# Can I make a circuit that responds to the number of volts in the input ?

#### Michael George

Joined Feb 8, 2015
53
Hello everybody, Would you give me a hand ?
I need to make a circuit that has Inputs and outputs. I assume that the circuit has 3 LEDs as outputs.
1- When I connect the Input to 1 volt the first LED will light.
2- When I connect the same Input to 2 volts The second LED will light, In this case it doesn't matter if the first LED is on or off.
3- When I connect the same Input to 3 volts The Third LED will light, and also I don't care about the first and the second LED.

The voltage step should not be 1 volt only, It may be 2 volts or any value. I mean if I connect 2 volts the first LED will light. If I connect 4 volts the second LED will light and so on..

Is it possible to make this circuit using transistors ?
What is the schematic of the circuit ? ( Just give me a link or a reference that I can learn how to make this circuit )

#### bertus

Joined Apr 5, 2008
21,896
Hello,

Have a look at the LM3914 chip.

Bertus

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#### GopherT

Joined Nov 23, 2012
8,012
Hello everybody, Would you give me a hand ?
I need to make a circuit that has Inputs and outputs. I assume that the circuit has 3 LEDs as outputs.
1- When I connect the Input to 1 volt the first LED will light.
2- When I connect the same Input to 2 volts The second LED will light, In this case it doesn't matter if the first LED is on or off.
3- When I connect the same Input to 3 volts The Third LED will light, and also I don't care about the first and the second LED.

The voltage step should not be 1 volt only, It may be 2 volts or any value. I mean if I connect 2 volts the first LED will light. If I connect 4 volts the second LED will light and so on..

Is it possible to make this circuit using transistors ?
What is the schematic of the circuit ? ( Just give me a link or a reference that I can learn how to make this circuit )
Yes.
Look at a chip called LM3914

It is like a volt meter with 10 outputs. It powers up the 10 outputs step by step.

There are other options like a window comparitor but will take several chips.

#### tracecom

Joined Apr 16, 2010
3,944
It can also be done quite easily with a microcontroller; the PICAXE is one that is suitable.

Joined Jan 15, 2015
6,585
As you are seeing there are a number of ways to go about doing what you are looking to do. The LM3914 is a good choice and using a micro-controller is another good choice. Another option as Gopher mentioned is using a comparator type chip like the LM 339 which gives you 4 comparators in a single chip. Give this a read as to using comparators. Deciding what works best for your application and needs is a matter of really defining them. For example I want to turn on a LED at 1 volt, 1.5 volts and then 3 volts but I won't always use those voltages, I want to be able to easily change and adjust what voltages I turn a LED On / Off at.

Ron

#### GopherT

Joined Nov 23, 2012
8,012
@Michael George
The circle on left is your input.
This is really what is happening inside the LM3914 chip. This option simplifies because you would need a 10 volt power source to make the LEDs turn on at 1, 2 and 3 volts exactly (or you would have to somehow play with the high input limit. This seems easier if you want a versatile option for when the LEDs turn on.

#### cmartinez

Joined Jan 17, 2007
7,703
Hello,

Have a look at the LM3914 chip.

Bertus
Great chip... didn't know about it, must have hundreds of different uses. Thanks.

#### WBahn

Joined Mar 31, 2012
26,398
Hello everybody, Would you give me a hand ?
I need to make a circuit that has Inputs and outputs. I assume that the circuit has 3 LEDs as outputs.
1- When I connect the Input to 1 volt the first LED will light.
2- When I connect the same Input to 2 volts The second LED will light, In this case it doesn't matter if the first LED is on or off.
3- When I connect the same Input to 3 volts The Third LED will light, and also I don't care about the first and the second LED.

The voltage step should not be 1 volt only, It may be 2 volts or any value. I mean if I connect 2 volts the first LED will light. If I connect 4 volts the second LED will light and so on..

Is it possible to make this circuit using transistors ?
What is the schematic of the circuit ? ( Just give me a link or a reference that I can learn how to make this circuit )
Well, you've given conflicting specs.

In #3 you stated that if you connect it to 3V then the third LED should be on and you don't care about the first two. But then you say that if you connect 4V that the second LED should be on (and, presumably, the third should be off).

Which is it?

How is the circuit supposed to know what voltage step you have in mind?

If that is part of the spec, namely that you apply the amount of the step voltage to one input and the input voltage to the other, then fine. But you need to make that part of your spec. Similarly, if you set the step voltage using a switch or a pot or something else, that needs to be part of your spec.

#### #12

Joined Nov 30, 2010
18,223
One really slick feature of the 3914 is that you can choose a bar graph display or just one LED at a time.

#### GopherT

Joined Nov 23, 2012
8,012
Great chip... didn't know about it, must have hundreds of different uses. Thanks.
And the lm3915 has a log response to the input that is perfect for audio.

#### cmartinez

Joined Jan 17, 2007
7,703
And the lm3915 has a log response to the input that is perfect for audio.
When I was kid I wanted to build a circuit to monitor a water tank's level, either with one or two wires, or with coaxial tubes... I didn't know back then how difficult it would be for someone with zero knowledge about electric signals... back then datasheets were extremely hard to get (and keep)... maybe I would've persevered and experimented lots more if I had known that a single chip had the essential function for a bar-type display, like this one.