- By Thursday night, 2014 Jan 23, e-mail your photospectrometer design to the class e-mail list, so that everyone can share the designs.
- For Monday, 2014 Jan 27, as individuals (not groups), find a data sheet for the phototransistor WP3DP3BT. Also, select a cheap photodiode that is available in the same size and shape of package as the WP3DP3BT phototransistor and look up its data sheet.For the photodiode and the phototransistor, report the dark current, the voltage drop across the device (that would be collector-emitter saturation voltage for a phototransistor and the open-circuit voltage for a photodiode), and the sensitivity (current at 1mW/cm2 at λ=940nm, which is the wavelength where silicon photodiodes and phototransistors are most sensitive).Find a plot of the spectral sensitivity of a silicon photodiode or phototransistor (it need not be from the data sheets you found—all the silicon photodiodes and phototransistors have similar properties, unless the packaging they are in filters the light).We want to make a circuit so that the full-scale (5v) reading on the Arduino corresponds to an irradiance of 204.8μW/cm2 at 940nm, so that each of the 1024 steps corresponds to an increment of 0.2μW/cm2. Remember that 1000μW=1mW. (We may not be able to use the full range, as the circuit should saturate at a somewhat lower value, depending on the saturation voltage or open-circuit voltage of the photodetector.)Update 2014 Feb 6: Q1 is intended to be an NPN phototransistor, not PNP as shown here!
For the circuits above, figure out what values of R1 and R2 to use to get the desired voltage range at A1 or A2. Look up what standard resistance values are available with 2% tolerance, and pick the nearest one. (Hint: Google is your friend for finding tables of information.)
In class on Monday, we’ll try building this circuit and seeing how it works with the Arduino Data Logger.
- Before Monday 2014 Feb 3, get an Arduino board (I recommend Uno Rev 3, but any ATMega Arduino board should do), install Arduino software (more instructions in the Getting started guide), and start doing some of the on-line tutorials.