amplify the power (infrared)

[ryanleung]

I would like to use this circuit to transmit the audio signal.

[audioguru]

Hi Ryan,
It looks like a 1MHz oscillator that is amplitude-modulated by the input signal.
A photo-diode or especially a photo-transistor doesn't work at such a high frequency as 1MHz, so I recommend using a lower frequency such as the 38kHz to 40kHz used by remote-controls.

Please attach the schematic of the receiver for us to see how to improve it.

[ryanleung]

Sorry, this circuit is old version.  I changed the frequency to the 38kHz already.  But the distance between the transmitter and the receiver is very short.  How can i increase the distance?  increase the power? I don't know how to increase it.

[Staigen]

Hi ryanleung

Maybee you should switch to FM instead, and at the same time have it pulsed at a somewhat higher current. A schematic would be fine.

//Staigen

[ryanleung]

but my project requirement must use AM to transmit the audio signal.  How can i amplify the power and increase the distance???

[audioguru]

What is the DC modulation input levels?
Please attach the schematic of the receiver.

[ryanleung]

R1 = 470ohm; C1 = 0.1uF; R2 = 1kohm; R3 = 10kohm; IC = LM324

[audioguru]

Hi Ryan,
Your LM324 doesn't have its negative power supply pin 4 connected to a negative supply, no wonder it doesn't work. If pin 4 is connected to ground then the opamp would be an AM detector.
Also, the LM324 has crossover distortion and a very lousy high frequency response, it drops above only 5kHz. I use TL071 single, TL072 dual and TL074 quad low-noise wide-bandwidth opamps for nearly everything because they are inexpensive, have very low distortion, have low noise and operate well to 100kHz. If a TL074 quad opamp is used in your circuit then it would need to have its inputs biased and a diode added in its negative feedback loop.

[ryanleung]

I connected the pin4 to ground already and the signal was amplified.  The problem occur at receiver or transmitter?  Or the output power of the transmitter insufficient?

[audioguru]

Hi Ryan,
Due to a severe impedance mismatch, your IR receiver has an enormous loss.

R1 is the high value of 470k so that the phototransistor has a high gain. But the value of R2 is so low at only 1k that the output of the phototransistor is nearly shorted.

The opamp should be a type that works well at 38kHz, be in a non-inverting configuration so that its input bias resistor can be as high as 2.2M so the phototransistor's high impedance output is not loaded down. To avoid having a negative supply, the opamp should be biased to half-supply. The opamp can have as much gain as you want and can feed a passive AM detector diode or an active one that uses an opamp.

Your choice of using AM instead of FM for modulation means that your receiver will pickup mains hum from incandescent and fluorescent lights. 

[ryanleung]

1. can I change the value to R1 =100kohm ,R2=2.2kohm,R3=1Mohm and C=0.01uF???
2. can i use the TL072?
3. should I construct the band pass filter to filter the fluorescent light and the noise?

[audioguru]

1. can I change the value to R1 =100kohm ,R2=2.2kohm,R3=1Mohm and C=0.01uF???

A 2.2k load on a 100k collector resistor is still a huge attenuator. My suggestion of using a high impedance non-inverting opamp allows the photo-transistor to have its full gain.
I just noticed that R1 was 470 ohms which is much too low. It should be a much higher resistance for good sensitivity and the input impedance of the opamp that it drives should have an impedance at least 5 times higher.

2. can i use the TL072?

Yes, it works much better than a low-bandwidth LM324 at 38kHz. But the circuit needs to be changed for it to work.

3. should I construct the band pass filter to filter the fluorescent light and the noise?

You could make a highpass filter to reduce the low frequency mains hum but its harmonics would still pass and sound like a buzz. New compact fluorescent lights have a 38kHz oscillator that would cause severe interference to an AM modulated 38kHz system.

[windoze killa]

You could also try using more IR LEDs to increase the amount of radiation.

[ryanleung]

I used two IR LEDs already.

[audioguru]

Provide a high resistor value for the phototransistor's collector resistor, a high impedance load for it from the high frequency opamp's input impedance and more gain from the opamp.
Then the range will be plenty with your two IR LEDs.

[ryanleung]

how to check the opamp which can satisfy the 38kHz operation?

[windoze killa]

I used two IR LEDs already.

Try 4.

[audioguru]

how to check the opamp which can satisfy the 38kHz operation?

I already told you that the LM324 is lousy and showed a graph from its datasheet.
I also said that the TL07x family of opamps performs well up to 100kHz.
The TL07x family of opamps is low noise, low distortion, wideband and inexpensive.

Here is how the opamps compare:

[audioguru]

Doubling the number of IR LEDs won't make much difference when the receiver circuit results in such a high loss instead of a high gain.

[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?

[audioguru]

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.

[ryanleung]

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

[audioguru]

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?  :)