(N.B. No
help is available on this unit today. This article is purely for
historical interest and the Colour Reversal Club cannot be held
responsible for any problems should you attempt to built this unit).
A lot of interest
has been shown recently in “doing A/V on the cheap”, and I would like
to help with a scheme that I worked out a few years ago and has been well
tried in this area. The obvious thing is to get the projector lamp to dim
using the low voltage side of the projector, and not getting involved in
the mains side of it at all, for safety reasons. I sent the circuit to
Pete Roberts some time ago, and he said that he had not been able to dim
the lamp on the 24V side properly, and that he would use my system
and report back to me, but unfortunately he went abroad to one of the
Colonies and I have not heard from him.
I understand that
some projectors use an “Auto Transformer” in the lamp circuitry, and
if your projectors are of this type, then this circuitry is definitely not
for YOU. So before trying to use this circuit then check if yours is in
this type.
Using two
projectors you will need to have two separate controllers both of which
are contained in the one simple control box. The circuit given is for the
one projector, and will need to be duplicated.
The drawings will
show how this is intended to be used. In operation the sliders are moved
from one end to the other, together that is and one lamp should dim at the
same time as the other one comes up to full brilliance, then the button at
the end where the sliders are located should be depressed to make the
appropriate change. A double ganged slider could be used, using one in
reverse, but using the single sliders both projectors can be brought up to
full brilliance so as to align them.
On Rollei type
projectors it has been found possible to mount the Triacs suitably
insulated to the metal case of the projector, using this as a. heat sink.
The Triac needed is one that will be able to pass the full lamp current,
i.e0 for a 150 watt lamp a 6.arnp device is needed,
and for a 250 watt lamp a 10 amp Triac must be used. I have not tried this
on a 250 watt lamp. A certain amount of fiddling with the pre set
potentiometer will be needed so as to get a smooth control, and with one
lamp completely out.
The 5K pot. is
one that is used as the control, and the 3K3 pot is the pre set ones. As
can be seen on the circuit the Triac is in parallel with the lamp on/off
switch, so if needed the switch can be used in the normal manner. Some
projectors do not have a separate switch for the lamp as do the Rollei, so
it will have to be in series with the wiring to the lamp, which means
unsoldering some of the wiring, beware of this, and make sure the joint is
not a “dry” joint because of the amount of current that will pass
through this joint.
If this circuit
is used on a Rollei you will find spare pins on the hand controller, and
these can be used for the extra. wires, the push button for the slide
change is wired in parallel with the. fixed button on the projector body
or use the sarne pins as are used in the remote control.
If further
information is needed, then write to me at my address at the article end.
As can be -seen it will be possible to make this device for two projectors
for £10-12. Looking at the R.S. catalogue I see that an 8A Triac is
cheaper than a 6A one.
Good luck to you
all who try it, and don’t forget that in back issues of the Library
there is a circuit for auto changing one projector. from a stereo tape
recorder, using a low frequency pulse that will not interfere between
tracks.
Diagrams on following
page.