When shooting using a light tent a low intensity light source is quite useful. The CCFL (cold cathode fluorescent light) found in LCD screens is perfect for this purpose. CCFL and the associated light scattering panels can be found in broken laptop and lcd screens for virtually nothing.
This Instructable shows how to use the a salvaged panel, a DC power source and inverter to create a large, low intensity light.
Some Words Of Warning
This project involves electricity, high voltage and soldering. If you do not feel confident about working with any of these things, do not attempt this project.
Step 1: Materials
- Broken LCD panel with a functioning Cold Cathode Fluorescent Light tube
- DC-AC inverter for your LCD panel and hookup harness
- DC power source capable of producing at least 12V
- Soldering Iron
- Selection of resistors (for a 12V power supply a 70K Ohm and 50K Ohm)
- Single Pull, Single Throw switch (SPST)
- Hookup Wire
- Screwdrivers and other implements of destruction
- COMMON SENSE FOR WORKING WITH HIGH VOLTAGE
Step 2: Locate a Broken Lcd Panel
Locate an LCD screen that still faintly lights up, but is otherwise non-functional. If the screen does not light at all, it is most likely suffering from a worn out CCFL or inverter. In this case, you can try to purchase a replacement inverter or tube, but typically this is fairly expensive.
Broken LCD screens can be found on EBay. Look for 15"-17" monitors.
Step 3: Extract the LCD Layer
- LCD - the black colored panel that actually produces images (upper most layer)
- dispersion layers - there are usually three plastic layers that help disperse the light from the CCFL evenly over the entire panel
- a reflective panel - the last layer of the panel - the CCFL bulb is usually embeded or attached to this layer. Use extreme care when handling this portion of the screen. The CCFL is very thin tube of glass that is quite delicate. Also, it is filled with mercury vapor which is not so good for you or your brain. Don't break it.
Sometimes the dispersion layers refuse to stay in the frame without the LCD layer in place. A small amount of clear packing tape around the edge of the frame will help solve this problem.
Step 4: Locate a DC-AC Inverter
The CCFL requires a fairly specialized circuit to drive it. LCD inverters can be found on EBay for about $12. Generic inverters will do the job just fine.
If you're salvaging a panel from a broken whole laptop or monitor, locate the small board that the panel plugs directly into.
If possible salvage the wire harness that is made for your inverter. You can remove the stock connector to provide bare wires that are easy to work with. The harness that connects to the DC in side of the inverter is most useful. Unless you have been extremely careless with the panel, the AC harness should still be connected to the CCFL tube.
In theory an inverter should be closely matched to the CCFL it will be used with. This will typically extend the life of the tube and board. This shouldn't be a problem for this type of project, however. As long as the inverter is for a tube of approximately the same size CCFL, it should work just fine.
Step 5: Create a Voltage Divider Circuit
From my experimenting I've discovered that many inverters expect 12V+ input to drive the inverter and and around 5V+ to "enable" and set the "dimming" level. Check this generic spec sheet for some guidance: http://www.lcdinverter.co.uk/MH-1405A04-spec.htm. From my testing, the circuits are quite robust and can accept between 4.5 and 7 volts on the enable and dim pins and work properly. Above 7 volts a suspicious whining tends to be emitted.
A 12V DC power source can be easily adapted to power the inverter by using a voltage divider circuit. In a voltage divider circuit, use two resistors to drop the voltage as needed. In this circuit R1 drops the voltage 7V then R2 an additional 5V. Notice that 7V+5V=12V. The schematic below shows the circuit I used to create the voltages I needed to drive my inverter.
In this circuit, C is Ground, A is 12V+, B is 5V+. For an inverter that has enable and and dim pins, connect those to 5V+ to turn the screen on.
Prototype your circuit using a breadboard. Measure the voltage between C and A; it should be 12V+. Measure the Voltage between C and B and it should be 5V+. If you get values within 10-20%, you should be OK.
If you need assistance in choosing resistors for your particular voltage source, check out the Circuit Design Tutor.
Some tips to keep in mind:
- If the sum of the ratio of the resistors equals the input voltage, your design will be much simpler. For example Vin=12V, ratio of resistors is 50:70 or 5:7 - 5+7=12.
- Remember you can simply add resistors together in series to create a single resistor (read up on Ohms Law for help here.
- If your input voltage is 18V, R1 should be 130K Ohm, R2 should be 50K Ohm.
R1: 70K Ohm Resistor
R2: 50K Ohm Resistor
K1: SPST switch
Step 6: Test the Circuit
A word of warning here: The inverter creates extremely high voltage output. While it is rather low amperage, it can still do some damage. Don't even think about handling the inverter while it is on. Burns, shocks and maybe even death can occur.
Plug in your voltage divider, power source, inverter and panel and see if everything works. If your inverter is making a high pitched whining sound, it is either damaged or something is receiving too much voltage. Unplug everything quickly and double check your voltages and wiring. If voltages are correct and the whining continues, your inverter is most likely damaged. Find a replacement. Damaged inverters have been known to overheat and start small fires.
Step 7: Enclose the Circuit
Enclose the circuit - I like Ice Breaker Sours Gum boxes. They're super cheap, self sealing, easy to cut and just the right size for small projects.
Step 8: Complete and Use the Light
I used the following parts to make a more complete light:
- original screen housing with hinges
- heat dissipation frame
Step 9: Some Shots of the Light in Action
These are some shots that I took using the light. The light was oriented to the right side of the light tent and was the sole source of light for most of the shots.
The shots have been post processed, but only for black/white balance and contrast.
Some Closing Thoughts
I'd like to get a panel with 2 CCFL tubes (top and bottom) and cut a hole in the center and use the panel as a ring flash.