Hi, I am currently working on a opacity meter that measures the thickness of smoke coming out of the exhaust of automobiles. We have a laser pointing at a photo-diode receiver and they will be about 2 ft apart. The receiver circuit outputs a voltage depending on the intensity of the laser (the thicker the smoke, the smaller the voltage. And vice versa). The problem is that there are particles still burning in the exhaust that emit light which interfere with the laser reading. So we are trying to have the laser modulate at 100Hz and record voltages when the laser is on and off. We have the laser hooked up to a function generator. When the laser is on, this would measure the voltage of the intensity of the laser plus the burning particles. Then we would record voltage when the laser is off; This would measure the voltage of the intensity of the burning particle only. Then we want to take those two voltage readings and subtract them, this would result in (Laser + Burning particles) - Burning particles = Laser. And we need to have the reading and calculation continuously. The problem is I have no idea on how to record voltages continuously, and subtract 2 recording continuously, and output the resulting voltages. I am thinking this will require implementing a microprocessor with an internal clock and ADC, and some C coding. Anybody have any good ideas or suggestions?