integrated-circuituln2003
i made an water level indicator using IC: ULN2803
It works well when used on glass of water, but when connect to my rooftop tank all the leds slowly diminishes and turns off. I think this has to do with high resistance of the tank water, but i doubt it since it worked well for first few days.
i am not any expert (clearly) so any help is appreciated.
Perhaps this image would help.
As you can see the metal pressing to connect the pins to the chip have a central bar that is exposed at the end of the resin.
ULN2003A are usually quite a bit cheaper, but only 7 channels.
BTW, they're not really good for 500mA, if you read the datasheet critically, you'll see it's more like 50 or 60mA per channel with all channels active, assuming a reasonable temperature range.
From the Toshiba datasheet, the lower graph is for Ta = 85°C
Best Answer
I'm not big in chemistry, but I believe that it has stopped working because of electrolysis. In other words your contacts have corroded to the point that the resistance is too high. You could prevent this by using AC, but then the circuit would be a lot more complicated. I would opt for capacitive sensors, Analog Devices have an interesting app note about Capacitive Liquid Sensors Or you can follow this Instructable