Optical Trigger
Module
This optical trigger
is a general purpose module for use with 'standard' 3mm package infrared diode
emitters and corresponding matched phototransistors. The emitter terminal on the
module will source about 27mA to ground with a 1.2 V drop across the diode. The
detector terminal will source a maximum of 3.4 mA to ground (short circuit). The
circuit operates by sensing the voltage difference between the potentiometer (ADJ)
level and the detector level. The circuit diagram is.

The circuit operates
by detecting the voltage at the Detector terminal and comparing it to the preset
voltage level. When there is nothing blocking the light path from the emitter
to the detector, the phototransistor is conducting and the voltage at the detector
terminal is low. In this state the output from the opamp is low. When the light
path is blocked by the projectile, the phototransistor conduction decreases and
the voltage at the detector terminal increases. This creates a positive difference
between the opamp inputs and the output goes high. The ADJ 2.2K trimmer pot allows
some adjustment of the triggering point to accommodate variations in the emitter-detector
setup. Power is supplied via a 3mm/1/8" jack socket. The supply is internally
regulated at 12V. The LED indicates whether the output is high - a helpful feature
if a fault occurs in the optical gate. The component parts and assembled module
are shown below.
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While this is a
very basic module it sufficient for preliminary experimentation. The module is
limited by the switching speed of the phototransistor and the slew rate of the
opamp. Typical switching speeds of phototransistors can be anything from ten to
a hundred micro seconds depending on the circuit arrangement. This system is suitable
for velocities up to about 50m/s. If a faster device is required then a photodiode
system can be implemented. The switching time of a photodiode is in the nano second
range but the photovoltage is very small. This requires more critical signal processing
than the phototransistor setup. Opto modules for future high velocity coilguns
will be based on photodiode detectors and high speed comparitor opamps.
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