Defcon 2009 Target Design

 

Design Goals

• Mounts on the car
• Detects airsoft hits
• Does not get triggered by car movement
• Easy to make
• Provides 2x IR beacons for targeting

 

Mechanical

I have seen targets shatter into pieces from the force of airsoft bullets. I definitely do not want that. So my target material is 3/8 inch polycarbonate. Polycarbonate is the stuff that the actual bullt-proof glass is made of, so it should be strong enough. The 2x4 piece of smoked poly was $1 at TAP Plastics, and that was mostly cutting charge.

 

To detect a pellet hit, we need to have a sensor. Again, I decided to be cheap, and bought 1 3/4" piezo speaker aat my local surplus store for $0.25. This is just a metal disk, with some pizeo material and wires sticking from one side. They are sold in many, many places and most of them will work without any modifications in circuit.

 

A piezo speaker / sensor (with USB cable for scale)

 

Once I had both, I just applied generous amount of superglue, and attached speaker to the center of plastic. This was it! A quick test with a scope proved that there was some signal coming out of it in response to taps.

 

To finish the device, I needed to also have IR leds and mounting brackets. So I drilled two holes for IR leds, and 4 holes for mounting brackets. The holes were slightly smaller than LEDs, so the friction holds the LEDs tightly.

 

Photos of the finished device:

 

 

Note that I did not have IR leds on hand, so I have inserted a single yellow LED instead. Also, this is a debugging version of the device with serial ports attached and no LED drivers.

 

Electrical

 

Piezo speakers generate charge when bent. What that means is that they can 'potentially' generate huge voltages, but those voltages could be easily and safely reduced by connecting a parallel resistor. Thus the simpliest circuit is just speaker and resistor in parallel. I have found out that 100K resistor is enough to keep the signal level to +/- 1V for most cases.

 

Speakers generate both positive and negative voltage. Most circuits cannot work with voltages outside the power supply range. So if our power is 0V and +5V, how are we going to work with -1 volt from piezo? Well, we could require 0V, +5v and -5v, but it is much easier to move the common for piezo. Thus, instead of generating -1 volt to +1 volt, it will generate 1.5 volt to 3.5 volt, and we can easily work with that. We can achieve it with the two resistors in series.

 

I wanted to keep schematics as simple as possible, and for that I chose AVR ATtiny85 chip for microcontroller. The chip could be re-programmed in circuit, and most importantly, it has built-in differential amplifier. This means that whatever the common voltage for piezo is, it will automatically substract it before doing measurement. It is also small (8 pins), so the circuit is compact. The chip is the only active component I need.

 

The device consits of:

• MCU and interface
  • ATtiny85 + 8-pin socket
  • 10uF tantalum capacitor (C1)
• Input circuit
  • 1 resistor for setting the scale (R1)
  • 2 resistors for setting the center point (R2, R3)
  • 2 diodes to limit maximim voltage and protect the MCU (D1, D2)
• IR beacon circuit
  • 1 resistor for base current setting (R4)
  • 1 transitor (Q1)
  • 2 resistors (R5, R6) for LED current limiting
  • 2 IR LEDs (D3, D4)

 

Schematics and electircal drawings:

 

The total is just 14 parts for fully functioning detector, including IR emitters

 

Tests

 

For tests, see DC09TargetTest1

 

Component pricing

• $1.00 Polycarbonate piece, 3/8", 2" x 4"
• $0.25 Piezo speaker
• 2x $0.80 90deg brackets for mounting

 

• 2x $0.50 IR LEDs
• $2.73 ATtiny85 micro
• (less than $2) Resistors, capacitors, diodes, transisor, screws

 

Total: less than $9

 

 

-- Mikhail