0-90ohm input to pic, 25ma limit

Thread Starter

Dragn Wagn

Joined Jul 25, 2008
Hi. I'm working on my first pic automotive project and need to get a 0-90 ohm fuel sender to a 0-5v input. My thoughts were to use a 7805 regulator and resistor voltage divider to get the required voltage but I am stuck on the 25ma max input limit. Can anyone suggest an alternate arrangement that would meet both criteria? perhaps a noninverting op amp? I appreciate any advice!



Joined Apr 5, 2008

One solution could be a constant current source.
The voltage over the sensor will be (the resistance of the sensor) X (current from the source).
Example : when the current source is 10 mA, your range will be 0 - 0.9 Volt.
Put this trough an amplifier that mutiplies it by 5 and your range is 0 - 4.5 Volts.



Joined Jul 17, 2007
Nice idea, Bertus :)

Tried it in a simulator, seems to work pretty well.

The LT1006A I used is rated for inputs down to ground, but the offset gets skewed a bit when the input is down at zero; output is off by 50mV. Otherwise, it's good. You couldn't use a common opamp like a 741 or TL081 or LF353 without using a negative supply.

I decided the current limiter needed a voltage clamp on it as well; you wouldn't want 13 or so volts making arcs inside the fuel tank :eek:

I used a couple of diodes in series to clamp the sender signal level at around 1.4v. Little chance of arcing if the level sender is dirty or intermittent.