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Need help on transmiter and receiver

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holmeschang

New Member
I am still a first year student in E & E...first time looking at this...

I had been given a project but nothing detail is stated...Only a circuit diagram...But after looking at it, suppose to be a transmitter and receiver...But I am not sure this circuit works or not or what exactly the output sounds like...Can anyone help me???:( It due soon...I working on it almost a week...

9096-4059279611_552c5a0eb5_o.jpg
 

MikeMl

Well-Known Member
Most Helpful Member
Won't work if pin 4 of left opamp is grounded...
 

MikeMl

Well-Known Member
Most Helpful Member
Down load a data sheet of the LM386; yours is missing some connections
 

Chippie

Member
which part of circuit can step up the volume?:confused::confused::confused:

For starters,R5 in the right hand circuit adjusts the volume level, but you need to sort out the feedback components that are missing from the LM 386
 

mbarazeen

Member
the circuit is for converting audio into lgiht signals(modulate) then the receiver convert it back to audio and gives the output to the speaker. you can say a short of line of sight wireless link.
741 needs dual supply, you can try other alternatives, the audio from mic is amplified by the opamp and modulates the bias of the transistor. so the current through the LED varies thus the light changes.
the 2nd crcuit works in reverse to reproduce the audio.
 

audioguru

Well-Known Member
Most Helpful Member
Your microphones are not connected correctly.
Your opamps are inverting which makes their input impedance too low. They should be non-inverting then they will have a high input impedance.
The LM386 audio amplifier is missing important parts.
 

holmeschang

New Member
<Your microphones are not connected correctly.
Your opamps are inverting which makes their input impedance too low. They should be non-inverting then they will have a high input impedance.
The LM386 audio amplifier is missing important parts. #7>

inverting opamps? then how can I make it non-inverting? I had successfully run the circuit on the bread board but the outpt somehow very low...I man the volume is very low...

About the LM386, I look thru the datasheet for missed component... it said that connect a resistor and capacitor from pin 1 to pi 8 can varied the Gain, what does it mean?

Pin 7 needed to connect a capacitor to ground, what is BYPASS mean? How it help my circuit? What happen if I did not connect them?

Sorry...I know it sound very easy question but I'm really just beginner and firt=st time touch this field...I will be grateful if someone can explain for me... :)

By the way...Thanks for the replies...they help a lots!! :D
 

audioguru

Well-Known Member
Most Helpful Member
=holmeschang;807354inverting opamps? then how can I make it non-inverting? I had successfully run the circuit on the bread board but the outpt somehow very low...I mean the volume is very low...
Of course the output is low. The low input impedance of your inverting opamps are shorting the high impedance signals to ground.
You need to learn about basic opamp circuits to see the difference of inverting and non-inverting circuits.

About the LM386, I look thru the datasheet for missed component... it said that connect a resistor and capacitor from pin 1 to pi 8 can varied the Gain, what does it mean?
The gain is 20 but can be increased to 200 if a capacitor is connected between pin 1 and pin 8. A resistor in series with the capacitor will allow the gain to be selected from 20 to 200.
But your circuit is missing the extremely important 0.05uF capacitor in series with a 10 ohms resistor from the output to ground to prevent high frequency oscillation.

Pin 7 needed to connect a capacitor to ground, what is BYPASS mean? How it help my circuit? What happen if I did not connect them?
The bypass capacitor at pin 7 is used to reduce hum if a cheap power supply is used. If the amplifier does not produce hum then the capacitor is not needed.
 
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holmeschang

New Member
Of course the output is low. The low input impedance of your inverting opamps are shorting the high impedance signals to ground.
You need to learn about basic opamp circuits to see the difference of inverting and non-inverting circuits.

So to change the opamp circuit to non-inverting circuit, does it mean I have to change the pin 3 connection and pin 2 connection of the 741 in reverse way? I,m not so sure bout inverting and non inverting works...can provide me a link? That clearly shows the difference and how should it work on my opamp...:) thx again...
 

audioguru

Well-Known Member
Most Helpful Member
Every article and tutorial about opamps show the difference of an inverting circuit with a low input impedance and a non-inverting circuit with a high input impedance.
 

holmeschang

New Member
I had complete the circuit and the sound emmited is somehow not so clear. Can I have a clear explanation the variable resistors' role? R2,R5 in transmitting circiuit and R5 in receiver circuit... Thanks...
However, I still not clear how the circuit work base on replies of you all...can explain more detail?? Thanks again.
 

audioguru

Well-Known Member
Most Helpful Member
The gain of the transmitting opamp is calculated by the value of R2 divided by the value of R1 plus the impedance of the mic which is about 3k ohms (if the electret mic has two wires). But the value of R1 is too low so it loads down the output of the mic.
If the value of R1 is higher then the gain is reduced. The inverting opamp with the low input impedance should be a non-inverting amplifier with a high input impedance.

R5 in the receiver is its volume control. if it is set too high then the LM386 little power amplifier clips the signal because its max output is only 0.45W when it has a 9V supply. Clipping causes severe distortion.
 
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