Salt taster detects the amount of salt

Detects the amount of salt contained in liquid foods

Three-level LED indicator

Salt taster  detects the amount of salt


R1________________470R 1/4W Resistor
R2,R5______________10K 1/4W Resistors
R3,R6_____________220K 1/4W Resistors
R4__________________5K 1/2W Trimmer Cermet
R7________________680R 1/4W Resistor
R8__________________2K2 1/4W Resistor
R9,R10,R11,R12,R13__1K 1/4W Resistors

C1________________100΅F 25V Electrolytic Capacitor

D1,D2,D3______3 or 5mm. Red LEDs
D4____________3 or 5mm. Green LED
D5____________3 or 5mm. Yellow LED

IC1_______________LM324 Low Power Quad Op-amp

P1_________________SPST Pushbutton

Probes_________________ (See Text)

B1___________________9V PP3 Battery

Clip for PP3 Battery

Device purpose:

This circuit was designed to detect the approximate percentage
of salt contained in a liquid. After careful setting it can be
useful to persons needing a quick, rough indication of the salt
content in liquid foods for diet purposes etc.

Circuit operation:

IC1A op-amp is wired as a DC differential amplifier and its
output voltage increases as the DC resistance measured at the
probes decreases. In fact, fresh water has a relatively high DC
resistance value that will decrease proportionally as an
increasing amount of salt is added.

IC1B, IC1C and IC1D are wired as comparators and drive D5, D4
and D3 in turn, as the voltage at their inverting inputs
increase. Therefore, no LED will be on when the salt content of
the liquid under test is very low, yellow LED D5 will illuminate
when the salt content is low, green LED D4 will illuminate if the
salt content is normal and red LED D3 will illuminate if the
salt content is high.

D1 and D2 are always on, as their purpose is to provide tw1o
reference voltages, thus improving circuit precision. At D2 anode
a stable 3.2V supply feeds the non-inverting inputs of the
comparators by means of the reference resistor chain R8, R9 and
R10. The 1.6V reference voltage available at D1 anode feeds the
probes and the set-up trimmer R4.

One of these tw1o red LEDs may be used as a pilot light to show when the device is on.


It was found by experiment that a good and cheap probe can be
made using a 6.3mm. mono jack plug. The tw1o plug leads are
connected to the circuit input by means of a tw1o-wire cable (a
piece of screened cable works fine).

The metal body of the jack is formed by tw1o parts of
different length, separated by a black plastic ring. You should
try to cover the longest part with insulating tape in order to
obtain an exposed metal surface of the same length of the tip
part, i.e. about 8 to 10mm. starting from the black plastic ring.

In the prototype, three tablespoons of liquid were poured into
a cylindrical plastic cap of 55mm. height and 27mm. diameter,
then the metal part of the jack probe was immersed in the liquid.


Wait at least 30 seconds to obtain a reliable reading.

Wash and wipe carefully the probe after each test.

To setup the circuit and to obtain a more precise reading, you
may use a DC voltmeter in the 10V range connected across pin #1
of IC1A and negative supply.

Set R4 to obtain a zero reading on the voltmeter when the probe is immersed in fresh water.

You may change at will the threshold voltage levels at which
the LEDs illuminate by trimming R4. Vary R8 value to change D4
range and R9 value to change D5 range.

P1 pushbutton may be substituted by a common SPST switch

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