# 555 pump controller not triggering

I built this 555 pump controller and it works correctly if I short the wires, however, when trying to use the electrodes in water it doesn't do anything. I am using a 12V 1A power supply so I don't think it's a current problem.

The description with the diagram says increase the 100k resistors for more sensitivity, so I made them 1M but still nothing. Why would increasing them help? Surely that will lower the current. Also is 1M enough?

• DO you have a multi-meter? How much resistance do you get between the probes in water? – Trevor_G May 5 '17 at 21:12
• higher resistors mean a weaker pullup- less to pull down via water. i would get a float switch; this isn't something you want to fail because of corrosion... – dandavis May 5 '17 at 21:26
• I suspect the problem, looking quickly at the schematic, is that it's not a managed design. To make a water detector (worried about an overflow, in my situation), I had to do some careful measurements. In general, I found that I had to plan on currents in water near $250\:\textrm{nA}$ or so, to trigger the circuit. This suggests to me that the $100\:\textrm{k}\Omega$ resistors are simply too small a value and that the 555 may not really be suitable for a managed design given leakage specs that I haven't tried to read today. (There may be other problems.) I used 6.8 M$\Omega$ but no 555. – jonk May 5 '17 at 21:45
• Suppose your water is very clean. It's resistivity will be high (as high as ten or twenty M$\Omega$-cm.) Depending on your probes, it may not be able to pull down the 555 pins strongly enough to overwhelm your pull-up resistors. This is why using larger values might help. So I'd increase them still further given your written experiences so far. Or increase the surface area of the probes and their proximity to each other. – jonk May 5 '17 at 22:07