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Wily Widget

  Previous Issues

Wily Widget at Work  --  Shooting Range Hit Indicator

Wily received an email message from the owner of a shooting range, not far from his home.  The guy wanted a unique indicating device which would detect when a steel target was hit with a bullet and flash a bright light. 

The guy said that when hit the steel targets he used at his range make a loud “ping” sound.  But, when his shooting range was busy, often a shooter could not always hear the ping sound from his target above the other sounds along the range.  Could Wily come up with something?  Wily was intrigued and scheduled a visit to the guy’s range the next day.

Nearly all the targets at the shooting range were made of heavy steel plates, painted white. They were supported on heavy steel struts.  Wily noticed that the targets did indeed make a loud ping sound when struck by a bullet.  Wily knew that he could easily detect the strike of the bullet to the plate with a piezoelectric device mounted on the bottom back side of the heavy steel vertical support, close to the ground.  That was the easy part.  But, how was Wily going to indicate when the target was hit. 

A flash of light might work, but where would Wily place the light source, so it would not be in danger of being hit with a bullet itself?  Wily had an inspiration.  Maybe he could use a bright green laser pointer.  Those things are highly visible, even in daylight.  Wily could position the laser low to the ground and aimed so its beam would hit the center of the white target, triggered by the ping of the bullet against the steel.  The laser could be positioned slight off center in front of the target, low to the ground and protected by a thick steel plate or perhaps hidden behind a small boulder.  If Wily was careful with his design, he could make the system battery powered.  The laser itself didn’t need much power and Wily could use an efficient shock sensing circuit to trigger the laser.   

Green Laser Pointer Steel Targets  Target With Green Laser Spot 

The system that Wily came up with is shown below.  The system is broken into two separate parts.  A small box containing a piezoelectric shock sensor and a trigger circuit is 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.

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.
Hit Indicatordesigned by David Johnson
Green Laser Driver Circuitdesigned by David Johnson

First Quarter,  2012    


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