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amplify the power (infrared)


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Hi Ryan,
Your opamp doesn't have its pin 4 connected! It also doesn't have a supply bypass capacitor.

Your circuit doesn't have much gain:
1) The collector load resistor for the phototransistor's value is much too low.
2) The inverting opamp's input impedance of 10k is loading-down the phototransistor.
3) The output resistance of the phototransistor is reducing the gain of the inverting opamp.
4) An opamp type isn't spec'd that operates well at 38kHz.

If your transmitter works, the range is reasonable and the phototransistor isn't picking up stray light or IR then my modified circuit should work well.

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Your IR receiver probably has a strong output at 38kHz now but you can't hear a frequency so high. Therefore the receiver needs to feed an AM detector diode and filter to recover the modulation.

The transmitter circuit has two very important resistors that need adjustment. See my sketch.

You are using Amplitude Modulation. Noise is fluctuations in amplitude that the circuit is designed to receive. You should be using FM instead, for low noise.

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Your IR receiver probably has a strong output at 38kHz now but you can't hear a frequency so high. Therefore the receiver needs to feed an AM detector diode and filter to recover the modulation.

The transmitter circuit has two very important resistors that need adjustment. See my sketch.

You are using Amplitude Modulation. Noise is fluctuations in amplitude that the circuit is designed to receive. You should be using FM instead, for low noise.



two resistors?
the AM detector means the envelope detector?
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should I amplify the signal again?(tansmitter and receiver)

The two resistors that I boxed with red in my receiver circuit reduces the gain. They should be adjusted so that the circuit doesn't overload with too much signal.

Did you make an AM detector (envelope detector) to demodulate your AM modulation?
What range are you trying?
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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

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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.

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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?
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