unique problem

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rathin2j

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hello,
i have prepared IR CORDLESS HEADPHONE as a project in my college.my circuit is devided in two parts 1)transmitter 2)receiver.transmitter is connected to audio device through audiotransformer.and on receiver side i have used "ir photo transistor"as receiving device.a transistor(BEL187)is used to drive the audio output i.e.head phone.now the problem is that when i turn ON the circuit it doesnt work.means i dnt here any sound on my RECEIVER.but when i touch one RESISTOR the sound starts coming.in the following link my circuit is there.and the resistor R7(470kΩ)in the receiver circuit is the resistor by touching which the sound starts coming.please help me.i dont know why it is not working but i think it is not a systamatic fault.do u think it is fault in desiging the circuit?pl.tell me franqly. thank u very much**broken link removed**
 
if you literally touch a resistor on your circuit and it starts working, sounds like a short or bad connection to me.

try breadboarding the circuit, checking connections with a continuity tester to be sure, and see if it works then.
 
thank u very much for ur precious reply.now i got the output the problem was lying in the AUDIO TRANSFORMER.the shopkeeper cheeted me.he was not having the AUDIO X'mer so he gave me general supply purpose X'mer.thats why the output was not sufficient to transmitter.
 
I have seen the extremely simple circuit as an old project on a website in India.
Isn't the sound extremely distorted?
Isn't the range only 1m?
 
yes

hello,
yes u r right it is very old one.but it gives range of 5 meters.but now i have improved my circuit.i have made following changes.
i have replaced
audio X'mer with Op-amp.
darkington pair amplifier with power anplifier ic(LM386)
head phone with a speaker(as impedance was not matching)
IR photo transistor(germenium) with sillicon(iron cap).
now u want belive but i m getting range of 10meters,unbeliveablelly.i have attached the theoryotical diagrams.and thanks for the reply
 

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  • IR photo receiver Gen-II.jpg
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May we see the receiver, please?

I like the second version if I understand it

You can bias the diode to be emitting all the time to achieve lower distortion, or is it just a gain adjustment?
 
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The original very old tx and rx circuits are horrible.
There is an LED in the audio path of the transmitter so its swing is extremely low. They used a transformer to try and "boost" the signal.

The receiver is just a few transistors then they put DC through the headphone to burn it out and cause distortion.
 

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soryyy

hellooooo,sorry for the mistake in giving the schematic.i m ataching the receiver circuit with this.
 

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  • Copy of IR receiver gen-II.jpg
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yes diode can b biased for that and it is but due to that whe v keep the transmitter near to the receiver,than it give a strill sound.
 
The photo-transistor needs to have a fairly high value collector resistor and a very high impedance load to have high gain. Your preamp opamp is an inverting type with a low input impedance. The opamp should be a non-inverting type with a high input impedance.

The lousy old 741 opamp is too noisy (hissss) to be a preamp. Use an audio opamp instead like a TL071.

If the preamp is a non-inverting type so that the photo-transistor has high gain then the capacitor at pin 1 and pin 8 of the LM386 amplifier is not needed to boost its gain then it won't have hiss.

Of course you have acousical feedback howling (shrill sound) when the microphone can hear the speaker and the sound goes around and around in the amplifiers and in the air.
 

This one is the original article **broken link removed**
some years before I have planned to build but couldn't!
Its a headphone burning circuit, me also think so.

Another one **broken link removed**
designed in much better way, at least you can use the receiver section.
 
ok,thanks for the tip.i will try it and will share the experianc.but i would like to ask that my PCB is ready.so can i replace my old OP-AMP(741) with new one(TL071)in the same circuit.will it cause any damage?
 
The TL071 has the same pins and functions as a lousy old 741 opamp.

The inverting type of opamp circuit has a low input impedance (10k in the music receiver) which loads down the output of the photo-transistor and reduces its output. The photo-transistor has a 10k collector resistor instead of 100k so its gain is low anyway.

Because the gain is low, the LM386 has a capacitor at pin 1 and pin 8 to boost its gain. Then the hiss at its output is the noise amplified a lot from the LM386.

The circuit is a mess of errors.
 
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