Fluid level detector - with a choice of three level indications

7-62 - elektor july/august 1982 The number of applications for this circuit is enormous, ranging from a level control for hydro cultures to the-kitchen-is-under-water-because- of-the-washing-machine-detector. It must be pointed out that the title is not quite correct as the LM 1830 from National Semiconductor will only detect conductive fluids, but, as most common liquids are conduc- tive, this should not present a problem. The frequency of the internal oscil- lator of the IC is 6 kHz (determined by capacitor C1). The oscillator out- put amplitude is approximately 2.4 V peak to peak and is fed to the probe via an internal resistor of 13 k and capacitor C2. When the probe is immersed in a conductive fluid the output of the oscillator is effectively "shorted" to earth via the fluid. If the fluid level then falls below the end of the probe, the detector input (pin 10) will be provided with the 6 kHz out- put of the oscillator. Transistor T1 will conduct and switch on one of the thr...