Interfacing Thorlabs photodiode with arduino

Hey All.

I’m trying to interface a thor photodiode ( with my 2560.

I have the output from the BNC aligator clip of the diode going to an analog in.

When I use a regular photodiode (like the one that comes with uno kit), i get nice responsive peaks. When I use the thor diode, i get fast response times to light source-on but very slow response to light source-off.

BUT - when I connect the diode to a oscilloscope I get nice square waves with response to a fast flashing light source, aka it works perfectly.

My EE friend said there might be some issues with signal reflectance, that the arduino’s resistance and capacitance doesn’t match up, but this is a bit beyond me and would love it if anyone could shed some light on that.

Thanks in advance for any advice.

Please supply a clear drawing of how you are connecting the device to the Arduino


very simple circuit

The datasheet says 0-10V output from sensor so how are you dividing this down to 0-5V range. Also you should be using a 50ohm cable with 50ohm termination on the end.

It is advisable to read the manual for the photodiode. Here is step number 2 of the installation procedure:

  1. Attach a 50 Ω coax cable (i.e. RG-58U) to the output of the DET. Select and
    install a terminating resistor to the remaining end of the cable and connect to
    a voltage measurement device. See Chapter 4, page 5 to determine resistor
    values. Thorlabs sells a 50 Ω terminator (T4119) for best frequency
    performance and a variable terminator (VT1) for output voltage flexibility.
    Note the input impedance of your measurement device since this will act as
    a terminating resistor.

so I put a 50ohm resistor in parallel with the arduino's ground and analog in. With this configuration it does not get a signal at all.

Step 3 from the manual is

  1. Power the DET on using the power switch. To check battery voltage, see the Battery Check section on page 8.

Is the detector turned on? Is the battery good?

You will need a voltage divider to protect the ADC against input voltages > 5V, otherwise you may destroy it.

yes, fresh battery. detector is on. i get no signal at all, it goes to absolute zero, not even noise is present.

i checked the imedance of the bnc cable; it is basically zero.

I ran a 50ohm resistor in series with the BNC signal, and then ran another 50ohm between the signal in and black aligator clip, in parallel with the arduino. I also get no signal at all in this configuration.


the strange thing is that with any voltage reader, oscilloscope or volt meter, the signal ramps up as fast as it goes down, but with the arduino it has a very slow fall down time.