Flashlight LED Bulb
Looking for online schematic for light brightness?
I am converting a big old flashlight from incandescent bulb to LED. I would like to measure the brightness before and after to make sure I get same amount or more light afterwards. I can use AD conversion 0-5V, and 10 bit resolution from PIC. My plan was to make this circuit and be able to adjust so that in sunlight I would get the full 5V, and then put in a dark room with my flashlight and measure brightness.
Thanks for reading all the way, hope this makes sense.
Use a photo-diode or small solar cell, not a Cadmium Disulphide photo resistor. I prefer the small solar cell for this - it should be postage stamp size. Your method including calibration is fine. With the cell square on to the sun it provides some sort of reference. The torch needs to be measured at a reference distance. The sun can be taken as 1000W per square meter, but the torchlight has a different spectrum so not all that meaningful to start assigning values. It is just something meaningful and should be repeatable to get the circuit working properly. Normally a light meter with weighted human eye response (called photopic response) is used, and it measures Lux. You could probably approximate this with a clear plastic tinted yellowish green. You can buy photo-diodes with this filter built in (search photopic photo diode), or you can buy a low cost light-meter ready made.
The solar cell and the photo-diode are similar, but having a larger area will take into account the variations in the beam better. Normally you would use a diffuser in front, something like ground glass or opal glass. Clear plastic frosted by sandpapering will be a start. Mount the detector firmly anchored in a light tight box (small tin can?) painted flat black inside. Measure the current from the diode. If you have a resistor it should develop something like 20mV in the sun, exactly square on. The exact figure is not important, so long as you know what it is, but 40mV is too high, it will be less linear. The resistor is to measure close to the short circuit current, which is proportional to illumination in a linear way over a large range. The value of R depends on the area of the cell. Determine by trial and error. If you get 400mV that means R is way too high, approaching open circuit.
Amplify the 20mV to 5V or just use a DMM. Using the ADC setup makes it easier in the dark part of the measurement. If you look up photo-diode amplifiers, they will generally show current to voltage converters. The solar cell has too much output for these. So use the resistor and a non-inverting amplifier using an op-amp - look that up.. To go from 20mV to 5V is a gain of 250 times.
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