Turbidity, an optical technique for assessing suspended solids in liquid -- is an important water quality parameter. Typically, light is shone into a liquid sample, and the amount of light scattered back out of the sample (off of particles suspended in it) is measured; the intensity of this scattered light is considered to be related to the amount of suspended solids in the sample.
Avoiding ambient light effects. The circuit design in this repository uses a modulation and filtering technique in order to dimish the effects of ambient light, reducing the need for elaborate enclosure designs.
The basic idea, described here, is to send a pulsed light signal into the liquid at a frequency F that is significantly higher than the anticipated frequencies of light changes due to ambient lighting effects; the light that scatters back off particles in the water is then measured using electronic techniques that filter out any signals with a frequency much less than F. The intended result is that only the scattered light from the emitter, and not scattered light from other ambient sources, is detected.
The circuit consists of two main parts -- an emitter (the light source shone into the water) and a detector (the light measurement).
The emitter is a photodiode (with wavelength around 800 nm, as this is considered to have better scattering properties for turbidity measurements [REF]), pulsed via 'pulse width modulation' [REF] via a pin on the Riffle (a PWM Arduino IDE pin) at a frequency of about 2 kHz.
The detector consists of a transimpedance amplifier photodiode circuit, followed by a rectifier (to only allow 'positive' amplitude pulses through) and a high-pass filter (to suppress frequencies lower than the emitter frequency -- in this case, with a cutoff of about 1.5 kHz).
- Op Amp (JFET) AD820 datasheet
- Emitter: SFH 213
- Detector: HIR204
- Two 1 kiloOhm resistors
- One 1 megaOhm resistor
- Two 0.1 microFarad capacitors
