A Liquid-Level Indicator for the Blind

Have you ever wondered how a blind person knows when to stop pouring liquid into a glass or cup? The blind people have known usually insert a finger into the cup to the desired depth. Then they pour until they feel the liquid with their finger tip. Unfortunately, this procedure can cause discomfort if the liquid is hot, and can be socially awkward if a blind host is serving sighted guests.

The simple circuit shown in Fig. 1 will help your blind friend or relative pour liquids with relative ease and without having to touch the liquid. The circuit is inexpensive to build and uses an LM3090 LED-flasher chip as an audio oscillator. The circuit emits a tone whose frequency in part depends on the value of C1 When the probe leads, which parallel C1, are bridged by a low-to-moderate resistance, the frequency of oscillation suddenly increases. Such a probe, shown in Fig. 2, can be connected to the audio generator by a suitable length of flexible two-conductor wire.

With the value of C1 as shown, the frequency of oscillation is about 1.6 kHz. When the probe clips were attached to the rim of a plastic cup and tap water was poured into the cup to the level of the probes, the frequency of the tone increased to approximately 3 kHz. The frequency change that your circuit exhibits might differ from these values. Different beverages have various values of conductivity and can produce different frequency changes. In any event, you will hear a distinct tone change when the liquid bridges the probe tips.

If you build this circuit for a blind friend, house it in a small plastic box. Make sure the battery holder is readily accessible, as the cells must be easily replaced by one who cannot see.

Be sure to place a raised marker at the positive end of the battery holder to indicate the correct battery orientation. A drop of paint or glue will do. Your blind friend should have little difficulty replacing the cells if you can do it blindfolded.

Incidentally, the liquid-level indicator can be designed to emit a tone only when liquid bridges the probes. I prefer the two-tone approach because

the blind user always knows when the unit has been switched on and there fore will remember to turn it off when he has finished using it.

Going Further. You can add a volume control or alter the frequencies that the circuit generates or both. The former is easily accomplished by reducing the value of R1 to 47 ohms and inserting a 1 K ohm potentiometer in series with R1. To alter the tone frequencies, try different values for C1. Increasing the value of C1 will lower the frequency and reducing the capacitance will raise it.

The entire circuit should be self contained and complete with battery and miniature acoustic transducer. The LM3909 can even be powered by a single 1.5-volt silver-oxide cell of the type used to power digital watches. This will allow you to assemble a miniature unit. The audio output could be provided by a miniature earphone salvaged from a discarded hearing aid. Alternatively, you can use a midget transistor-radio earphone.