:

  • Play a sound each time a new minute starts.
  • Play a sound to indicate only 5 minutes left.