User Manual

Pico Technology ADC-20/ADC-24 Terminal Board User Guide
DO117-5 7
2.3 Light level
You can use the ADC-20/ADC-24 Terminal Board with the ADC-20 or ADC-24 to
measure light levels. You will also need to use a Light Dependent Resistor (LDR) and
a fixed resistor.
Use the +5 V output to supply power to the circuit. Use a resistor of around 1MΩ for
R
A
, and place the LDR in location R
B
.
2.4 pH
pH sensor circuit
You can use the ADC-20/ADC-24 Terminal Board with the ADC-20 or ADC-24 to
measure pH. The circuit shown below allows the use of any standard pH probe,
including the one available from Pico Technology (part number DD011). If you use
this method, you will have to calibrate the probe using two or three buffer solutions
(solutions of known pH values).
Note: The op-amp should have very high input impedance. An LT1114 is suitable.
Beware - the pH of a liquid can vary widely with temperature.
A much simpler and more complete way to measure pH is available. Known as the
Pico DrDAQ pH Logger (PP274), this is an optimised version of the above circuit. By
using the temperature sensor included, it will compensate for variations in pH caused
by temperature fluctuations.