Shooting Range Hit Indicator

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. 

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.

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.

DC Magazine, 2nd Quarter 2012, Page 7:  Shooting Range Hit Indicator