How do you design this LED project?

Thread Starter

austinstudent

Joined Jan 28, 2020
24
So I'm not entirely sure how to design this. Could anyone help me with the calculations needed to modulate as well as the circuit design? Please and thanks.
 
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Thread Starter

austinstudent

Joined Jan 28, 2020
24
Show what you've done so far.

Don't expect anyone on this site to do your homework for you.
HI Mr. Dennis,

I apologize I wasn't try to milk anyone of their time. Genuinely not sure how to go about this. I thought it would be similar to the circuit I drew down below. I don't know how to do the calculations for modulation in all honesty, really was seeking help for that.

Thanks


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Thread Starter

austinstudent

Joined Jan 28, 2020
24
Hi Mr. Dennis,

Put some effort in at your suggestion, and I also came up with this. Does this look ok to you?

Thanks in advance,

I appreciate any feedback/critique, I know your time must be super valuable.

IMG_6952.jpeg
 

dl324

Joined Mar 30, 2015
10,541
Put some effort in at your suggestion, and I also came up with this. Does this look ok to you?
In your first circuit, you're not modulating the current in the LED.

In the second circuit, the power source and the signal generator will be fighting each other.

It would be helpful if you told us what types of circuits you've studied. Have you used opamps?
 

Thread Starter

austinstudent

Joined Jan 28, 2020
24
In your first circuit, you're not modulating the current in the LED.

In the second circuit, the power source and the signal generator will be fighting each other.

It would be helpful if you told us what types of circuits you've studied. Have you used opamps?
Mr. Dennis,

Yes I've worked with opamps, again not an expert on circuits so not too sure how that would play into this.
 

dl324

Joined Mar 30, 2015
10,541
Yes I've worked with opamps, again not an expert on circuits so not too sure how that would play into this.
You're required to be able to show DC and AC current independently for your demonstration. Can you think of an opamp circuit that will allow you to do this?

Could you post datasheets for the LEDS?

When is this assignment due?
 

Thread Starter

austinstudent

Joined Jan 28, 2020
24
You're required to be able to show DC and AC current independently for your demonstration. Can you think of an opamp circuit that will allow you to do this?

Could you post datasheets for the LEDS?

When is this assignment due?
Mr. Dennis,

It's due Monday so I have some time. I'm not quite following you there. I'm not sure how to utilize the OPAMP for this. Ya I'll send LEDs, hold on.

Thanks
 

dl324

Joined Mar 30, 2015
10,541
As with most things, there are many ways to implement the circuit you've been asked to design. Pick something you've studied that you think will satisfy the requirements. Try them and post any questions you have.
 

dl324

Joined Mar 30, 2015
10,541
The LED data is for a random red LED:
1580321211980.png

You're going to need to know the forward voltages of the LEDs so you can determine LED currents.
 

dl324

Joined Mar 30, 2015
10,541
Anyone have any thoughts on this?
  1. What is the purpose of the series combination of 100pF and 22 ohms in parallel with the 300 ohm resistor?
  2. What is the purpose of the coupling capacitor for the AC signal?
  3. Why are you using an inverting summing amplifier?
  4. Why are you using different gain for the AC and DC inputs?
  5. You forgot to label circuit ground.
Parts of the problem unclear to me.
  1. Why are you modulating the current instead of simply turning the LED on and off? Is it just a contrived situation to teach you how to do it?
  2. Were you given any requirements for the DC and AC currents?
 

Thread Starter

austinstudent

Joined Jan 28, 2020
24
  1. What is the purpose of the series combination of 100pF and 22 ohms in parallel with the 300 ohm resistor?
  2. What is the purpose of the coupling capacitor for the AC signal?
  3. Why are you using an inverting summing amplifier?
  4. Why are you using different gain for the AC and DC inputs?
  5. You forgot to label circuit ground.
Parts of the problem unclear to me.
  1. Why are you modulating the current instead of simply turning the LED on and off? Is it just a contrived situation to teach you how to do it?
  2. Were you given any requirements for the DC and AC currents?
1/2/3/4 This was something I found online for modulating LEDs. I genuinely had no idea how to do it otherwise, again not familiar with the term. If you know of a similar solution to this and could draw/explain it I'd appreciate it. Again spent quite a few days just googling/asking TAs and not getting other answers. Next monday I just need to show a circuit diagram and explain the design, trust me I have put some time into this project but again not making much progress. Thanks for taking the time to continue this conversation though, that means volumes tbh.
5. My apologies on that

My TA had this to say, not sure if I sent it already.
 

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dl324

Joined Mar 30, 2015
10,541
  1. Why are you thinking you have to drive both LEDs at the same time?
  2. Are you planning to use the same detector for both LEDs?
You should also note that most opamps won't sink/source more than about 25mA.
 

Thread Starter

austinstudent

Joined Jan 28, 2020
24
  1. Why are you thinking you have to drive both LEDs at the same time?
  2. Are you planning to use the same detector for both LEDs?
I assumed you had to do it that way, how would you suggest using the second LED then
Yes, but I believe we are given an additional detector.
 

dl324

Joined Mar 30, 2015
10,541
I assumed you had to do it that way,
If you can talk to your instructor or TA, there's no need to make assumptions. If you couldn't ask questions, then you can simply state your assumptions as part of your solution.
how would you suggest using the second LED then
Since the LEDs have different maximum currents, I'd make two variants of the modulate circuitry and do the experiment once for each emitter/detector combination.
Yes, but I believe we are given an additional detector.
What do you intend to do with the detector results? One LED is visible light and the other in infrared.
 
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