# Use of transistors to turn on an LED

## Homework Statement

Hello,
I have got input from comparator op amp which gives HIGH or LOW. Based upon that, one of two LEDs needs to light up. Two LEDs should't be ON at the same time.
Calculations are not needed, only the schematics.

None

## The Attempt at a Solution

I am not really familiar with the transistors, so I used logic gates.

Input = 1
AND 1 | NOR 0

Input = 0
AND 0 | NOR 1

The use of logic gates should be avoided here, thus the use of transistors comes to my mind.

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berkeman
Mentor
Hint -- Use Google Images and search for Emitter-Follower Transistor LED Drive and Common-Emitter Transistor LED Drive (two separate searches). Show us what you find, and tell us which one of those is an inverting configuration and which is a buffer configuration (for NPN transistors).

Hint -- Use Google Images and search for Emitter-Follower Transistor LED Drive and Common-Emitter Transistor LED Drive (two separate searches). Show us what you find, and tell us which one of those is an inverting configuration and which is a buffer configuration (for NPN transistors).

Thanks for looking at the thread, I will be looking into that a bit later, Busy right now.

Original text may be a bit vague. So, to avoid confusion, one LED is always ON. If input is logic 1 , first LED is ON. If input is logic 0, second LED is ON.

NascentOxygen
Staff Emeritus
I have got input from comparator op amp which gives HIGH or LOW. Based upon that, one of two LEDs needs to light up. Two LEDs should't be ON at the same time.
Calculations are not needed, only the schematics.
You have a comparator whose o/p can be taken to be logic HI or logic LO. Can it be somewhere in between HI and LO? You want this to drive a pair of LEDs, with one LED glowing to indicate the comparator o/p is HI and the other LED similarly indicating but on o/p LO. Is there a requirement for buffering, or is it okay to use a comparator that is able to drive the LEDs directly?

You have a comparator whose o/p can be taken to be logic HI or logic LO. Can it be somewhere in between HI and LO?
I am using comparator with hysteresis, I don't think it can be somewhere in between.
You want this to drive a pair of LEDs, with one LED glowing to indicate the comparator o/p is HI and the other LED similarly indicating but on o/p LO.
Yes
Is there a requirement for buffering, or is it okay for the comparator to drive the LEDs directly?
I think it would be okay.

I only had one electronics course so far and it was introductory. In this course we do some introductory circuit designing with a lot of variables ignored like system response or accuracy of measurements. This is the last part of the circuit.

NascentOxygen
Staff Emeritus
Do you know the voltage levels of the comparator's output? What supply voltage/s do you have available?

Do you know the voltage levels of the comparator's output? What supply voltage/s do you have available?

Well, calculating the values of all the components used is the part two of the project, first I need to complete the schematics. I won't actually build this. Project is only done on the paper. Meaning, I can use any part that can be bought from some store or ordered online, every part in existence is available.

NascentOxygen
Staff Emeritus
Have you been given enough hints to be able to finish this now? Just make sure to specify the gate driving the LEDs is of a type that has buffering.

Have you been given enough hints to be able to finish this now?
To be honest, by myself no. No reason for me to lie. Like I said, I know really basic stuff in electronics. But browsing the internet, I stumbled upon some thread stating that using a transistor is not needed and "overkill". Use of transistor here is not obligatory.

And this was provided

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NascentOxygen
Staff Emeritus
Does that look like it will do what you want?

Does that look like it will do what you want?
On the first glance it looked weird, but explanation is provided and there is a video in which it is seen working. I would have to show it to my professor so he could tell me if that solution is acceptable.

NascentOxygen
CWatters