This electrode formed the other side
of a capacitor plate. When the need made physical contact with the fluid, the
40KHz signal was injected into the fluid. The signal was then routed through
the highly conductive fluid and would induce a similar 40KHz signal at the copper
electrode, through capacitance coupling. The copper plate electrode is
connected to a simple receiver circuit.
The fast rise and fall times of the
square wave signal produce narrow pulses at the receiver input. A low power
Schmitt trigger acts as a voltage comparator and converts the short pulses into
logic pulses. The logic pulses are then connected to a retriggerable one-shot type
circuit with a 50ms time constant. The logic output of the one-shot is then
fed to the system’s microprocessor.
The result is a nice clean logic
signal whenever the needle made contact with the fluid. The logic signal was used by
the system’s computer to index the needle’s position, keeping just below the fluid
typical sequence starts with the vertical indexing system moving the needled to an upper
most “home” position. The system then moves the needle downward, using its stepper
motor. When the needle comes in contact with the fluid, the sensor circuit sends a
logic signal to the computer. The needle is then slowly indexed downward as the
fluid is drawn off. The circuit is shown below.