According to the input current to turn On/Off LED.

Thread Starter

Tejasvi471

Joined Jun 9, 2020
35
Hello there,

Is this the best way to design this circuit
its designed according to this table

INPUT
OUTPUT
0ma​
LED OFF​
1ma​
LED ON​
2ma​
LED ON​
3ma​
LED ON​
4ma​
LED OFF​

1659089295527.png

Thanks
 

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BobTPH

Joined Jun 5, 2013
5,489
You did it in LTSPICE. what does the simulation say?

What I see is a current source that feeds a resistor and two PN junctions. Most of the current will go through the PN junctions, making the resistor redundant. Both transistors will be turned on, and the LED will be off always, since there is no voltage left across it.
 

crutschow

Joined Mar 14, 2008
30,454
It's obviously not the best way (or even an acceptable way) as your simulation shows.
Tell us your understanding of the circuit.
 

Irving

Joined Jan 30, 2016
2,991
Its do-able with the addition of 2 more resistors but its poor circuit design as it won't be stable over temperature and will need transistor selection. Here's the trace to show what's possible:

1659109880132.png
 

dcbingaman

Joined Jun 30, 2021
682
That is by far not the best way to accomplish the goal. A window comparator IC would work much better. Yes it has a lot more parts but is much more reliable and by changing R1, R2 and R3 you have better control of the window area. You could also replace the comparator IC's with one 'Window' comparator IC and most likely get down to one IC chip and no transistor needed.
 

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Last edited:

Thread Starter

Tejasvi471

Joined Jun 9, 2020
35
You did it in LTSPICE. what does the simulation say?

What I see is a current source that feeds a resistor and two PN junctions. Most of the current will go through the PN junctions, making the resistor redundant. Both transistors will be turned on, and the LED will be off always, since there is no voltage left across it.
I simulated and it is running, your analysis that both transistors would remain ON seems right, but adding series resistor to current source and taking output from it turns it into voltage source.

1659333765007.png
 

crutschow

Joined Mar 14, 2008
30,454
Do you want a real circuit that will work well in practice, or just something you can simulate?

If you want a real circuit, what would be the source of the input current?
 

crutschow

Joined Mar 14, 2008
30,454
I want real working circuit
Then a transistor circuit will not give very stable or accurate operation.

Below is the simulation of a typical window comparator circuit using the LM339/393 comparator IC, similar to that posted by dcbingaman in Post #7, to give stable trip points:
R6 converts the 0-4mA phototransistor signal into 0-2V.
This voltage is detected by the comparator to turn the LED on (red trace) at 1mA and off at 4mA.

1659535537093.png
 
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