# Building a DC Voltage tester that test specific voltages (10,12,14,15 volts)

#### Tavio1610

Joined Nov 8, 2018
7
Hey guys I need your help. How would you go about building a dc voltage tester that is capable of testing 10,12,14 and 15 volts only. It has to be done by illuminating an LED at each voltage input.

#### ericgibbs

Joined Jan 29, 2010
10,767
hi Tavio,
Welcome to AAC.
As this is Homework, we would like to see your attempt at solving the problem, we can then point out any errors.
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#### Tavio1610

Joined Nov 8, 2018
7
I have a bit of ideas, my first was the use of zener diodes rated at different voltages and so as each breaks down it lights up the LED. This however has a flaw in that that a 10v can also be broken down by 11 volts lighting up the LED. My second guess was using relays but the electronics assistant suggested it best no to as they are used for higher voltage supplies. I then results to the use of gates where a one input would always be biased at like 5v giving a logic one input and using varying resistors when a supply of 10 is attained the gate receives another input of 1 and lights the LED, this though isn't simulating after the 2nd of 4 lights.

#### ericgibbs

Joined Jan 29, 2010
10,767
hi T,
Look at the LM3914 IC for one option.

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#### MrChips

Joined Oct 2, 2009
21,843
You need to specify a window for each voltage.
For example, for 10V LED to be lit the window could be 9.5V to 10.5V.

#### drc_567

Joined Dec 29, 2008
882
There is a similar project in a book by John Boxall ...Arduino Workshop.
However, another approach would be to use comparator circuits, forming specific voltage 'windows', as suggested previously.

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#### Tavio1610

Joined Nov 8, 2018
7
Anyone willing to help with a schematic?

#### MrChips

Joined Oct 2, 2009
21,843
Anyone willing to help with a schematic?
There is no schematic until you specify the criteria.

#### Tavio1610

Joined Nov 8, 2018
7
The criteria is I need to build a DC Voltage tester that test 10,12,14 and 15 v. One LED is to turn on for each voltage tested. So say I'm testing, a green LED would indicate 10v is being tested, a red LED would then turn on when it is 12v(and all other LEDs) would be off.

#### ericgibbs

Joined Jan 29, 2010
10,767
hi,
Have you explored the possibilities of the LM3914 IC I have posted.?
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#### dl324

Joined Mar 30, 2015
11,506
The criteria is I need to build a DC Voltage tester that test 10,12,14 and 15 v. One LED is to turn on for each voltage tested. So say I'm testing, a green LED would indicate 10v is being tested, a red LED would then turn on when it is 12v(and all other LEDs) would be off.
What is the circuit supposed to do for 9.99V, 10.01V, 11V, etc? Are there restrictions on the components you can use?

You should post the full text of the problem.

#### Tavio1610

Joined Nov 8, 2018
7
All lights are supposed to remain off. When and only when 10v is tested should one of the LEDs turn on, the next tested voltage would be 12 and at that point the LED indicating 12 volts should come on, the previous LED indicating 10v and ALL other LEDs should remain off. The LEDs are to turn of only at the specific voltages and no other in between values

#### MrChips

Joined Oct 2, 2009
21,843
All lights are supposed to remain off. When and only when 10v is tested should one of the LEDs turn on, the next tested voltage would be 12 and at that point the LED indicating 12 volts should come on, the previous LED indicating 10v and ALL other LEDs should remain off. The LEDs are to turn of only at the specific voltages and no other in between values
You are still missing the point.

There is no tester that will test for 10.00000V
You must specify a range of voltages.

Furthermore, to be more precise, you could specify a tolerance, for example,

10V range is 9.5V to 10.5V ±0.1V.

Stating 10.0V ±0.5V would be sufficient for this purpose and would be a lot better than just stating 10V.

#### Tavio1610

Joined Nov 8, 2018
7
In that case I guess I'll work with the range being 10.0v +/- 0.5v. Thanks for the suggestion

#### crutschow

Joined Mar 14, 2008
25,398
In that case I guess I'll work with the range being 10.0v +/- 0.5v. Thanks for the suggestion
For that you can use a window comparator (look it up) for each voltage.

#### MrChips

Joined Oct 2, 2009
21,843
You could still use LM3914 and skip the 11V and 13V output indicators.

#### bertus

Joined Apr 5, 2008
20,752

#### Alec_t

Joined Sep 17, 2013
11,640
....... but the LM3914 has a small overlap between consecutive output actuations. Would that be acceptable?

#### dl324

Joined Mar 30, 2015
11,506
Maybe we should let the OP figure out the implementation details. That could be one of the learning objectives of the problem.

#### sc0tch

Joined Nov 6, 2018
64
You can also use a LM393 dual comparator for each voltage range you want to test for. Set comparator 1 to your minimum voltage for the range by creating a reference voltage to compare against by using a divider or a zener diode. Likely set comparator 2 to your maximum voltage for the range. Invert the output of comparator two and then use an AND gate to express as AND(A, B'). If this is true then light the LED using the output of this gate.

Remember you can only test up to the voltage that the circuit is powered by. Therefore you may need to use a voltage divider to half or third your test voltage.

I did not add any pull-up/pull-down resistors in the schematic above but should be enough to give you an idea how to use a comparator to test voltages and build the logic gate accordingly