Build Instructions

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• 1

  Circuit Design

  To build a device that achieves my specification with a microcontroller or small form factor computer would have been trivial. It's far more fun to design the circuit with a combination of 74 series logic chips and the compulsory and venerable 555 IC . I used a circuit simulator, Multisim, to build and test my circuit before I committed to a PCB design.

  Creating the first part of the circuit, the flashing LED, is pretty easy. I decided to build the classic 555 Astable circuit to give a frequency out put of 1 Hz. This gives our aspiring Sydney Bristow or James Bond a 500ms window to diffuse the Logic Bombe. To ensure the a nice even 500ms charge and discharge time I used 2 diodes, one to charge the capacitor and one to discharge the capacitor. The charge and discharge time can be calculated with the following simple equation:

  T = 0.69 x (R x C)

  Using a a resistor value of 15K ohms and a capacitor value of 47uF the charge and discharge times are:

  T = 0.69 x (15k x 47uF)  = 0.49s

  This gives us a period of 0.98s, therefore frequency = 1/P = 1.020Hz. I was aiming for 1Hz so this is fine, especially when you consider the resistors are +-5% tolerance and capacitor tolerances are far more. (ok for government work..:-).

  The output of the oscillator goes to both a blue LED and two AND gates.  There's an option to cut either the blue wire or the red wire, and then a further option to cut the wires when the LED is flashing On or Off. 

  The output from the blue wire is fed to these two AND gates. While the blue wire is "complete" it feeds a LOW signal to the AND gates however when the blue wire is cut the connection to 5V through a 1K resistor comes into play and a 5V signal is fed to the AND gate. One AND gate is looking for a positive input from cutting the blue wire AND a positive input from the 555 1 Hz Oscillator (the blue LED is ON). In this scenario the bomb will be defused, the oscillator is stopped and the green LED is illuminated. The other AND gate is again looking for a positive input from cutting the blue wire, however this time it's looking for a LOW signal from the 1Hz oscillator (the blue LED is OFF). In this scenario the bomb will be activated, the oscillator is stopped and the red LED is illuminated.

  Cutting the red wire is a bit more simple to deal with as it does not take into account of the clock signal. Cutting the red wire at any time means the bomb will be activated, the oscillator is stopped and the red LED is illuminated.

  There's a series of SR latches used through this circuit to enable the "Armed" and "Disarmed" LEDs to come on and stay on. An SR latch is also used to make the reset input to the 555 go LOW and stay low once either wire is cut. The SR latches are built using NOR gates. I also use a resistor / capacitor network on the SR latches to initially Reset the latch so I know that it is in the correct configuration when the circuit is energised.

  I use a couple of OR gates in the circuit. I actually build these using a NOR gate and Inverter. This is simply to keep the chip count down and I have unused Inverter and NOR gates available and don't want a dedicated OR gate IC on the PCB.

  Below is a video walk-through of the circuit simulation showing how it all works.
• 2

  PCB Design

  To design the PCB I simply transferred the schematic design from the Multisim Simulation design to a KiCAD Schematic. I then placed all of the components and laid out the board. Once complete I ordered 5 PCBs from JLC PCB. Even though the circuit worked in simulation, you always worry that you've made a mistake once you've done all the hard work and ordered your PCB for manufacture. These boards came back pretty quick and I had them in about 7-days.

  The full KiCAD design file is zipped up and in the files section of the project if you want to build your own PCB (BooobyTrap.zip).

• 3

  Dirty Bomb

  I wanted a harmless kinetic event to occur if the Logic Bombe was incorrectly defused. After giving it some thought I decided that a "dirty bomb" effect was the safest, practical and reusable method.  To achieve this I used two ultrasonic humidifier modules as shown below.

  If the Logic Bombe is incorrectly defused then there is an output that can supply 5V to a device via the NPN transistor. This output was used to supply two ultrasonic humidifier modules. The modules are usually supplied by a USB connection and are turned on by pressing a button. I wanted the ultrasonic humidifiers to come on as soon as voltage was applied, to this end I had to undertake some light hardware hacking. I simple soldered some power wires to the USB power input which would be fed from my 5V supply from the PCB. I then bridged the small push button with a small length of wire soldered in place. The image below shows how these connections were made.

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