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Part 2 - Project Specifications

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Project Features

A good starting point with any design is to decide what features one would like the finished entity to have. In the case of this project, a few features would be particularly desirable:

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  1. The ability to adjust the maximum brightness of the tube;

  2. A simple ON/OFF switch, as opposed to having to unplug it every time;

  3. The ability to adjust the frequency of pulsation of the tube ('breathing' frequency);

  4. A 'soft-start' to reduce the initial current surge through, and hence the stress on, the filament;

  5. The power source should be a standard USB input.

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This was all to be achieved without spending a ludicrous amount of money on specialist components and with a final control circuitry solution that was compact since the tube itself was only small.

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Solution Overview

To achieve the low cost and small footprint, a microcontroller-based solution was the only real option for the control circuitry. This also had the benefit of giving me access to the multitude of peripherals (PWM, ADCs, timers etc.) available on even the smallest of microcontrollers these days.

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The main obstacle, however, was in generating the correct voltage for the anode of the tube. The filament, since it requires a voltage below that of the 5V provided by the USB source is relatively easy to acquire, a simple series resistor or a PWM (Pulse Width Modulated) signal applied to it would be sufficient to drop the voltage and current down to a safe level. The anode requires a voltage (much) higher than 5V and will need to be boosted by means of a DC-DC converter. This higher voltage should also be adjustable to provide the adjustment in brightness as stated above.

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The ON/OFF switch should be as convenient as possible, the last thing anyone wants to be doing is struggling to find and flick a tiny SMD slide switch in the pitch black. This limits the selection to either a tactile push-button switch or a capacitive touch switch.

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Finally, since a microcontroller is being used, the 'breathing' frequency can easily be varied simply by using a potentiometer and an ADC (Analogue-Digital Converter) to set the period of a timer within the microcontroller.

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In the next part, I will begin exploring how to generate the high voltage for the anode of the IV-15.

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