This circuit, which uses a CD4538 retriggerable monostable multivibrator and a CD4011 NAND chip, can sense fluid levels using a variable capacitance (see the figure). Such a sensor can be as simple as two adhesive copper foil strips on a saline or blood-supply bag or on another nonmetallic container. These strips should be as close together and as long as possible, since C ≈Ε (A/L).
IC1, C\[subscript\]X, R1, and R2 form a simple oscillator. Using the third gate of this chip adds stability.
When the fluid level is low (CX = C0), the oscillator's frequency is high enough so that the trigger input (+TR2) keeps that half of IC2 retriggered, i.e. \[overbar Q2\] is kept low. As the fluid level rises, so does C , and in turn the frequency goes down.
Potentiometer R3 with capacitor C3 sets the level at which the retriggering is no longer continuous. Therefore, what might look like positive pulses at \[overbar Q2\] are actually "dropouts" from the low state.
As long as the frequency is low enough, these pulses will appear at +TR1, keeping the second half of IC2 retriggered for appropriate values of R4 and C2. Typical values of 50 to 100 pF for CX will produce an oscillator frequency of 10 to 30 kHz, depending on stray capacitance.