amplify the power (infrared)

[ryanleung]

hello audioguru~~
I have are few question about my project.
1.  I constructed your receiver already.  And I amplified the signal using opamp, then it was connected to the audio amplifier.  But the sound is very weak.  I try to adjust the volume but the sound is also weak.  I don't know how to increase the signal.  And the maximum distance only have about 5cm that is very short.
2.  I constructed the envelope detector (R=40ohm, C=0.1uf) and connected to your receiver output but the envelope detector output have not output signal.

thanks

 

[audioguru2]

I constructed the envelope detector (R=40ohm, C=0.1uf) and connected to your receiver output but the envelope detector output have not output signal.

No Ryan.
A resistor and capacitor is not an AM envelope detector. Also, a 40 ohm resistor and a 0.1uF capacitor is like a dead short at 38kHz to the output of an opamp. The minimum load for most opamps is 2k ohms.

An AM envelope detector is a diode followed by a filter capacitor. Look at the circuit of any AM radio to see.View attachment 38864

 

[ryanleung]

how about the distance, the signal is very weak

 

[audioguru2]

how about the distance, the signal is very weak

Did you try it with the AM envelope detector circuit I sketched?
I think you need to use an oscilloscope to see if the transmitter and receiver parts are adjusted properly for full modulation.

 

[ryanleung]

Ok, I will try it.

thanks

 

[ryanleung]

The red circle voltage measured is not 5.9V, it is 9V same as the VCC
Is it no current though the IR LEDs?

View attachment 38866

 

[audioguru2]

The red circle voltage measured is not 5.9V, it is 9V same as the VCC
Is it no current though the IR LEDs?

Correct. Then the voltages for Q2 must also be wrong.
I notice now that the voltage divider of R4 and R5 has its values too high for 1.9V on the base of Q2, when Q2 has 31mA of current in the LEDs. R4 should be 5.6k and R5 should be 1.5k. The resistor that I added in series with the base of Q2 must have its value reduced also.
But then the total resistance might be too low for Q1 to drive. Try it.

How do you know if Q1 is oscillating and what frequency it oscillates at?

 

[ryanleung]

The circuit can sucessfully received the signal.  If I want to add the Automatic Gain Control(AGC) to the receiver, how can I do????

 

[audioguru2]

The circuit can sucessfully received the signal.

 

[ryanleung]

How can i increase the transmit distance?
now, it can work 6-8 inch only

 

[audioguru2]

How can i increase the transmit distance?
now, it can work 6-8 inch only

Maybe the phototransistor is being saturated by visible light. Is the range better when it is dark?

 

[ryanleung]

Maybe the phototransistor is being saturated by visible light. Is the range better when it is dark?

Yes

And I have a some question to ask you.  My oscillator can not adjust the frequency to 38kHz.  I use 220nF capacitor and 180uH inductor and I can not decrease the frequency anymore.