Laser Communication

[shaiqbashir]

Hi guys!

well im posting below two diagrams. The 1st one is of the transmitter and the other one is for the reciever. The laser light will be used as the signal carrier. My question is this that as it is written in the diagram that the condenser mic is to be used. Now is it correctly written?

what will happen if i use a electret mic here. I m thinking of using an electret mic with a preamplifier using TL071 opAmp. will it work?

please do tell me about the technical defficiencies of this circuit if exist.

Thanks a lot and Good Bye!

 

[audioguru2]

Hi Shaiq,
Your receiver circuit is nice and should work well. It uses negative feedback around its transistors to cancel differences between transistors.

Your transmitter won't work with a condenser mic, it must be an electret mic. Many people confuse the two.
The transmitter will kiss the laser diode goodbye. A laser diode's brightness depends on its current, like an LED. In a laser pointer, the internal resistance of its small button battery cells limits the current. Your circuit is a voltage source to the laser diode, with 3.8V applied to it. The current will be extremely high. Change the laser diode driver circuit to a variable current source. A PNP transistor with an emitter resistor and negative feedback from the emitter resistor to the opamp for variable control of the current.

The transistor in the transmitter probably won't work because it doesn't have DC negative feedback provided by an emitter resistor to ground. The BC548 transistor has a current gain range from 110 to 800. If the transistor's current gain is 800, then its base current of about 4.6uA will try to result in 3.68mA of collector current. The transistor will be saturated and its collector voltage will be zero. You can calculate the collector voltage for other values of current gain, then select a transistor that has lower gain. Or you can add DC negative feedback:
Add a 1k resistor in series with the emitter to ground to provide DC negative feedback. Bypass the resistor with a capacitor to keep the transistor's high AC voltage gain. Then calculate the transistor's collector voltage with transistors having a current gain from 110 to 800 and it will be fine.

The transmitter's transistor has a very high voltage gain since it doesn't have AC negative feedback. Therefore it will be very distorted. 

 

[shaiqbashir]

Hi audioguru!

thanks for that valuable information. Now here i want to point out one thing that where is the laser diode in this circuit?

there is a simple laser torch 3V powered. So if the circuit is supplying it 3.8V then i think it should work. Please explain this point.

Secondly! i will be very very thankful to you if you would tell me the difference between DC negative feedback and AC negative feed back.  ???

Thanks a lot once again!

Regards,

 

[audioguru2]

Now here i want to point out one thing that where is the laser diode in this circuit?

It is drawn looking like an LED.

there is a simple laser torch 3V powered. So if the circuit is supplying it 3.8V then i think it should work. Please explain this point.

A Laser pointer uses a 2.2V Laser diode with a resistance of about 38 ohms. The resistance of the button cells is about 24 ohms. Therefore the total resistance of 62 ohms limits the current from the 3V battery to 13mA. My pic shows the spec's of a typical laser diode. Its absolute max current is only 20mA. Your circuit would apply 3.8V to it, therefore its current will be 42mA. Its peak current in your circuit will be 126mA!

Secondly! i will be very very thankful to you if you would tell me the difference between DC negative feedback and AC negative feed back.View attachment 38216