Team:IISc-Bangalore/OptoMatic/Impedance

Impedance Method

Measuring Impedance of the Sample

Impedimetric analysis is one of the methods we are using to obtain analytic data for the concentrations of the two bacteria. It has been in used widely in the food industry[1], and has hence been thoroughly studied.

Measurement

The impedance of the bacterial culture at 10KHz was measured. The impedance of the culture (at frequencies much smaller than 1 MHz) decreases with growth of the bacteria[2]. Voltage divider circuit was used to indirectly measure the impedance by measuring the voltage at the junction.

Basic Diagram representing the working of the circuit
Fig.1 - A basic diagram showing the working principle of the circuit used.

Circuit Design and Major Components Used

Circuit Diagram for the Measurement apparatus
Fig.2 - Circuit Diagram for the Measurement apparatus

Gold traces on glass slide

We know that,

Here, Z0 is an intensive property of the culture, and δ is a unitless geometric factor i.e. δ depends on the geometry of the apparatus. Thus, by decreasing the dimensions, sensitivity of our sensor was increased. Hence, instead of using plate electrodes, gold traces were used on glass slides to improve our sensitivity, which also helped us fix our geometric constant, which would have otherwise been sensitive to motion.

HCSR08 AD9850 Module

It is a versatile and small package solution for generating sinusoidal waves with very high accuracy. For the module sheet, click here. For the datasheet of AD9850, click here.

ADS1220 by Texas Instruments

ADC ADS1220 by Texas Instruments was used (in a breakout board by Protocentral), using a peak hold circuit in order to convert the sinusoidal voltages into DC voltages. For the datasheet of ADS1220, click here.

References

  1. Monika Wawerla, A. Stolle, Barbara Schalch, and H. Eisgruber, Impedance Microbiology: Applications in Food Hygiene Journal of Food Protection 1999 62:12, 1488-1496

  2. Grossi, M. Lanzoni, A. Pompei, R. Lazzarini, D. Matteuzzi, B. Ricc`o, Detection of microbial concentration in ice-cream using the impedance technique. Biosensors and Bioelectronics 23 (2008) 1616–1623