Bike fuel meter to MCU need help

[ltsharma]

the above picture is schematics of bike fuel meter,

As in NOW figure
I Disconnected the fuel meter connection of bike & connected it to NON-INV amp, for PIC project with digital fuel display, Digital display is perfect by ADC of PIC
Now, i need to enable both analogue and digital meters, how can i do?
any idea??

 

[barry]

Your analog meter measures CURRENT; your ADC measures VOLTAGE. So, it would seem that the simple approach is to hook everything back up the way it is in the top picture, and measure the voltage at the junction of the meter and the sensor. Get rid of that 3K resistor on the opamp input.

 

[ltsharma]

i dint get it ,, can you elaborate ??

>> at junction of meter and sensor there is lot of voltage fluctuations, depends on battery charge, and also its more that 5 volt, i can't just fed it to ADC

im using lm358, its having 2 op amp, is there anything to do with 2nd opamp to this?

 

[barry]

If your battery fluctuates, then your analog meter reading will fluctuate, too, right?. The current through that meter is battery voltage divided by the sensor resistance. You can put a voltage divider across your sensor (the voltage divider will have to have much higher resistance than the sensor). Or, if the resistance is too high you can use one of the opamps as a voltage follower and divide the output voltage before sending it to the ADC.

 

[ltsharma]

meter is not fluctuates anymore, i saw that. but battery voltage will fluctuates very much, 12 to 15V, is not effects to sensor voltage?

 

[barry]

My only guess is that the meter is dampening out the fluctuations due to the battery. I'm sure that if the battery fluctuates, so does the voltage at the junction. So, just add some filtering to your opamp, or in your software.

 

[ltsharma]

ok, by adding capacitors, pi?.
and would you know, how much is the resistance of fuel meter?

 

[barry]

ok, by adding capacitors, pi?.
and would you know, how much is the resistance of fuel meter?

How would I know the resistance of YOUR fuel meter? Maybe if I read YOUR schematic where it shows 100 ohms? But that's just a wild guess.

 

[ltsharma]

simply asked, is there any standard resistance of moving coil meters.

 

[barry]

simply asked, is there any standard resistance of moving coil meters.

Simply answered, no.