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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Hi all, here's an unconventional frequency counter circuit I've designed. I'll try and explain how it works: (This circuit does not include a prescaler)
A schmidt-triggered inverter gate is used as a relaxation oscillator. You adjust R and C to get 512Hz. I call this signal "REFERENCE" because the displayed frequency will be proportional to this.
The 512Hz REFERENCE signal is divided by 128 using a CD4024 7-stage divider chip to give a 4Hz signal. I call this "REFRESH", because this inherently defines the refresh/readout rate of the LED displays.
I take REFRESH and squish it down from a 4Hz squarewave into a 4Hz pulse with a pulsewidth of more than 200nS. This is done using my pulse-squishing circuit; a diode, resistor, capacitor, and another schmidt-triggered inverter gate. The resulting pulse is called "RESET" because for those 200nS, the display will go blank and the counter will reset to zero.
I take the positive edge of RESET, and allow it to positively trigger another schmidt-triggered inverter (with the input floated at V/2 so it maintains its state). I take the negative edge of REFERENCE, and allow it to negatively trigger the same gate. The output of this gate is now the "INHBIIT" signal.
To count frequency:
Say we are measuring a 1MHz signal with no prescaler:
1) The "reset" pulse hits the counter, blanking the display and resetting it to zero. I will consider the 200nS infinitesimally small for purposes of clarity. 2) The positive edge of the same "reset" pulse "uninhibits" the CD4026s and they start counting. 3) The negative edge of the next "reference" pulse "inhibits" the CD4026 and they stop counting. The CD4026s will have counted (1/512) * 1,000,000 pulses, and the clock will end up displaying "1953" 4) The clock will hold and display that value for 1/4 of a second, until the process repeats.
Now, to calibrate the frequency counter, you tweak the frequency of the reference oscillator (which affects the display refresh rate too, but who cares) until the display reads the appropriate value.
You can add a prescaler for higher frequencies. I recommend a CD4020 or a 74LS93.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Sulaiman, I cannot use a crystal with my design unless its frequency lines up perfectly with any prescaler and the width of the pulse-counting part of the 'read' cycle. In due course, I may make a version with the pulse-squisher and dividers to bastardize it so it works, but for now, it has to be manually calibrated. I guess that's OK for me, because I don't mind a +/-1% reading.
Hazmatt, I haven't made the circuit yet -- I don't even know if it will work. I'm just doing the PCB right this minute. :)
Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
It sounds like it will work and be moderaterly stable, minus maybe having to calibrate it every time you turn it on, but since that won't bother you its ok. As for the PCB desing looks nice except I don't see how your gonna be able to do the white jumpers that in between some pins, also these 3 red traces (I'm presuming) unless they were meant to make contact with those other or you still have some trace editing to do, all in all it looks great and sounds like it'll work.
Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
I had a similar problem with my capstone, or final project for college ( It was a clock that displayed miliseconds as well as the usual time frames), even though it was based off a ucontroler, I solved it by doing a "Chained display" (Can't remember the acyual term for it). i.e since the clock displayed 12:00:00:00, lets say the clock showed 12:34:56:78, the actual display would light up 8 then 7 then 6...etc, i believe every display was shown for ~2-5ms, any faster and the display wouldn't light any slower and it would hang on a digit.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
WhiteArc, it only has to be calibrated once, ever. Then I'll record the values for Rfreq, Cfreq, Rsquish and Csquish, and I will be able to crank them out repeatably. Crystals are also expensive. The CD4026s should be good to at least 8MHz counting frequency, but with a prescaler, they won't need to count that fast.
Dangermouse, a 32,768Hz crystal will cost me a few dollars here, and it is imperative that I significantly change the reference frequency in order to calibrate the meter. (The initial readout value will have a completely arbitrary proportion to the input frequency -- you have to unbastardize it (and the prescaler) by adjusting the timing period)
Those white jumpers I will carefully stretch between pins. The 'red traces' will in fact be actual wires on the bottom of the PCB. At some point in time, when I can make these up commercially and get a double-sided board done, I won't need any links.
Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
Oh ok, I was just concerned about the jumpers because In a tech prep course I took in High school, we had to make PCB's, design, layout, print, etch, and test them, and putting solder points that close to each other just makes me nervous, so I was concerned, but I guess if you know how good you are at doing that kind of stuff then by all means go for it.
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