I'm making an air/fuel ratio gauge out of one to read my o2 sensor. i've screwed around with the chip for three days and it wont behave with my tolerances. I want the chip to light led1 at .37V and led10 at .55V, that is a .18V range. is the chip stable with such a low voltage range? I want led6 to light at .47V(optimum A/F mixture signal), each light is .02V apart basically. how can i do this? will i need to amplify the input signal to something the chip can detect better? how would you do it?

 

No circuit only guessing ...

 

I'm making an air/fuel ratio gauge out of one to read my o2 sensor. i've screwed around with the chip for three days and it wont behave with my tolerances. I want the chip to light led1 at .37V and led10 at .55V, that is a .18V range. is the chip stable with such a low voltage range? I want led6 to light at .47V(optimum A/F mixture signal), each light is .02V apart basically. how can i do this? will i need to amplify the input signal to something the chip can detect better? how would you do it?

Hi

That input voltage seems quite low. The data sheet indicates the voltage across the divider can be as low as 200mv with "expanded scale" circuitry.

I would boost the input signal with an opamp to increase the input voltage level to the LM3914.

Please post a schematic of your circuit.

eT

 

Here's the schematic I drew up.

 

I will put one up as soon as i can. 200mv across the resistor network might be too big for my application. seeing as i'm asking the circuit to have a range of 180mv between Rlo(pin4) and Rhi(pin6). im making my millivolt signal to pin 5 out of a voltage divider from my power source, i figured it wouldnt hurt anything.

 

im making my millivolt signal to pin 5 out of a voltage divider from my power source, i figured it wouldnt hurt anything.

Can't you use a different divider ratio to get at least 200mV?

 

Hello,

I have rotated and scaled your image.
I have some questions about the input signal.
(see the remarks in the image).

Bertus

 

The notes I made at the signal, 0-1v is the input that would be transmitted by the O2 sensor ultimately, the 9v demarkation is the voltage of the wall wart I'm using to power the chip and supply the signal, instead of the 1k for the signal divider i'm using a 1k trimmer pot parallel to a 50 ohm fixed resistance.

 

Hello,

Where does the O2 sensor come from?
As you might know, we do not allow automotive modifications over here.
See the Terms of Service section 6 for more info.

Bertus

 

I believe it to be a generic narrowband O2 sensor mounted in the exhaust header of my carbureted engine. I'm unaware of the terms about the car modification, but seeing as it would be a liability to the site if I took someone's advice and ended up bricking my car. This project is a test instrument to probe the voltage output of O2 sensors. Its no different than measuring the voltage of a lead acid battery or using a timing light.

 

I will put one up as soon as i can. 200mv across the resistor network might be too big for my application. seeing as i'm asking the circuit to have a range of 180mv between Rlo(pin4) and Rhi(pin6). im making my millivolt signal to pin 5 out of a voltage divider from my power source, i figured it wouldnt hurt anything.

hi

An opamp buffer would simply raise the voltage level input to the LM3914, the signal from the sensor would remain the same (0-1v or whatever). Then adjust the voltage across the LM3914 divider to get the correct scale. Should be straight forward as long as the O2 sensor output is linear.

eT

 

Thank you ET, I will bring the signal range into more visible tolerances. I have a whole tube of op amps ready to go. I'll give it a one volt bump up I think.