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simple pulse counter

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voltorben

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hi!

Im doing an exam project at school. My idea is to build a simple electronic combination lock (here) , but i want to use a rotary dial (like on an old phone) to determan what buttons are pressed (so no buttons really), instead of actually pressing buttons. So i read on some other site how a rotary dial works, and this is how:

Code:
Timing Diagram. number 5 is dailed

* * * *| Dial is stationary | Dial is rotated to number |* Dail is released* | Dial stops
* * * * * * * * * * * * * * *________________________________________________
latch ______________________/* * * * * * * * * * * * * * * * * * * * * * * * \____________
* * * __________________________________________________* *_* *_* *_* *_* *_______________
pulse* * * * * * * * * * * * * * * * * * * * * * * * * *\_/ \_/ \_/ \_/ \_/ 


3 simple rules:
- If pulse changes from low to high, increment a counter
- If latch changes from low to high, reset the counter
- If latch changes from high to low, the number is added 
* to a circular buffer and the buffer is compared to the pin code
But my problem is: how do i count the pulses so i can find out what number is dailed? I have found some PIC solutions out there, but the problem is i have to keep it really simple (and cheap!) using IC's so i easily can explain how it works at my exam (and get a good grade ;)). Does anybody have any ideas? or links to other simelar things? any help would be appreciatet.

Andy
 
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