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.

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).

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.

Once all of the component parts had been sourced and the PCB soldered together it was time to build up the Logic Bombe. I didn't want the Logic Bombe to look professional and slick, it needed to look as though it was cobbled together by the compliant hacker of a super villain. I decided to start with the payload, this is simply jam jars filled with water and food colouring to give the effect of being a noxious substance. The jam jars were taped together with some black duct tape. 20mm holes were cut in each the jam jar lids  and a 20mm cable gland was used to support the humidifier elements. A 5mm flashing LED was also mounted in each lid to add to the effect. 

The PCBs were stuck to the payload with a combination of cable ties and double-sided tape. The power supply was a small USB power bank, I stripped a USB cable end and fed the positive and ground wires into the PCB power supply terminals. To finish off the Hollywood movie look, I made sure the red and blue defusing wires were over exaggerated coils sticking out of the side of the device.

With it all together it was time to test it. I cut each of the the wires with the LED in the high or low state to check it worked as designed. Take a look at the video below showing how to defuse the Logic Bombe.

In conclusion, I really enjoyed this project. It was great learning about the Raspberry Pi Pico and I will definitely be using it in future projects. It was fun to use some 74 series logic chips after a small hysteresis of a few years where I always reached for a microcontroller instead.

Thanks to my daughter for producing some great art work for the project.