How to modify this circuit?

[erique82]

How do i modify this circuit(s) so that i could obtain the wavelength on the oscilloscope? Or rather, how do i connect this circuit to the oscilloscope to obtain and analyse the wave?

 

[audioguru]

Hi Erique,
You should be able to see the transmitter's output on a 'scope.
The transmitter frequency-modulates a 44kHz carrier. The Gwahaha receiver tries to amplify the 44khZ but the phototransistor, opamp and your hearing don't go that high. The receiver needs a fast photodiode and an opamp with a much higher bandwidth than just 9kHz like the old 741. A modern opamp like a TL071 has a 100kHz bandwidth and would work fine as a transimpedance amp. The receiver also needs an FM demodulator, not a differentiator.

 

[akg]

the 555 produces PWM signal. so we should use an integrator instead of diff: in the demodulator section.

 

[audioguru]

the 555 produces PWM signal. so we should use an integrator instead of diff: in the demodulator section.

I disagree. The 555 is freerunning, not triggered by another oscillator.
National calls this circuit a "Pulse Position Modulator", but its output sure looks like FM to me.

 

[akg]

ok.. 'audio' .. the first pic is definitely a pwm .. but in the second the OFF times are fixed right ?.only the ON time varies .. so is it eqvivalent to fm ?

 

[audioguru]

I think a CD4046 phase-locked-loop IC would demodulate its FM pretty well.

 

[Roff]

Both the receiver circuits need to have either dual supplies or a voltage divider to satisfy the common mode input voltage requirements of the op amps.

 

[audioguru]

Both the receiver circuits need to have either dual supplies or a voltage divider to satisfy the common mode input voltage requirements of the op amps.

Good point! :lol:
If a voltage divider is used then the output of U1 in the Wahaha receiver needs a coupling capacitor to feed the volume control. Or else you won't hear the 40kHz. :wink:
Even my little dog can't hear that high. Bats?

 

[erique82]

Let's say I'm removing the audio input for the transmitter. Will I still obtain 44kHz on the scope? I think I can just play around with the capacitors and resistors to obtain an acceptable frequency. Instead of a speaker, i'll just connect the output of the receiver to the scope instead.

 

[audioguru]

The laser diode might melt if you change the duty-cycle of the 555 that drives it without any current-limiting.
The receiver won't have an output with the opamp's pin 3 connected to pin 4.
Most of the projects on that guy's site don't work. Now his Pentium2 server doesn't work.

 

[erique82]

Nth too technical plz...

This driver circuit is actually designed for the first receiver. If i wanted to hook it up to a 100Mhz, what op amps should i use? Are there any major modifications needed for the receiver as well? I needed to get the waveform on an oscilloscope so that i could analyse the waveform.

 

[audioguru]

Re: Nth too technical plz...

This driver circuit is actually designed for the first receiver. If i wanted to hook it up to a 100Mhz, what op amps should i use? Are there any major modifications needed for the receiver as well? I needed to get the waveform on an oscilloscope so that i could analyse the waveform.

Please save and attach schematics as a clear GIF or PNG file type, instead of a fuzzy JPG that is used for photographs.

I don't know any opamp that works as high as 100MHz. A TL071 or TL081 works as high as 100kHz, but won't properly amplify a triangle waveform above about 10kHz. A TL072 and a TL082 are dual 100kHz opamps.

Since the 1st opamp has its non-inverting input grounded, then it needs a dual supply.

R8 in the 2nd opamp circuit has a value so high that it limits the max laser diode current so low that it might not work. The 2nd opamp circuit will work with a single or dual supply.

The receiver circuit doesn't show how its photodiode is connected so I don't know if it will work. It also has the non-inverting inputs of the opamps grounded so also needs a dual supply. A TL074 or a TL084 are quad 100kHz opamps.