My device is based on article by a A. Morpurgo "A low-cost instrument for environmental particulate analysis based on optical scattering".
Note that the OOPA350 schematic in original paper has some mistakes: OP legs are incorrectly numbered and photodiode leg connected to the + input is also grounded.
First OP is a transimpedance amplifier. It keeps photodiode input at 0V using the 1M feedback resistor. The output voltage is very linearly proportional to the photocurrent. Photodiode generates negative current on OP inverting input that is compensated to 0V by feedback resistor. So OPamp output becomes positive.
I use Texas Instruments OPA350 that was recommended in the italian paper about dust detection. It is single-supply rail-to-rail OP. Dust produces weak scattered light, so amplifier has to be low noise.
Input noise of OPA350 is 5 nA. One of often used photodiode amplifiers in physics laboratories is OP27 that has twice smaller input noise but requires bipolar power supply.
Circuit has to be fast because the light burst is short. I dimension circuit to some 50 kHz bandwidth that is suitable for taking several consecuitive samples with ADC. It is not practical to use larger feedback resistor than 1 Meg as circuit becomes slow. Better is to add one more OP stage. As a rule of thumb OP DC gain should be below 100 and resistors used below few hundred kOhm.
The circuit self oscilates without the capacitors across feedback resistors. So I added a 10 pF capacitor across 1 Meg and a 100pF capacitor across 100k. Low pass filter is dimentioned for 1M+10pF=100k+100pF=100kHz.
At output is a high-pass filter to eliminate continuously scattered light and 50 Hz. 10k+100nF=1ms=1kHz.
Circuit diagram is drawn using Eagle. Both opamps are assembled on a small SSOP chip breakout board. See the photo. The amplifier board is covered with insulating tape and aluminum tape for electric pickup shieldieng.
Microcontroller runs in a loop to find out when ADC voltage has reached some threshold. Then it samples and looks for maximum value. ADc peak value I think is proportional to the particle size. Microcontroller bins particles into small medium and large bins. And once in 10 seconds reports to display and serial.
Photodiode can be 1...2 mm active area size. I ordered from Farnell BPX65. I had in drawer older FND100 photodiode and it worked too.
Laser diode I soldered to a hole on a copper heat sink. I glued a glass lens with f=5 mm at some distance to make the beam focus at couple centimeters. There are 2 foil diafragms to reduce scattered light.
Be carefful with lasers. It is class 4 laser. You can damage eyes. (Electricity is more dangerous as you can loose life!) During alignment run laser at low power. Fix everything steadily on the table. Keep other people away from the room. According to safety measures should wear color protective googgles. Laser beam should be directed at some steady surface. Focused beam can start fire and smoke.
Finally the beam will be enclosed in the metal enclosure. That is considered safe.
Beam dump is a 90 degree copper plumbing piece sprayed with furnace black paint.
Photodiode is looking at 90 degrees to the laser beam into another 90 degree black piece.
Air is sucked into the box through a tube. About 10 cm long tube is practical for checking air coming out
from air ducts and filters. A small fan is placed not on the input but on the exit of the box because some
dust sticks on the turning blades. Number of particles counted in a time interval depends on air flow speed.
As there is relatively much heat from the laser diode and LM317 I had to use a cast aluminum enclosure by
Hammond that I got from Farnell. As a power supply I use 9V supply rated at 1A.
First diode lasers available were near infrared around 800 nm as used in CD players. Then red lasers appeared and now blue diode lasers are used in blueray disks. Light scattering increases quadratically for shorter light wavelength. So I think it is advantageous to use blue laser. Si photodiodes have sensitivity peak around 900 nm, but around 400 nm signal is only 10%. This might be why
red lasers are still used. I use a blue laser because I have many 1W laser diodes from an old Casio projector. To extend the lifetime it is adviceable to run laser diode at a half of maximum power. I made current controller for laser diode from a LM317 regulator and 2 ohm reistor that stabilizes maximum current at 0.6A. Similar circuits can be found elsewhere, for example, here: