December 2012
Does anyone have a schematic for a simple light meter? I'm trying to determine the difference in illumination provided by CFLs as opposed to incandescent light bulbs. I'm not too interested in the absolute value of the light hitting the surface, just the relative difference between the two. Thanks in advance. Regards, Pete
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I did an experiment a few months ago using a serial port as a light meter that can log brightness data to a file. I posted it at www.cedarlakeinstruments.com/blog/?p=10. The software I wrote to display data isn't online, but I can email it to you if you'd like. Just contact me at www.cedarlakeinstruments.com. Here is the posting:
The lowest cost way I could think of was to avoid a microcontroller or other logic component and use whatever was already on a modern PC. My idea was to use a Cadmium Sulphide (CdS) photocell. CdS cells are used in many camera light meters and work by changing resistance in response to ambient light. By using the current through a photocell to charge a capacitor, we can tell the light level by measuring the time taken to charge. In days of old, we could have used a joystick port to do this, but those are long gone, now replaced by USB-connected joysticks.
On PCs without joystick ports, we could have done the same thing with the printer port. However, those are also a thing of the past.
So far, most PCs still come with serial ports, so we'll try those. The Clear To Send (CTS) and Data Terminal Ready (DTR) lines can be controlled from a Win32 or .NET application. There is some concern about the responsiveness of Windows, but as long as we avoid very short time constants, we can get usable data.
The concept is to build an RC circuit from a photocell and a 200 mF capacitor. The photocell is driven from the DTR line, and the junction of the capacitor and photocell is read by the CTS input. Here's the schematic; it's pretty simple:
The results are surprisingly useful. Discharging the circuit for two seconds and then charging it while polling the CTS line every five milliseconds shows a clear difference as the photocell is pointed at various areas of a lighted room.
All you need is a small solar cell mounted in the end of a "toilet paper" tube. Make sure the cell's face faces inward and you seal the tube end containing the cell so it's (more or less) "light-proof". If you wish, paint the inside of the tube FLAT BLACK before mounting the solar cell (reduce any "internal reflections, etc." of the tube).
To use: Connect the solar cell's leads to the DMM (polarity is not important: all you want is the "absolute" display value) and set the DMM to the 2V scale. Darken the room and turn on the CFL you want to "measure". Wait a few minutes to allow the bulb to fully "warm up" and point the open end of the tube at the glowing CFL. The voltage reading on the DMM display will give you the "relative brightness" indication you're looking for (i.e., "higher" voltage reading = brighter lamp). IMPORTANT: Make sure you use the same "measuring distance" (i.e., 1 inch) between the open end of the tube and each CFL you're measuring for best accuracy! Repeat for all CFL's you want to measure.
Hope this helps.
