The Perfect Shade: an Simple Self-regulatory Optoelectronic Circuit

In the following I would like to describe a very simple, optoelectronical self-regulatory circuit.

It allows to keep the brightness in a room relatively constant, nearly independent of the brightness outside, using a specific "window". What I can demonstrate here in a small model system, is already used in a similar way in modern housing. Light valves are also used in LCD displays, TV screens and several other things.

The intension of this instructable to build a simple and inexpensive example for an optoelectronics regulatory circuit that could be used for educational purposes.

The system consists of:

- a Liquid Crystal Light Valve / LCD Controllable Black-out Panel (Adafruit Product ID: 3330. 8.40 €, via Exp-Tech,Germany; 96,5 x 38,0 x 2,0 mm)

- a photoresistor (SEN-09088. SparkFun, via Ex-Tech, 1.70 €)

- a rotary or a slide potentiometer (I used a 10 kOhm potentiometer; any type may do, 20 kOhm might be better)

- I used some jumper cables and a breadboard

- a housing, to set up the "room" (I used some of my son's LEGOs for this purpose)

- a power source, 4.5 V or 5 V (e.g. 3x 1.5 V batteries pack or USB)

So how does it work?

The Liquid Crystal Light Valve becomes gradually darker with increasing voltage applied in the range of 1-4 V (Transluminecence vs. Voltage response curve see scatter blot). At 5 volt its transluminescence less than 5% of the maximum value.

The resistance of the photoresistor depends on the amount of light reaching its surface. In our case it will have a resistance of about 8-20 kOhm in a dim lit room, and about 1-2 kOhm in a bright lit room (for details see the data sheet).

The photoresistor and the potentiometer are arranged to give a voltage devider, so that a decreasing resistance of the photoresistor, due to more light, results in an increasing output voltage, which then will darken the light valve and dim the room.

As the photoresistor is placed behind the light valve, a brighter illumination will activate the photoresistor, activating the light valve, reducing the in-comming light, enhancing the resistance of the photoresistor, opening the light valve, ..., and finally the system will settle at an equilibrium point. So in the ideal case, the brightness behind the light valve would be kept nearly constant over broad range of brightness values on the outside and the turnable potentiometer would allow to tune and set the in-door brightness level.

While it seems to be working in principle, this very simple solution will not be perfect and, depending on the sensor and the settings, it may over- or undermodulate. As you may see in the attached videos, my setting results in overmodulation, making the room even darker upon strong illumination.

A more advanced system would use a light sensor, a microcontroller and a digital potentiometer to stabilize the brightness in the room. I may come back to this in a later instructable. Just a hint: a PWM signal may not work well with the light valve.

Please be aware that I am neither a native speaker of English nor an electronics guy, and I hardly know the correct technical terms even in German. So please excuse any errors, and any help and corrections are appreciated.

- Build your "room".
My LEGO construct is a small room with front windows and a glass ceiling, the latter consisting of the light valve. The front windows will allow you to look into the room and to estimate the brightness inside. The photoresistor is placed near the glass ceiling, looking upwards. I intentionally placed it in one corner, so you better can see the difference with or without activated photoresistor. If LEGO is not at hand, you may use a cardboard box or similar.

Take care not not to break or damage the light valve. Its just two sheets of thin glass.

- Setup the electronics
Connect the wiper and a terminal port of the rotary/slider potentiometer, to result in a tuneable resistor. One terminal port is connected to ground, the wiper port (the middle one) to a common base with the photoresistor. Attach the photoresistor to the cables, place one at the common base the other with "plus".
Attach your cables to the light valve as indicated. I would recommend to solder the cables to the connectors, but the female ends of jumper cables might be used either.
Place one cable to the common base of the resistors, the other to "ground".

Place the photoresistor in the "room" looking upwards and place the light valve "ceiling" above it.
Connect your power source to the breadboard.

- Play
Try to find a appropriate setting for the turnable potentiometer. I have been using a strong LED torch as light source. See what happens if you block the light sensor, or you just light either side of the room.
For example, in the second and third image at "introduction" I blocked the light on the sides with and without the sensor, at the same illumination level. As you can see, the lit left side in third image (sensor blocked) is brighter than the right side on the second image (sensor not bocked).

Be aware that the light valve has a memory effect, so it may remain dark for a while even with power off.

Attached you find some sample videos how the system behaves if the light strength is modified or the sensor is blocked. As you may see, the system is overmodulating, making the room very dark at a strong illumination.