Pulse Width Modulated LED Stimulator
The end user wished to apply up to 100 Hz sine waves of different wavelengths to ground squirrel retina as well as photostimulate channelrhodopsin, the light gated membrane channel. The two disparate requirements were met with the flexibility offered by a microcontroller based system. The current through each LED when enabled is set by a digital potentiometer, with a maximum of ~600 mA. To enable fast switching the current path is switched from a 20 Ohm resistor alone to include a 5 Ohm resistor in series with the LED. This prevents "load lag" inevitable with voltage regulators and enables LED response linearity with pulses as brief as 100 ns.