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# Help with a 4 level voltage detector circuit design

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#### Sunny55

##### Full Member level 3
Hi, can anyone tell me how to calculate the resistance values for R1, R2, R3, R4 and R5 for a 4 level voltage detector circuit using LM339 please? The measured voltage is 12.7V(input voltage).

Thanks and Happy New Year ! :grin:
Sunny55.

Hi, can anyone tell me how to calculate the resistance values for R1, R2, R3, R4 and R5 for a 4 level voltage detector circuit using LM339 please? The measured voltage is 12.7V(input voltage).

Thanks and Happy New Year ! :grin:
Sunny55.

First you have to make assumptions:
Assume the current I through R1, ..., R5 is 1mA and R1=R2=R3=R4=R5 ...
Then:
U[total] = I * (R1+R2+R3+R4+R5)=I * 5R
Now you have simple equation with one unknown: R ...
Good luck and happy New Year .....
:wink:
IanP

Hi, thanks for reply. Why do we need to assume the current as 1mA? Could you explain how you get 1mA please?

Thanks
Sunny55

Why do we need to assume the current as 1mA? Could you explain how you get 1mA please?

1mA is an arbitrary value, not to big, not to small ...

You are free to choose any value you like, say1uA, ....,5mA, 100mA, ...,100A etc.
But if the chosen value is unrealistic there are problems.
For example, if the current is to small it may be comparable with the bias current of the LM339 inputs, if the current is to big it will load power supply to hard or it will unnecessarily heat up components.
So once again, my experience tells me that 1mA is just right .......
:wink:
IanP

it is nice to have round-number voltage readings. so you can choose 2.5, 5.0, 7.5, 10.0 as your thresholds.

let's assume all resistors are in series, and R1 is at the top.
let's choose R2=R3=R4=R5 = 2.2K. This is a common value resistor, but try and select for exact 2.2k, or use 1% resistors for accuracy.

then the current through R2..R5 will be 1.13636mA

Next we find what should be R1. here voltage drop is 2.7v, and current is 1.13636ma. so R1 = 2.376Kohm
You can use a 5K preset for R1, and fine-tune for correct voltages. Or you make R1 with series/ parallel combinations. simplist is to use 2.2K + 180ohms in series.

You can choose other voltages and resistor values and do the calculations like this

it is nice to have round-number voltage readings. so you can choose 2.5, 5.0, 7.5, 10.0 as your thresholds.

let's assume all resistors are in series, and R1 is at the top.
let's choose R2=R3=R4=R5 = 2.2K. This is a common value resistor, but try and select for exact 2.2k, or use 1% resistors for accuracy.

then the current through R2..R5 will be 1.13636mA

Next we find what should be R1. here voltage drop is 2.7v, and current is 1.13636ma. so R1 = 2.376Kohm
You can use a 5K preset for R1, and fine-tune for correct voltages. Or you make R1 with series/ parallel combinations. simplist is to use 2.2K + 180ohms in series.

You can choose other voltages and resistor values and do the calculations like this

Hi, the resistors value are as follows for my design. R1: 12K, R2 to R4: 220ohm and R5 : 7.5K. The potentiometer is 2.5K.
How can i find out what is the lower and higher limit of the measured voltage the circuit can measure?

Thanks.
Sunny55.

....The potentiometer is 2.5K.....

which potentiometer is this ? are you posting a new question ?

The potentiometer is connected in series with the resistors and i think its for setting the upper and lower limit of the measuring circuit. Yes, this is my question.

here is a magic formula which you can use for doing all your calculations --

V = I x R

allthe best

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