Salt Tester Circuit

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. This circuit is using LM324 as main components. This is the figure of the circuit;

IC1A op-amp is wired as a DC differential amplifier and its output voltage increases as the DC resistance measured across 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 increases. 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 two 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 two red LEDs may be used as a pilot light to show when the device is on.

Parts:

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________________100µF 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