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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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## Discussions

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

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

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

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

Steve

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?

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

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?

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

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

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

Steve

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