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Does anyone know how to set up a circuit by using counters, decoders, 7 segment displays and a 555 timer to show the revolutions (pulses) per minute from a photo-transistor? I'm doing this as part of my own project but I am not sure of what other components I need and how it could be wired.

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Yes, lots of people I suspect.

However, using a microcontroller would be a simpler way. In a micro you can count pulses in a known period, or measure time between pulses. Counting pulses within a period is simpler, but can take a while to get a meaningful reading or you have low resolution. Most of the time you measure time between pulses and then invert to get speed.

Many micros have a capture capability where the value of a free running counter is captured in hardware when a pulse is received. By subtracting the previous capture value each pulse, you get a high resolution measurement of the period between pulses. I'd low pass filter that a bit, then invert when needed to update the user interface to show speed.

The 7 segment display encoding is also something the micro is well suited to do. It can then perform the multiplexing on the multiple digits, or you can get a micro with lots of I/O lines and hook up each segement individually.

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  • \$\begingroup\$ "Does anyone know how to..." mean something different among today's youth.. It's like "can you tell me if you know?". :) \$\endgroup\$ Feb 10, 2013 at 16:19
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    \$\begingroup\$ @abdullah: The answer to your reworded question is then "yes, I can". Part of the point is to get people to phrase questions properly and write what they really mean. Too often people seem to fall back onto idioms without stopping to think. I recently had someone tell me to "take my cotton-picking hands off of...". This person isn't racist, and was really embarassed when I asked why it should matter whether my hands had previously picked cotton or not. She had just used the expression and never stopped to think about what it really means, which is actually pretty offensive. \$\endgroup\$ Feb 10, 2013 at 16:28
  • \$\begingroup\$ Yeah, thanks for editing that comment and making my previous comment sound silly, too :) \$\endgroup\$ Feb 10, 2013 at 17:23
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The best (simplest and cheapest) is to use a microcontroller, its internal clock and a capture pin. The hardware way, based on a 555, needs a counter, a register to hold the count and maybe some logic here and there to reset the counter, etc. And... if you want a minimum of accuracy, a 555 is not the first choice. On top of that, I suppose your count, being a number, would be mathematically processed, which is much simpler in a microcontroller register than in pcb traces.

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What you're looking for is a frequency counter, a basic bit of lab test equipment. The various ways of making them are all over the internet. Your easiest option will be to buy a cheap multimeter with a frequency counter (Hz) function, it will be easier & more accurate than anything you're going to nail together on breadboard.

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