The Firework Ignition System

My project began several months ago as a conversation with my brother in law Tommy. He had found plans on the internet to build an enclosure with a set of light switches that could be used to set off fireworks by passing current through a strand of nichrome wire. I told him we could do much better then that. After minimal convincing he agreed and we started scrounging for parts.

The fireworks ignition system has control over 30 unique channels. Each channel consists of a transistor circuit which switches the power source on and off. The system is powered by a 12 Volt 1.3 Ah sealed lead acid battery. When not in use there is a DC input jack mounted that allows for recharging through a wall wort. An arming key was incorporated as a safety mechanism (When unarmed no power is applied to the transistors). Control of the system is accomplished by a serial port set at 38.4 kbps. Through this port the user can set the state of any of the channels, check the status of the arming key and get the voltage of the battery. The entire project is housed in an aluminum suitcase.

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At the heart of my firework ignition system is an SX48 running at 50 MHz. I was able to make full use of the SX’s 36 IO pins. Thirty lines are dedicated to the 30 channels. Two lines connect to a MAX233 chip for serial port control. Another line is connected to a 7805 (TO-92) which monitors the state of the arming key. The remaining 3 lines are used to communicate with the PIC A/D converter via an SPI interface. The system was built with cost as an important factor. At $10 the SX48 Proto board was an easy choice with its incorporated 5V regulator, led and all the difficult surface mount soldering already done for me. I also designed 2 pc boards using the freeware version of Cadsoft’s Eagle. The interface board connects the SX48 Proto board and contains the MAX233 chip, A/D converter and 30 LEDs to represent the state of each of the 30 channels.

The driver board contains the transistor circuits for 10 channels. There are 3 driver boards in the system. Once the boards were designed I printed the design onto glossy magazine paper. I then transferred the toner from the paper to the 1 oz copper clad boards using an iron and etched the boards using ferric chloride. I drilled out the through holes and vias on my drill press and mounted all the components.

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Once the boards were populated it was a matter of creating a panel (I used the plastic from an old x-ray viewer and painted it black), mounting the rest of the components and wiring everything up.

The channels are brought out using speaker terminals. From here the user would run wires from the terminals to the fireworks. A small strand of nichrome would wrap around the fuze and connect between the wires. A cutout is provided in the panel with clear Plexiglas over the interface board and SX48 Protoboard. This provides visibility of the LEDs and the “brains” of the system.

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This project was a lot of fun. As with any similar project I learned a lot. This was the first time I etched my own boards and the first time I soldered a surface mount component. Using SX/B made programming for the SX much simpler. The serial and shift functions made communications especially simple.