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\technote{7}{Dan Ports}{2004/04/29}{drkp@mit.edu}
\tntitle{An Inexpensive Remote Projector Shutter}


\section{The rationale}
\label{sec:rationale}

Video projectors are incapable of displaying a true black, i.e. an
absence of projected light. Having any light projected when the
projector is inactive is frequently unacceptable in the theater, such
as in a blackout. Since projector lamps generally cannot be switched
on and off without adequate warm-up and cool-down periods, the light
must be physically blocked with a shutter. This is commonly done
simply by manually placing a piece of paper in front of the lens. But
this is impossible if the projector is far away. Various manufacturers
will happily sell you a DMX-controlled shutter for \$275 or so
\cite{wahlberg-shutter}. Those of us without unlimited budgets are
forced to be a bit more creative.

This note presents a design for a remotely-controllable motorized
projector shutter that can be built for almost no cost --- it requires
only a (possibly broken) CD-ROM drive and some readily available
electronic parts. It is based on an idea mentioned by two posters to
the Stagecraft mailing list \cite{stagecraft-shutter}.


\section{The design}
\label{sec:design}

A computer's CD drive can easily be converted into an effective
projector shutter. The tray serves as the shutter; the hole in the
tray will need to be covered with blackwrap or some other suitably
opaque material. Helpfully, the CD drive already contains a motor that
can open and close the tray. We need only control it ourselves, since
we will not be using the drive electronics.

The first step is to take apart the drive. Exactly how to do this, and
exactly what the innards of the drive will look like will vary
depending on the model --- and, given that we're dealing with
scrounged-up parts here, we really can't be too picky about what model
drive we're using --- but the general set of parts will be the same.
At this point, it's possible to disembowel the drive if desired,
removing the main circuit board, laser\footnote{If power is on, do not
  look into laser with remaining eye.}, spindle motor, etc. It might
also be useful to remove the front bezel, if adding the blackwrap
makes the tray no longer open.

The tray motor is generally located near the front of the drive
(though the exact position will vary with the drive), and
should have two solderable contacts. Solder a long wire to each one;
this will be the attachment from the CD drive to the electronics. It
doesn't much matter which wire is which; the only effect will be to
reverse which switch direction is forward and which is reverse.

A schematic for the electronics is provided in
Figure~\ref{fig:schematic} (also provided separately as a PDF
file). The required components are listed in
Table~\ref{tab:components}.

\begin{table}[htbp]
  \centering
  \begin{minipage}{5in}
    \centering
    \begin{tabular}{|rl|r@{.}l|}
      \hline
      Num. & Component & \multicolumn{2}{c|}{Price} \\
      \hline
      1 & CD-ROM drive & \multicolumn{2}{c|}{---\footnote{Conveniently found
      wherever old CD-ROM drives are disposed of}}  \\
      1 & LMD18200 H-bridge\footnote{It's cheaper and probably just as
      good to use
      something else instead --- see the note} & \$11&69
      \cite{digikey} \\
      1 & 74LS123 monostable & \$0&56 \cite{digikey} \footnote{Or ``borrowed'' from one
      of the Course VI labs --- not that anyone would do anything like
      that, of course} \\
      1 & LM7805 5V regulator & \$0&57 \cite{digikey} \\
      2 & 100 $\mu$F capacitor & \$0&56 \cite{digikey} \\
      1 & 50 k$\Omega$ potentiometer & \$0&44 \cite{digikey} \\
      3 & 10 k$\Omega$ resistor, 0.25W & \$0&42 \cite{digikey} \\
      2 & 560 $\Omega$ resistor, 0.25W & \$0&42 \cite{digikey} \\
      2 & LEDs, 1 red and 1 green & \$0&23 \cite{digikey} \\
      2 & Push-buttons, normally-open & \$0&94 \cite{digikey} \\
      1 & Selector switch, SPST & \$0&73 \cite{digikey} \\
      1 & Power jack, $2.1\times 5.5$ mm & \$0&42 \cite{digikey} \\
      1 & 12V 1.5A wall transformer & \$15&30 \cite{digikey}
      \footnote{Or cannibalize a spare wall-wart from wherever you can
        find one --- it can't be hard to find a cheaper one} \\
      \hline
      & \textbf{Total} & $\le$ \textbf{\$36}&\textbf{48}
      \footnote{Generally much less, since so many of these parts can
      be found for (nearly) free if you look in the right places} \\
      \hline
    \end{tabular}
  \end{minipage}
  \caption{Required parts}
  \label{tab:components}
\end{table}

\begin{figure}[htbp]
  \centering
  \includegraphics[width=6in]{21m735-tn7-schematic}
  \caption{Schematic diagram}
  \label{fig:schematic}
\end{figure}


\section{Some notes on the electronics}
\label{sec:electronics}
This circuit provides three controls to the user: a \textsf{Position}
switch and \textsf{Go} and \textsf{Stop} buttons. When the \textsf{Go}
button is pressed, the shutter moves to the position selected by the
\textsf{Position} switch. The \textsf{Stop} button can be used to stop
the tray (it's not totally clear to me why you'd want to stop the tray
while it's partially opened, but you can do it.)

Power is supplied via a 12-volt AC adapter (``wall-wart'') through a
female 2.3 mm power jack. This
seemed appropriate, since this would likely be used in a
semi-permanent environment, and it would be easier to connect it to AC
power rather than have to deal with changing batteries. It could be
easily converted to run on battery power (e.g. a 9V battery). The only
complication is that the particular H-bridge used here, the LMD18200,
requires at least 12V power. Also, the tray motor is designed to run
on 5V power, but seems to work for me at 12V (and very quickly!). The
speed of the motor operation is proportional to the voltage applied,
so it's necessary to adjust the operation time of the motor
accordingly (see below).

A voltage regulator is used to provide a regulated 5V supply to power
some of the electronics. A bypass capacitor connecting the 5V supply
to ground is necessary to eliminate transient noise when the motor is
activated (this would interfere with the monostable operation). I used
a 100 $\mu$F electrolytic capacitor because it was what I had around,
but a smaller capacitor should work too.

The circuit controls the motor using a ``H-bridge'' chip. This chip,
whose internals are roughly like the circuit in
Figure~\ref{fig:h-bridge}, controls a DC motor by connecting each
motor terminal to either power or ground. This means the direction of
the motor is reversible, which is essential to this application. The
chip I used for this purpose was a LMD18200 H-bridge chip. This chip
provides some nifty features like thermal sensing and current overload
protection --- none of which can reasonably be claimed to be necessary
here. That is, it's total overkill for this application; I just used it
because I happened to have one. Alternatives would be to use a cheaper
H-bridge, or to build one from discrete transistors as in
Figure~\ref{fig:h-bridge}.

\begin{figure}[htbp]
  \centering
  \includegraphics[width=3in]{21m735-tn7-h-bridge}
  \caption{H-bridge internals}
  \label{fig:h-bridge}
\end{figure}

A 74LS123 monostable multivibrator\footnote{This is not obscene.
  Really.}  (``one-shot'') solves the problem of how long to turn on
the motor. The ideal solution would be to detect when the tray is
fully open or closed, but I just couldn't bear the thought of poking
at the drive electronics to find out how to do this (also, it would
probably be different on different drives). Instead, the monostable
serves as a timer --- whenever the \textsf{Go} button is pressed, it
runs the motor for about one second. The exact time depends on the
resistance of R$_2$ and capacitance of C$_2$; R$_2$ is a potentiometer
in order to make this time adjustable.

\nocite{scherz}
\nocite{horowitz-and-hill}
\bibliography{21m735-tns}
\end{document}