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Problem: Design a unique
indicating device which would detect when a steel target on a shooting range was hit with a bullet by flashing a
bright light. A crowded range masked the pinging sounds.
Solution: The circuit is shown below. The solution is broken into two separate parts - (1) a small box
containing a piezoelectric shock sensor and (2) a trigger circuit connected to the main
control box, positioned some 15 feet away.
The two assemblies were linked through a
coax cable. The coax cable has a thick jacket and is buried a couple inches below
the surface.
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The hit sensor contains a
simple ultra low power voltage comparator circuit, which is connected to a 500ms pulse
generator circuit. Once fired, the circuit’s transistor applies a resistive load to
the wire, which is also supplying power to the sensor circuit. The diode D1 prevents
the low impedance load from discharging the capacitor C1. The coax center conductor
therefore acts as both the power source and the signal source. When triggered, the
circuit causes the voltage on the wire to drop by about a half volt. That voltage
drop is detected by the main control circuit, which then strobes the green laser.
The hit detector assembly is attached to the back side of the target’s steel support,
using a strong magnet. The acoustical shock from the bullet hit works its way down
the steel support and through the sides of the plastic box.
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The other end of the coax cable is
connected to a second box containing a replaceable 3.6v lithium battery, the guts of
a green laser pointer module and a pulse generator circuit. When the shock
sensor detects the hit of a bullet, the 500ms one shot pulse from the hit detector
activates a 10Hz oscillator, configured to produce 20ms pulses. Thus, during
each bullet hit, the circuit pulses the laser module with about 5 pulses.
This visual effect is easy to spot, even over a
long distance. The laser is aimed at the center of the target. The box
containing the laser is installed low to the ground and protected by a boulder or a
thick steel plate, angled at about 45 degrees toward the shooter. A strip of
plastic is installed over the front of the box, to protect the laser from rain and
dust. |
Tadiran
3.6v Lithium Battery |
The battery selected was a
non-rechargeable 3.6v lithium battery, made by Tadiran. The battery is the
same size as an AA battery, so a standard AA battery holder could be used. The
2000ma-hour capacity of the battery should provide enough energy for over 100,000
laser pulses. The standby current for the complete system measured about 5
micoamps. The current was low enough that no on/off switch was needed. |
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