Intro : This simple, cheap, robust light meter is great for teaching labs for students to learn about light. And, it’s easy to make.
Features : Parts cost around $10.00 US. Gain switch has 5 positions, each an order of magnitude more sensitive from very dim room light to very bright sunlight. (Slight modifications allow extends sensitivity an extra factorof 1000, and neutral density filters can be used for bright light.) Sensitive from mid-IR to mid-UV. With filters can look only at UV, IR, Visible, individual colours, polarization, etc. Easy to build with tools at home – just use a drill, saw and soldering iron. Works with any cheap multimeter Permits quantitative measurements for an enormous number of existing experiments (eg. 1/R2 law,efficiency of different light sources, polarization of light sources, emission spectra etc.)
Parts and Tools
Circuit : Photo diode – VTS3082 or VTS3085 from VACTEC PHOTO or similar. LM324 Quad Op Amp 14 Pin Chip mount (recommended by not essential) 100, 1K, 10K, 100K, 1M Ohm resistors (1/4 or 1/8 Watt) Rotary Double-Pole Six Position Switch - C&K Supplies - Type A20615RNZQ Knob for Switch GeneralPurpose Power Diode – 1N4007 9 Volt Battery 9 Volt Battery Clip BNC Connector Coloured connection wire 2 (or 3) pin connector Case : 3.5 inch long pipe (Diameter 2, 2.5 or 3 inches) Straight coupling. End cap. Sheet of 1/32” inch (or similar) aluminium plate (large enough to cover the pipe end – eg. 3”*3”) Velcro-Adhesive tape strips. 5 minute epoxy glue. Black Paint (spray can works well) Smallpiece of white card-board and pen. 2 Number 3 (or similar) self-tapping screws. Tools : Soldering iron, solder, flux, wire clippers, pliers Hack Saw, file, bench clamp, drill, drill bit set (5/64”,1/8”,3/8”, rose), screw driver, punch, tin snips Notes : See the end of the document if you want supplier information. Most photodiodes will do the trick (eg. broken solar calculators). Just hook it upto your multimeter. It should give a voltage of at least 0.25 V when exposed to light. If you’ve got a sensitive multimeter (or you shine a bright flashlight on the cell) you should be able to get 100 micro to 10 milliAmps out of it when measuring the DC current. Note, if the photodiode has a coating on it this may absorb UV or IR radiation. The VACTEC PHOTO one has no coating, but SILONEX andother manufacturers make similar ones. Most OP AMPs that can run in single-rail mode will do.
Any multi-position switch will do. We waste one pole of the DP as a power switch, but you could easily use two switches (SinglePole/SingleThrow Power Switch + Gain) For piping, go with Schedule 40 or Drainage. 2 inches is a bit of a squeeze, but it works fine. 2.5 inches is probably optimal. Almost anyhardware store carries suitable piping.
Building the Case
Plans : The sketch below gives a pretty good idea of how it goes together. Figure One : Side View Schematic
(A – Straight Connector, B – Aluminium Face Plate, C – Photo Diode, D – Self Tapping Screws, E – Piping, F – 9 Volt Battery, G – End Cap, H – BNC Connector, I – Multi-position switch)
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A circle of aluminium isglued into the straight connector to hold the photo-diode. The 9 Volt Battery is held inside the tube with Velcro. The switch and BNC connector are mounted into the end cap. Number 3 wood screws keep the end cap and straight connector from sliding off the tube.
Steps for Building the Case Pipe : • Cut the pipe to about 3.5 inches in length • File the ends smooth and rounded so they don’t jaminto the connectors Connector Piece and Face Plate : • Push the pipe a comfortable distance into the straight connector piece (don’t push really hard or it will be a nuisance to slide in and out) and clamp the connector piece. • Punch the position for, and then drill a hole for the wood screw (5/64” works well for a number 3) to hold the connector piece (about a ½” from the bottom of the straight...