What do you get when you power a layer of liquid crystal material sandwiched between two sheets of glass? A Liquid Crystal Light Valve (a.k.a a LCD Controllable Black-out Panel) ! They're often used for electric welding helmets because they can protect the welder's eyes from the bright sparks. Or – if you aren't a welder – they can be used to provide a "reveal" effect for your project, as you can control the opaqueness of the LCD.
A Liquid Crystal Light Valve (LCLV) is a device that uses the properties of liquid crystals to control the level of illumination passing through an optical system. It's basically what goes on in your LCD monitor/TV/projector etc, but for each individual pixels.
In this product, Adafruit have one huge LCD piece, which will filter what you see through it. Normally, its a tinted gray color (because the light around Adafruit is not polarized, it is not completely clear). There are two sets of electrodes, on the left and right. The two pins closest to each other (on the edges) are the same electrode, and the LCD is symmetric so you can use either one as positive/negative.
By changing the voltage applied, it will block light more and more. Starting at about 1.0VDC the glass will start darkening. At about 4.0V the glass will be opaque. 5V is the max recommended voltage. In between you will get darker shades. Note that it is not capable of blocking 100% of light, but it looks like it blocks at least 95%. If you put a bright light behind it, you will see some light shine through!
Almost no current is used and one the LCD, once activated with a voltage, will stay at that darkness level even if the voltage is removed. So, if you want to have it turn on and off, you'll need to drive it with a 0V signal to 'open' it up, or connect a draining resistor between the two pins to leak the voltage off when not powered.
Please note that this is a thin piece of glass, you can't bend it and its very delicate! Use care when working with this item.
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