Help required for surface reflection circuit (IR-LED/ PHOTODIODE)

[milovisk]

Hi

I am new to this forum but have been interested in reading many threads and would like to know if any of the experts could help.

I am looking to make a circuit that I can measure different output levels of a photodiode from the amount of "signal" reflected onto it from a particular glass surface

My idea is to set the angle of the transmitter / receiver to get the optimum reading on a known surface coating on the glass, I would then like to measure a range of coatings and see if I can establish a significant measurable difference between them.

Ideally the transmitter and receiver would be about 5cm apart and I coulkd make some sort of light tube to optimize the reflected signal if possible.

I am looking for advice on a circuit so that I can measure the output of the photodiode as I do not want to use it as a switch like high or low , I want to be able to measure the output in mV so that I can then process the different levels into some meaningful data. I would also like advice if it would be better to use a very bright LED to reflect off the glass or an infrared LED?

Sorry if I have not explained myself very well but I hope someone can understand what I want to do.

Thanks in advance

 

[eljose79]

Hello and welcome to the forum! It sounds like you have a very interesting project in mind. I can definitely offer some advice on how to approach this.

Firstly, let's discuss the components you will need for your circuit. You will need a photodiode, which is a semiconductor device that converts light into an electrical current. You can choose between a silicon photodiode, which is sensitive to visible light, or an infrared (IR) photodiode, which is sensitive to IR light. Since you mentioned using a glass surface, I would recommend using an IR photodiode, as glass is more transparent to IR light than visible light.

Next, you will need a light source to reflect off the glass surface and onto the photodiode. You can use either a bright LED or an IR LED, depending on which type of photodiode you choose. Make sure to calculate the appropriate wavelength for your LED based on the sensitivity range of your photodiode.

Now, let's talk about the circuit itself. You will need to design a simple amplifier circuit to amplify the small current output from the photodiode. This can be done using an operational amplifier (op-amp) and a few resistors. You can also add a filter circuit to remove any noise from the signal.

To measure the output in mV, you can use a digital multimeter (DMM) to read the voltage from the amplifier circuit. You can also use an analog-to-digital converter (ADC) to convert the voltage into a digital signal that can be processed by a microcontroller or computer.

As for optimizing the angle of the transmitter and receiver, you can use a servo motor to adjust the angle and find the optimum reading. You can also add a feedback loop to the circuit to automatically adjust the angle for the highest signal output.

I hope this helps you get started on your project. Remember to always be safe when working with electronic components and have fun experimenting! Good luck!