Low voltage input arduino nano

Using Arduino General Electronics
1

Hi All

Really need your help ! I'm trying to read a 0.7v signal when it's high and about 0.3v when it's low.

I've tried a transistor switching 5V but the base emitter voltage is too low. Ive also tried an optocoupler and again the same problem. I've tried to analog read it......but that is too slow

Can anyone assist with a suggested circuit or point me in the right direction . ANY help would be appreciated as I'm not an electronic expert but I'm trying to have some fun :slight_smile:

2

Been thinking of using an lm339 comparator circuit......just scared as I can't afford to blow the circuit

3

Hi,
LM339 would do the job, where is the signal coming from?

Tom... :slight_smile:

4

Signal is coming from an SMD led. I really didn't know they could operate on such low voltages. Ive checked over and over again and it looks like the led is being driven by approx. 0.7v?

5

Signal is coming from an SMD led. I really didn't know they could operate on such low voltages.

They can't.

Could they be coming from the resistors in series with an LED? That would make more sense.

6
  • No, analogRead is not too slow.
  • You need to play the game - show the circuit of the device with which you are working.
7

I measured across it while it was on and got 0.7v. I removed the led from the circuit and still got 0.7v. I have measured all over and cant find a higher voltage. It might be possible that there is a series resistor on the supply to this pcb.

Would the attached work?

IMG_6411 (002).jpg

8

Paul__B:

  • No, analogRead is not too slow.
  • You need to play the game - show the circuit of the device with which you are working.

I would like to if it would add value. attached is a pic. Red mark is where i removed the led.

the flexible ribbon coming in has 6 tracks. The pcb has 6 independent buttons on it and 6 independent LED's on it.......yet there is no IC's on this. It looks like it is just buttons, leds, and a few smd resistors. Really confused how this can do all that :slight_smile:

IMG_6412 (002).jpg

9

Hi,
Swap the resistors over, at the moment your Vref is 5 - 0.49 = 4.51V
OPs circuit.
008f95964adcaff382c76218459b1c93304ab2eb.jpg

With the resistors swapped;
The output of the 339 will be LOW when input is higher than 0.49V and HIGH when input is less than 0.49v

Tom... :slight_smile:

10

I measured across it while it was on and got 0.7v.

You must not have measured it correctly. It is a matter of physics that you can not have a visible LED with a forward voltage of 0.7V. This is down to Planks constant.

removed the led from the circuit and still got 0.7v

That proves you are measuring the wrong thing.

11

Agree....just can’t figure why I am

12

Hi,
Can you post some pictures of the device you are measuring the voltage from?

Thanks... Tom... :slight_smile:

13

Bmurray:
Been thinking of using an lm339 comparator circuit......just scared as I can't afford to blow the circuit

Comparators aren't difficult to use.

If this signal isn't time critical you could simply read it with an analog pin though...

14

The LED could be part of a matrix, and driven pulsed instead of with DC.
You really need a scope to see this, not a DMM (that averages).
Leo..

15

Wawa:
The LED could be part of a matrix, and driven pulsed instead of with DC.
You really need a scope to see this, not a DMM (that averages).
Leo..

Actually that's a very good point, that would make sense of the observations. Wave the LED around and
see if its being PWM'd or multiplexed?