I want to see if there is a frequency in the air.I will take the frequency with a phototransistor.I will make a bandpass filter and i want to have high output(one) when the frequency exists or low output(zero) when the frequecy dosn't exists.The frequency is between 0 - 5 volts.Duty cycle 25%.If i put a capacitor in the output from the filter will i take the average (2,5Volts) from the square frequency or i will take zero because the the capacitor will go to ground(see the schqmatic).The frequency varies from 1 to 40 kHrz.Thank you in advance.

untitled.GIF

 

A phototransistor responds to light. AFAIK lumped circuit components like resistors, capacitors, and inductors are ineffective in dealing with frequencies that high (Red Light with a wavelength of 650 nm is approximately eqivalent to 461 THz). At that frequency a 1 uH inductor would have an impedance of 2900 Megohms and a 1 pf capacitor would have an impedance of 345 microOhms. Pretty tough to make a passive filter there let alone an active one.

The output of a phototransistor will be high if the incident light is in a band of frequencies around some center frequency. In photography filters are done with colored glass or plastic. You might want to tell us what you are trying to do rather than telling us how you think you should do it.

Let's start with the frequency range of interest.

 

My son (with my help) built a "light listener" that was described in a Radio Shack book - maybe 15 yrs ago or more. It was a photocell connected to an op amp (741) followed by an LM386 audio amplifier. With it you could listen to the hum or buzz of various lights - the output of an IR remote control - the "swish" of a candle flame. Maybe that is what the OP wants.

__________________
stevez

 

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the phototransistor i have detects 880nm wavelength (infrared).I send a frequency in this wavelength and i can detect this frequency.In front of the phototransistor i have a glass tha allow only infrared pass.The problem is that i want to know only if this frequency exist or not.So i want a demodulator (a simple one) that if the frequency pass through the filter the output is one(high) but if the frequency isn't in the space the output is zero(low).If i let the frequency pass to the transistor the problem is that the transistor starts to switching in this frequency.I want the transistor swith only if the frequency exists.Sorry for the long reply

 

I understand but i don't want to hear the frequency i only want to detect that this frequency exists in the space.If this frequency pass throug the filter then i want the transistor only to switch not to tswitch to this frequecy.Inagine a led that lights only when the frequency exists not swithing in this frequency.So is i put a capacitor is this possible to achive this?

 

hi,
Have a look at this:

http://www.farnell.com/datasheets/46674.pdf

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

You may also want to consider the old LM567. See:
http://www.national.com/ds/LM/LM567.pdf

You can set the detection frequency for the modulation you want and get a high/low output. John

 

I think what you are asking for is called an Envelope Detector. This is a simple circuit using a diode, a capacitor and a resistor to detect that the frequency is present after the filter.

__________________
RadioRon

 

OK. You want to detect the frequency of modulation which is carried by the light source. That is an entirely different proposition. If an IR source (880 nm) is modulated at 38 kHz then building a simple fileter to detect 38 kHz can be done. The suggestion to use a 567 is a good one, and you can also use a phase locked loop. You set the center frequency to the modulation frequency and you set the lock range to the desired bandwidth. If the frequency is present the loop will lock. If the frequency is not there the loop will drop out of lock. The lock indicator is the logic signal you require.

 

I have use this detector but the problem is that i send continous data (only a frequency).When this ir module detects continous it thinks that is noise and it is trying to clear the noise.So do you have any other suggestion?

 

Can you send a schematic.Thanks

 

I check LM567.I think it's ok thanks but in the data sheet doesn't have lots of circuts that explain the use of the lm567.Have you any idea where i have to find shematics?Can i use the LM567 as a filter also?Thanks

 

I see the datasheet but i don't understand the use of LM567.Do you have any other schematics to understand the use?Can i use it as a filter?thanks

 

hi,
You say continuous data, do you mean a simple continuous, unmodulated IR beam.
Post a sketch of the project so that we can see what you are trying to do.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Can you google for a schematic? Please try harder to find this yourself.

For example, if you google "envelope detector" and then take the first choice given, you will see a schematic.

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RadioRon