The mastermind of this design is a Python Script which handles the data.

The Diodemod itself holds a voltage source (12 Bit, 10 V, MCP4725) with a current measurement resistor (100 Ω) to supply the LED with a defined Power. The LED is connected to Port 1.

A similiar circuit can be found at Port 2. Here, the photodiode is connected. A reverse Bias voltage can be applied to the DUT (12 Bit, 10 V, MCP4725). The current is measured with 3 resistors (10 kΩ, 100 Ω and 1 Ω) which are selected by a relais. The voltage is amplified by two x10 amplifiers (MAX4238ASA+) which gives an overall amplification by 100.

As we use the ADC of the Arduino (10 Bit), an external Reference (REF02) was added to allow for stable and reliable readings.

The Voltage was swept in steps of 10 mV. The shunt used was the 1 Ω, as "high" currents were to be expected. The Testfixture used is described here.




What we always suspected : the Schottkydiode is the more energy efficient choice. And the 1N4007 is the worst. (from our non-representative treasure box pick)

The Voltage was swept from 0.0 V to -5.0 V in 0.5 V steps. Electrical Power of the LED was swept from 0 to 20 mW in 2 mW steps. The distance from the LED (611 nm, 15°) to the Photodiode was 40 mm. With BPX65!, you would get data like this :





With two columns from the above measurement, we can calculate the Responsivity of the Photodiode. This is just the slope of the curve.






In our case the value is slightly off, as not all Power of the LED hits the Photodiode. For a calibrated setup, it must be determined, how much of the optical power is available at the location of the Photodiode.





In this second graph, we just entered the measured values from above in an X-Y graph. On the horizontal axe, we see the Bias Voltage, on the vertical axe the current in Nanoampere. The top curve is representing the dark current. The distance from line to line represents a 2 mW increase in Power (LED).

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