[SOLVED] I need some help with a photometer

Hi

I have a tunnel photometer which has an output of 4-20ma and goes to a PLC to set the light levels in the tunnel,
it was send to a repair man and unfortunately the adjustments made are not correct.
It has 2 trimmers to set it up correctly,
I have a second same photometer that is perfectly set and i need to adjust the one that is not adjusted to have the same output.

The problem is that the input is a photo-diode and i need to substitute it with some non variable component so that i can measure the good one and set up the second one, the photo-diode is very sensitive even if the two devices are in the same room.

The power supply is +5/-5 , pot RV1 sets the voltage of the positive input of U2 to 0v, i have done that but the other trimmer RV1 sets the sensitivity of the circuit.

Can you please suggest any substitute component or circuit that will help me set up the device correctly?
I have no idea what to use in place of the photo-diode.



Thank you
Alex

I think that it is easier to measure the input and output of the second stage and calculate the amplification ratio,
then adjust the other photometer to have the same gain too by measuring the input and calculating what should be the correct output voltage.
This setting would be more accurate if the second stage is disconnected from the first but i will try first with the sensors covered so they are in total darkness.

Alex

You can use a short black tube to limit the sensor's field of view to an illuminated surface to create an illumination standard that you can swap from one unit to the other to make comparative measurements.

Yes it would help to limit the difference but these are so sensitive that i think the slightest angle difference will change the output.
Maybe i will use this trick and also measure the U2 input/output ratio and then set the second device to
have the same ratio too using the second u2 input and setting the output according to the gain.

Alex

Just adjust RV2 and get same output under same light.