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how to use a lm2907 to make an IR frequency reader? Answered

i want to use a lm2907 to make an IR frequency reader so that a multimeter reads the frequency from the output of lm2907

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steveastrouk (author)2011-06-26

If I were you, I'd put a pre-scaler on the input - divide the input by 4 by passing it through two J-K flip flops, like a 4027 to make the input frequency about 7 kHz, assuming an input of 30kHz.

From the data sheet for the 2907.

Vout = Fin x Vcc x R1 x C1

Lets let C1=1nF.... Let Vcc = 9V and Vout =2.5V at the frequency of interest.
2.5/7500/9 = R1 x C1
Make C1=1nF (as good as any)
Therefore R1=36K


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user

That could be clearer. The input from the IR sensor would be 30K, it gets divided down by 4, to 7.5 K at the input of the 2907.

Steve

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top.boy (author)steveastrouk2011-06-26

could you please draw a fast schematic for me you could send it by mail

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steveastrouk (author)top.boy2011-06-26

Just use the "minimum component tachometer" circuit in the datasheet, and change the 100K/0.1uF for 36K and 1nF.

Steve

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top.boy (author)steveastrouk2011-06-26

and change the "variable magnet pickup" with the IR reader?

and another thing : if i use a phototransistor and i wrap it with negative film(from old cameras) will it recieve IR only and not ambient light?

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steveastrouk (author)top.boy2011-06-26

Yes, put the IR sensor in where the magnet went. Don't worry too much about ambient light on the IR detector. You can always capacitively couple it to the tachochip.

Steve

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top.boy (author)steveastrouk2011-06-26

because i live in egypt i can only afford 2 kinds of detectors the first detects only 38Khz waves
and the second detects even poooor ambient light so is it a good idea that i wrap it with negative film?

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steveastrouk (author)top.boy2011-06-26

Try it !

Where are you in Egypt ? Cairo ?

Steve

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top.boy (author)steveastrouk2011-06-26

yes i'm in cairo
can we chat? i find this faster

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steveastrouk (author)2011-06-26

Use the datasheet and some of the application notes in there.

How interesting we have the nearly identical question from two people in a couple of hours.

A school project ?

Steve

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top.boy (author)steveastrouk2011-06-26

first: i saw the data sheet but i didn't understand

second : i'm the one who wrote the other question but this is before i knew the lm2907 and there is an error in the site so i'm messam and top.boy in the same time

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steveastrouk (author)top.boy2011-06-26

The only trouble with a 2907 is setting the frequency range of interest. What are you trying to read ? Something around 30kHz ?

Steve

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top.boy (author)steveastrouk2011-06-26

i don't know i want to make a TV remote jammer i know the circuit but it didn't work so i want to read the frequency out of the IRLEDs to increase or decrease the resistance

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