I am trying to build a water detection circuit that will operate for over one year using a 9 volt alkaline battery and therefore need to operate the detection cuircuit at the lowest possible current level. I am using two stainless steal electrodes, one at ground potential and the other electrode biasing a PNP transistor via a 4.7 mΩ resistor with a 10 mΩ hold off resistor from the base to Vcc. Since there is a timing circuit involved else where in the design, the operating voltage is regulated down to 5 VDC for stability. The nominal voltage across the electrodes varies from 0.5 to about 1.5 VDC. I assume that the voltage fluctuation is a result of electrolysis where hydrogen and oxygen tend to build up on the electrodes. A brief jolt of reverse polarity to the electrodes will drop the voltage across the electrodes back to about 0.5 VDC. Operating at µAmp bias levels, the circuit is sensitive to any form of leakage or contamination which is why I have included the 10 mµ hold off resistor. The collector output drives a 10 meg resistor. A CMOS invertor gate input is connected to the collector. The electrodes are nornally submerged in tap water where the PNP bias is at about .63 uAmps. When the water level drops below one of the two electrodes, the base is open (except for the 10 meg hold of resistor). The voltage fluctuation at the collector goes from esentiall Vcc to ground changing the inverter output state. After this wordy description here is my probelm. Data sheets on transistor don't specify gain at µAmp bias levels. What is the best choice for the PNP transistor and are the bias levels reasonable? Will the current levels be below the device cut off currents? I need to assure that going from submereged to out of the water conditions causes the collector voltage to exceed the inverter low and hi switching voltages.
THANKS!!!
THANKS!!!