lm339 led voltage meter build help

[twchapman63]

I am building this led voltage meter using the lm339 IC (link at bottom) and have run into the issue that only the first two leds will light up when measuring voltages 1.5v to 9v. I am curious whether I am using the probes correctly...I use Vin and the ground connected to pin 12 to check the voltage of a giving object and have the ground connected to R5 and +9v connected to a 9v battery pack. I also used a 100k ohm resistor in place of what I believe to be a potentiometer at R1. Any advice would be greatly appreciated.

Here is the link:
http://www.ecelab.com/circuit-simple-led-voltmeter.htm

 

[MrAl]

Hi,

If you used a 100k resistor in place of the pot then you cant adjust the voltage step for each LED.
If you short out the pot instead, you'll get steps of 9/4 volts each which is 2.25v each step.
Thus the bottom LED should light around 2.25v and the next around 4.5v etc.
However, the LM339 may not be able to take an input voltage at the full 9v...check the data sheet for this.

Sounds like you are using the probes correctly though.

 

[ronsimpson]

I think you should have all the LEDs on. Why only 2 I don't know.

Depending on how you wired in the 100k resistor; you probably have about 0.1 volt on pin 11, 0.2 volts on pin 9, 0.3 volts on pin 7 and 0.4 volts on pin 5. Therefor the 'meter' will measure 0.1 to 0.4 volts. R1 is a variable resistor so you can set where you want to measure. If R1=5K then your LED on points would be 1, 2, 3, 4 volts.

 

[twchapman63]

Thanks!

 

[twchapman63]

So this still doesnt work. I found other schematics for similar designs (like at bottom) with minimal changes to the circuit itself. When I build this all the leds are on but they are suppose to all be off with no input voltage, right? Anything on this would be of great use!

**broken link removed**

 

[Mikebits]

So this still doesnt work. I found other schematics for similar designs (like at bottom) with minimal changes to the circuit itself. When I build this all the leds are on but they are suppose to all be off with no input voltage, right? Anything on this would be of great use!

**broken link removed**

The LM339 is a high gain part, and can easily oscillate with poor layout and improper bypassing.