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frequency comparison

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New Member
rite... im trying to build a guitar tuner and ive come across a problem....
i need to compare two frequencys.... one provided by an oscillator and one provided by an external source (a guitar and mic).....
i need an output from the frequencys so i can turn an indicator on when the frequencies match as much as possiable....
ive thought about using beat frequencies but im running into a problems :( so i need some more ideas to consider....
so ne suggestions.....?????
:D :p :D

panic mode

Well-Known Member
I think I remember one such circuit...
You can amplify both signals till they are square (or use chips with Schmidt trigger to make them square), then put an AND gate and low pas filter (single RC element) and (analog) voltmeter accross capacitior.
When frequencies are diffrent, voltmeter will show high reading (ca. 4V).
When frequencies are similar, output will swing high-low-high-low...
The closer the frequencies, the slower the swing.



Active Member
I've seen simple audio mixer circuits where multiple microphones or similar inputs can be used. I wonder if that wouldn't work.


Well-Known Member
Mount the LEDs right next to each other. Tune until the beat frequency stops. If you don't have 5 volts available, select the values of R4 and R5 to give you about 10ma in each LED when it is on (the comparators can only sink about 10ma each). Otherwise, it should work on any voltage from 5 volts to 30 volts.
The LEDs have no individual significance. We could use one LED and a diode bridge, but this seems simpler.



Well-Known Member
Thanks, Claude. What if it doesn't work. :oops:
I was thinking of recommending a phase detector (XOR) for beat frequency detection, but then I started thinking about two big 3-phase, diesel-powered generators I that I once saw, and they simply had 100 watt light bulbs wired between the respective phases of the two generators (1-1, 2-2, 3-3). When they wanted to switch over, they fired up the off-line generator and tweaked the RPMs until all 3 light bulbs went dark simultaneously, then threw the switch. Sort of a phaselocked loop with a human in the loop.
I wanted to put a small incandescent bulb between the clock and the guitar comparator, but considering the amount of bidirectional current required (requiring push-pull emitter followers or some other contrivance), I decided the two LEDs were simpler.
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