provides an easy mechanism for laser operators to prevent laser output
when needed.
The interlock portion of the ISP standard is intended to help prevent
light from coming out of the projector if the projector is mistakenly
connected to a non-laser signal source.
Although it is shown in the diagram as a relay, which applies or
removes power from reaching the laser diode drivers, the interlock
system may be implemented in other ways, including as an additional
shutter within the projector.
The ISP specification allows for voltages up to 25 volts, and currents
up to around 160 milliamps to exist on the DB-25 pins. Thus, the
projector interlock must be facilitated in such a way that these values
are not exceeded. However, I recommend that you try to implement
an interlock that uses far less voltage and current – for example 5
volts and 5 milliamps. This could be done using an electronic relay
instead of an electro-mechanical relay. The interlock signals may also
be implemented using other methods that might provide an increased
level of safety – for example, by outputting a small sine-wave signal
on pin 4, and comparing it to the voltage received on pin 17.
It is also a good idea to put an LED or some other indicator
somewhere within the interlock system so that the user can see when
the interlock is enabled or disabled. In the diagram, we show an LED
connected as the Laser Emission Indicator.
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