Arduino driver library for LCLV like Adafruit 3627 and 3330
A single-element Liquid Crystal Light Valve (LCLV) display, such as the Adafruit 3627 or 3330, is an inexpensive device that uses LC material sandwiched between two polarizers to allow electrical control of opacity by applying a voltage across two leads. Measurements show that these LCLV always reduce transmissivity by at least 2EV (2 stops), but changing the voltage difference across the LCLV from 0V to 5V causes a further drop of approximately 9EV. Intermediate voltages allow smoothly varying transmissivity from -2EV to -11EV.
The light passing through an LCLV leaves linearly polarized, but with minimal diffusion. Thus, the LCLV can be placed in the optical path of a lens, acting as either a programmable neutral denisty (ND) filter or a global electronic shutter. Although somewhat longer shutter open times are recommended, shutter speeds faster than 1ms (1/1000s) were experimentally proven to be feasible using digital control.
This library for Arduino-compatible systems provides an LCLV class providing for AC control of LCLV displays, each using a pair of output pins. Although it is possible to drive an LC material by simply connecting one contact to ground and the other to a controllable output pin, always untwisting in the same direction will slowly degrade the molecular structure of the liquid crystal. It is awkward to drive a signal to a negative volatage, thus, in this library, the AC drive is simulated using two output pins such that the the polarity of the drive signal is alternated at approximately the frequency specified.
Arduinos typically support analog output using analogWrite(), which often is implemented using PWM (pulse width modulation) at between 490Hz and 980Hz. It was experimentally determined that the PWM output does indeed result in fairly consistent intermediate transmissivity for the LCLV. Using a 5V Pro Micro, going from PWM values of 0 to approximately 127 brought a roughly linear decrease in EV transmission, dropping from -2EV to approximately -10EV. PWM values from 128 to about 208 reduced transmission from -10EV to -11EV, and no further change was seen going to a PWM value of 255. However, requesting a PWM value of 255 might reach maximum opacity more quickly, so we recommend use of 255 (continuous digital logic 1 output) for making the LCLV behave as a closed shutter.
The LCLV class takes five parameters:
- pina: Output pin connected to one lead of the LCLV
- pinb: Output pin connected to the other LCLV lead
- v: Analog/PWM value from 0..255 or digital value 0 or non-0
- hz: Desired AC frequency, in Hertz (cycles/second)
- an: Analog control or digital?
The an parameter requires a little explanation. Analog outputs are commonly more linear in the middle of their range than near the minimum and maximum values, thus, if the an parameter is non-0, the PWM average value will be 127 instead of 0. For example, requesting a PWM value of 21 with an non-0 will alternate between 117,138 and 138,117 using analogWrite() to the pins. If an is 0, then v==0 will use digitalWrite() to output LOW, LOW and v>0 would alternate between LOW, HIGH and HIGH, LOW.
This driver was created by Professor Hank Dietz, [email protected], at the University of Kentucky in support of his Electronic Imaging 2021 research paper: "Programmable liquid crystal apertures and filters for photographic lenses." There is a web page, http://aggregate.org/DIT/LC, containing a preprint of the paper, presentation slides, and other relevant materials.
This software is distributed under:
Copyright Henry (Hank) Dietz, 2021.
Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)