Constant current source for ANALOG DATA Aquisition

[angryboy]

Hi All
I am trying to reduce the components in a circuit that is reading the level of fluid probes into ADC of PIC micocontroller. Please see the attached figure for the current circuitry. As shown it is a contant current source of apprx 220uA, Probe is connected between J1 & J3 and data is fed to ADC of PIC from J2. The circuit is working fine. It is used with two differnt kind of probes:
1) Reed switch with no series resistance
2) Reed switch with 10K series resistance (This is also simulated in the attached figure)
With switch ON & OFF, it gives two different voltage at J2 that in turn can be used to determine the level of fluid.
Is there an alternative cost effective way to use the concept in this ciruitry e.g., using current limiting diodes **broken link removed**
Please advise

 

[chuckey]

Are you relying on the fluid conducting to short out the probes? Will the liquid polarise?, because if it does then you must use AC.
Frank

 

[angryboy]

No it is a level activated probe, like a simple reed switch inside a magnetic foat that goes up and down to make and release the switch contact. It can be thought of as a simple switch connected to ADC input of PIC with a long about 5meters cable running from switch to PIC input.