bigal_scorpio said:
Thanks for the circuit mate, but do you mean that it would only sample the RPM every 3 seconds or so?
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BTW I can always make another spinning disk that has a different number of slots, even 1 slot if needs be if it made the sampling rate better?
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To reduce the time between readings you have to
reduce the time the counter is actualy counting AND
increase the number of pulses per revolution (slots in the disk).
The time the counter is actualy counting should be equal to (one minute / the number of slots) to read the speed in RPM.
on1aag's circuit is slow because it has too much "dead times": it is counting only for two seconds (from Q0's pulse to Q3) and uses a full second to latch up the count and reset the counter. (It is counting only 2/3 of the time)
To speed it up you may:
>> use Q7 (pin 6) to reset the FF instead of Q3 (pin 7) (the FF is formed by the two interconected gates)
>> use Q8 (pin 9) to latch the count instead of Q4 (pin 10)
>> use Q9 (pin 11) to reset the counter instead of Q5 (pin 1)
>> Leave the 4017's reset (pin 15) tied to ground instead of Q6 (pin 5)
This way you have an 8 cycles counting period and only 2 cycles of dead time, or 8/10 of the total cycle.
To test your circuit use a 555 as to clock your 4017 (pin 14) at a faster rate (on1aag used 2 hertz). The clock rate mat be obtained as:
Counting time = 60 seconds / number of slots in your disk = 8 clock cycles
Clock frecuency = number of slots in your disk x 8 / 60 seconds.
Later on, the 555 may be replaced with a frecuency divider that uses a cristal oscilator or the main's 50 Hz as a reference.