variable signal to lm3915

I have been trying to figure out how to test a diy oxygen sensor guage. The sensor's values range from 0-1volt. Here is a schematic of the guage

https://www.pelicanparts.com/techarticles/mult_air_fuel_monitor/FIG3.JPG

I am wondering what I can do to use the 7805's 5 volt output to give me different voltage outputs for the signal input.

I basically need to create a signal voltage of .450 volts. I have been trying to figure out what resistors, trimmers, etc to use but I am a noob. Truthfully only .450 volts is needed anything else will only give me estimates of the LEDs to voltage meaning.

Thanks.

You should be able to do it like this.

The circuit won't work:
1) The LM3915 has a logarithmic scale. You want to use the LM3914 that has a linear scale.
2) Pin 6 must be connected to a reference voltage like at pin 7. Then pin 4 must connect to a lower voltage.
3)The resistor from pin 7 to ground has a value way too low so the output current to the LEDs will be way too high. Use 470 ohms for an LED current of 22mA.
4) If you want the displays to be in series for a range of 0V to 1.25V with a resolution of 0.0625V per step then the pin4 of the first LM3914 must be grounded, the pin4 of the second LM3914 must be connected to pin6 of the first LM3914 and the pin6 of the second LM3914 must be connected to its pin 7. It is shown in the datsasheet for the LM3914.

audioguru said:

The circuit won't work:
1) The LM3915 has a logarithmic scale. You want to use the LM3914 that has a linear scale.

Click to expand...

Whats the difference between the logarithmic scale and linear scale on those drivers?

Why won't it work?



The resolution was to be within .05 volts and 1 volt. That is the resolution of the bosch o2 sensor I am using. The setup is basically just going to give me approximate values for the voltage output from the O2 sensor. This voltage output represents a lambda which give me air/fuel ratio.

Linear scale means the y axis will rise proportional to x axis. Log scale rises exponentially.

If two LM3915 ICs are in series with the 1.25V reference as the max reading, then the first LED will light when the input voltage is only 1.25mV which is way too low.
The second LED will light when the input is 1.77mV.
The 10th LED will light when the input voltage is 28.3mV.
The 11th LED will light when the input voltage is 40mV.
The 19th LED will light when the input voltage is 0.88V.
The 20th LED will light when the input voltage is 1.25V.
The voltage between the steps is different.

If you use two LM3914 ICs in series with the 1.25V reference as its max reading, then the first LED will light when the input voltage is 0.0625V.
The second LED will light when the input voltage is doubled to 0.125V.
The 10th LED will light when the input voltage is 0.5625.
The 11th LED will light when the input voltage is half the reference at 0.625V.
The 19th LED will light when the input voltage is 1.1875V.
The 20th LED will light when the input voltage is 1.1875V.
The voltage between the steps is the same.

You could use two LM3914's in series with the reference voltage set to 1.0V and then the difference between the steps will be exactly 0.05V.

does anyone know if I have to but a resistor on the o2 sensor to increase the load or draw?

Roff said:

You should be able to do it like this.

Click to expand...

How did you figure this out? I am so confused. I realize that you subtract .45 volts from 5 volts and use 2ma as the power/watts. But why do you have to add the 1k resistor to ground?

jkantelli said:

How did you figure this out? I am so confused. I realize that you subtract .45 volts from 5 volts and use 2ma as the power/watts. But why do you have to add the 1k resistor to ground?

Click to expand...

I'm afraid that what you realized is wrong. Where did you get the 2mA idea?
Use Ohm's Law. This is a simple voltage divider. See below.