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This is not a compete project but rather something that could be used as part of another NXT project that requires user input, such as a game. It consists of a keypad of 16 keys, in this case the numbers 0 to 9 plus a 'Backspace' key and an 'Enter' key. Obviously the keys can be labelled in any way required and you can have less keys or more if needed and you have enough parts.
It uses just one sensor, a HiTechnic EOPD sensor and it works like this: When a key is pressed, a series of levers raises a 'flag' in front of the EOPD sensor. Each key has its own flag and each flag is a difference distance from the sensor. The distance to the raised flag is measured and which key was pressed can be worked out from that. The associated program has two parts. The first allows the distances to each flag, and hence the value for each key, to be measured. These are stored in a file, so in theory you don't need to recalibrate the keys each time you what to use the keypad. The second part is purely to demo the use of the keypad and consists of a very simple maths program that asks for the sum of two numbers and allows the user to enter the answer. It seems to work rather well. The main problem I encountered is that the EOPD sensor seems to be pretty slow to accurately read the distances with the result that you have to press and hold each key for a couple of seconds to get a reading.
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