Our turbidimeter is so called “modulated four-beam” type. That means there are two separate light sensors. For more details see our previous post. What we need next is an amplifier for these light sensors. And here is the schematics for it.
I quickly point out some details:
- Two photodiodes have been connected to dedicated transimpedance amplifiers. They have constant gain and highpass cutoff frequency set at 2.0 kHz. (The pulsing frequncy of LED sources will be 1.6 kHz.)
- A CMOS switch is used to select which of the light signals is passed into the next amplifier stage which is a second order bandpass filter with center frequency at 1.6 kHz and gain of ten.
- The signal is selected by setting two pins of the CD4066. These control pins are wired to connector J1.
This is quite simple schema with not too many components. It will take one compact two-sided board to implement.
The other benefit of this solution is that we can use same bandpass filter and same ADC for both light signal channels and this should make their comparison easier.
We need to select proper capacitor type for C15 and C17 to avoid temperature drift of their values which leads to drift of the center frequency of the bandpass. Probably C0G ceramic type would be best here.
Experimental Photonics 