Current flow indicated by led.

[protowerks]

Thank you for your help!

I got it working with the following values:
R1 150
R2 & R3 47K
R6 100K
But i have 2 issues:
1. I'm only seeing 1.8v at the probes - i really needs as close to 3v as possible
2. When it has been sitting in the water for a while the localised resistance drops and the led goes off until i move it in the water and it comes back on.

Any suggestions on how to overcome these two issues - or am i better trying a different approach?

My three main requirements are:
1. Only use 2 probes
2. Have a led come on when probes are in the water
3. Have close as possible to battery voltage (3v ish) across the probes

Kind Regards,

Andrew

 

[alec_t]

I've just noticed R4 is shown as 100 ohms. Shouldn't that have been 100k ?

 

[alec_t]

I'm only seeing 1.8v at the probes - i really needs as close to 3v as possible

I presume you mean 1.8V between the probes. Is that when they're in the water?
Why do you need close to 3V? Ideally, the voltage between the probes would be zero when immersed.
The localised resistance changes (I would expect it to rise rather than drop) because of electrolysis effects at the probe surfaces.

 

[ronv]

What value do you have for R4?
Give us the voltages at each pin with the probe out and in the water once more.

 

[protowerks]

thanks ronv just got back.. will do some more meausrements and let you know the results..