Homemade Light sensors?

Does anyone know the resistance of the the photo resister inside?

If you do not i can test it with a volt meter but i really need to know so i can place an order on digikey to get a couple of photo resistors that are the same.

I just opened the vex light sensor and it is very simple, 1 photo resistor, 1 smd capacitor, 1 smd resistor, and vr1 which i thinking is a voltage regulator.

I need to know

The capacitors value uf and voltage

The smd resistor resistance in ohms.

The resistance of the photo resistor.

And what is component VR1 on the circuit board, i am confused as to what that is.

If i know this information then i can make all the light sensors i need.

I am sure that the engineers have these specifications at hand so please tell me, time is of the essence.

I think “VR1” stands for Variable Resistor 1 (Potentiometer) probably used to calibrate the photo cell.

That would make sense but i thought potentiometers were adjustable, this must be a set potentiometer i guess.

Can anyone help me here, i really need a whole bunch of these light sensors and at $20 a piece it is to expensive.

all you need is a CdS cell, these are about a dollar, then attach 5V to one end and the other to an analog input

CdS cells output resistance based on light so then experiment reading them with the analog input command

BTW you should probably read up on Ohm’s law ;):wink:

I have read ohm’s law and everything else but i wanted to replicate the exact vex light sensor down to the last component that way i could be assured that it would work.

You don’t have to make an exact duplicate of the schematics of the Vex Light sensor to make a homemade light sensor.

All you need is a simple photo-resistor and a second resistor to act as a voltage divider to make a light sensor for about a $1.

Depending on whether you put the photo resistor on the +Vin or ground side of the voltage divider, your output voltage will either increase or decrease respectively when the ambient light increases.

Using Ohm’s law, you can calculate the needed resistance value needed for a set resistor depending on the range of values that your specific photo resistor will fluctuate between in light or dark conditions.

The output is going to be an analog signal. Just plug this into the Vex controller, and in your code either treat it as a generic analog input, or if you use EasyC you can just use a light sensor block. The values you get back may not correspond directly with those of the Vex light sensor, but it will work just fine.

Folks - Don’t you think that Kirchoff’s laws, not Ohm’s law, are the ones that give you the info you need to analyze a pair of resistances in series used in a voltage divider?

Makes sense to me. Below is a schematic of the Vex Light Sensor that I made up based on the one that I have. I measured the resistances by getting the photocell as dark and as light as I possibly could. You can see it covers a huge range of resistances, and I discovered that it is extremely sensitive. If I had to pick a part from Digikey, I’d guess it was either PDV-P9200 or PDV-P9203. If you have time to experiment, buy one of each and compare each to a real Vex sensor to see which one comes closer.

Don’t worry about “VR1”, it is just a normal resistor. It could have been a variable resistor; if you look at the white outline, you can see a 3rd pad that would have been used. In the electronics industry, this is called a “Stuffing Option”, where a single design lets you opt for a variety of components at manufacturing time. A variable resistor in the VR1 spot would have allowed you to calibrate each sensor to match a specific curve, but a fixed resistor is cheaper and you skip the calibration step in manufacturing. The resistor stuffed in the “VR1” spot is just a normal 100K resistor. The resistor stuffed into the “R1” spot is a zero-ohm resistor, which is just a wire to you and me ;).


Good luck and let us know how it goes!

  • Dean