Turn on LED's after a specific voltage level

Thread Starter

Riddersholm

Joined Jun 1, 2019
9
Hi.

I have a circuit with 3 voltage levels: +40V, +8V and +5V. I want to detect if these voltage levels are present using 3 LED's.
My idea is that when the voltage levels are below 10% of the required levels, so +36V, +7.2V and +4.5V respectively, I want the LED's to turn on. When the voltage levels are above, they turn off.

Any idea how to do this, maybe using comparators?
 

jpanhalt

Joined Jan 18, 2008
9,663
Welcome to AAC.

A voltage comparator is often used for such purposes. You can use one reference voltage (such as a zener) and voltage dividers to get to the appropriate voltage of each source for the reference voltage. If you need to invert the output, a simple inverter will do it, but that can often be avoided.

From a digital prospective, you can use a voltage dividers and an ADC (analog digital converter). I assumed you wanted an analog solution.
 

ericgibbs

Joined Jan 29, 2010
10,171
hi,
A Quad LM339 comparator IC, could be used, resistive voltage dividers and say 3 potentiometers to Set the threshold voltages.
Do you have a sketch to post.?
E
 

Dodgydave

Joined Jun 22, 2012
9,197
Here is a circuit using the TL431, set the required voltage with the preset, use a 4.7K series resistor for the 40V supply. As the voltage drops below the set voltage, the led turns on.


IMG_20200619_124429_4.jpg
 

AnalogKid

Joined Aug 1, 2013
8,486
A quick innergoogle search found this. It is close to what you describe. In your case only 3 sections are used, and each has its own input.

Important: how is the monitor circuit to be powered? You could diode-OR the 5 V and 8 V inputs together on the assumption that both will not be out (or below 4 V) at the same time, or have some other, independent power source. How it is powered affects everything else, so it needs to be decided first.

http://www.circuitdiagram.org/battery-monitor-using-lm339-ic.html

ak
1592572908233.png
 

Thread Starter

Riddersholm

Joined Jun 1, 2019
9
Here is a circuit using the TL431, set the required voltage with the preset, use a 4.7K series resistor for the 40V supply. As the voltage drops below the set voltage, the led turns on.
Hi again Dodgydave.
Thank you again for your answer, the TL431 perfectly suits the job.
In the datasheet for the TL431, I came across the curcuit shown below, which is the same curcuit you draw, but with a upper voltage limit.
1592805563305.png
How do i calculate the values for R3 and R4, so that the current through the LED is 20mA? Let's say the LED has a forward voltage of 2.2V.
 

Attachments

jpanhalt

Joined Jan 18, 2008
9,663
Did you notice this:
1592818486371.png

At 20 mA, that will rapidly drain most batteries.* Modern LED's are easily visible at 1 mA to 2 mA. You might also want to configure it so no LED is on when "Low Limit < V(batt) < High Limit". Of course, that also can create problem by over discharging the battery.

*A 1000 mAh battery is fully drained by that LED in 50 hours with no other load. Moreover, such deep discharges may not be healthy for the battery.
 

Dodgydave

Joined Jun 22, 2012
9,197
Hi again Dodgydave.
Thank you again for your answer, the TL431 perfectly suits the job.
In the datasheet for the TL431, I came across the curcuit shown below, which is the same curcuit you draw, but with a upper voltage limit.
View attachment 210413
How do i calculate the values for R3 and R4, so that the current through the LED is 20mA? Let's say the LED has a forward voltage of 2.2V.
Use the circuit on post #9 that I gave you, it only uses one TL431 per led.
 

Thread Starter

Riddersholm

Joined Jun 1, 2019
9
Did you notice this:
View attachment 210423

At 20 mA, that will rapidly drain most batteries.* Modern LED's are easily visible at 1 mA to 2 mA. You might also want to configure it so no LED is on when "Low Limit < V(batt) < High Limit". Of course, that also can create problem by over discharging the battery.

*A 1000 mAh battery is fully drained by that LED in 50 hours with no other load. Moreover, such deep discharges may not be healthy for the battery.
The curcuit i want to detect the +40V, +8V and +5V voltages isn't powered by a battery, but i still want to detect the voltages.
If the current through the LED is set to 2mA, how do i then calculate the values for R3 and R4? And yes, the LED's is only ON if the voltage is too low/high.
 

jpanhalt

Joined Jan 18, 2008
9,663
And yes, the LED's is only ON if the voltage is too low/high.
That is not what the design you show does. According the the datasheet from which that circuit comes, the LED will be on only when the voltage is within the low and high limits. In other words, the opposite of what you just said.
 
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