amplify the power (infrared)

[ryanleung]

ok
thank you~~~

 

[ryanleung]

oh....sorry.....
I have some question about my transmitter.
I constructed the receiver already and I used the remote control to test the receiver.  The receiver can receive the signal.  If I used my transmitter to transmit the AM signal, the receiver can receive the signal but the signal is small.  how can i modify the transmitter?  the input signal only have about 1Vp-p, should i amplify the signal first?

View attachment 38791

 

[audioguru2]

I inverted the black background of your schematic.
The value of R7 in your transmitter is so high that the modulator's DC input voltage needs to be -144.5V to bias Q2 into a linear operating region. Also, a very high modulating voltage swing would be needed.

If R7 is 39 ohms and the modulating source is capable of driving such a low impedance, when the modulating input DC voltage is 0V then Q2 would be biased half-way. An input signal swing of 2.4Vp-p would fully turn on and off the IR LEDs. The carrier signal from Q1 to Q2 would need attenuation because then Q2 would have some gain.

I don't see how modulating the emitter of the transistor creates amplitude modulation. It seems that the carrier would just have its top or bottom clipped by the emitter's modulation.

View attachment 38793

 

[ryanleung]

umum......
How can I modify it now?
Thanks

 

[audioguru2]

I think the base of the oscillator should be modulated then it can connect to a common-emitter LED driver transistor.
An oscilloscope will show you what the transmitter is doing.

 

[SM2GXN]

Hi audioguru!

I think it will create AM and two sidebands if you inject a sine wave into the emitter,
if I don't remember wrong just about same configuration is used in RF but as collector load a tuned L/C circuit.
What do you think audioguru? 

 

[SM2GXN]

By the way audioguru, have ever tried to simulate an AM circuit in switchercad?

 

[ryanleung]

I think the base of the oscillator should be modulated then it can connect to a common-emitter LED driver transistor.
An oscilloscope will show you what the transmitter is doing.

sorry, I don't know your mean. can you draw a circuit to me, thanks!!!

 

[audioguru2]

This AM modulated IR transmitter circuit should work:

View attachment 38794

 

[audioguru2]

By the way audioguru, have ever tried to simulate an AM circuit in switchercad?

No, ever since I heard how wonderful FM broadcasts sound, I gave up with the narrow-band, distorted and full-of-interference sound of AM broadcasts.

 

[ryanleung]

hello audioguru.
My AM transmitter output signal is 9.2Vp-p. And I changed the R1, R2, C and R3 to 2.7kohm, 10kohm, 0.01uF and 1Mohm.

View attachment 38852

 

[audioguru2]

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.

View attachment 38854

 

[ryanleung]

Hello audioguru,
I constructed your transmitter and the receiver circuits already.

View attachment 38857

View attachment 38858

 

[audioguru2]

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.

 

[ryanleung]

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?

 

[audioguru2]

two resistors?

Sorry, I forgot to attach my sketch. The resistors are boxed in red.

the AM detector means the envelope detector?

Yes. Your receiver circuit needs one. View attachment 38859

 

[ryanleung]

should I amplify the signal again?(tansmitter and receiver)

 

[audioguru2]

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?

 

[ryanleung]

The resistor is 40ohm and the capacitor is 0.1uF. The frequency is 38kHz.
Is it right?

f=1/(2*pi*R*C)

 

[audioguru2]

The resistor is 40ohm and the capacitor is 0.1uF. The frequency is 38kHz.
Is it right?

f=1/(2*pi*R*C)

Yes, the capacitive reactance of a 0,1uF capacitor is about 40 ohms at 38kHz, but which resistor and which capacitor?