Evaluation of three ideas

Idea 1:

• Doesn't allow a variable frequency 
• Rather simplistic design for a second year project
• Use of solenoid adds an important safety feature
• Gas canisters are much too large for a portable device
• Does not employ a way to display the frequency  

Idea 2:

• Servo would still allow gas to flow if power was cut
• Does allow for a varied frequency
• Duty can also be altered unlike 555
• Allows for a way for frequency to be displayed
• Although cheap, it is the most expensive option
• Takes up a rather large and limited PCB space in a handheld device

 Idea 3:

• Variable frequency can be achieved
• No precise measurement or display of selected frequency 
• Frequency can be easily calculated
• Components are inexpensive
• The gas canisters/chargers are rather expensive, however are perfect for a portable application

Development of chosen idea:

For our final idea, we have decided to use push to make switches as our input to alter the frequency. Also, it will be the input to control gas flow. For the control we are going to use an 8 pin PIC. This will allow us to cut down on board space as it has the same layout as a 555 IC, also the 8 pin is a much cheaper option than its larger counterparts. This will keep our costs down an in-line with the 555 and Mosfet options. For the output we are going to source a normally closed gas solenoid, as this has an included safety feature, in addition to being easily controlled via PIC, unlike its servo alternative. For the display of frequency, we are going for 2 options, our first being an LCD display, which will change according to the user input of frequency. Since this adds a great deal of complexity to our design in terms of PCB size allocation and programming. We have decided on a back up of a more simpler LED which will vary in brightness if the frequency is either Low, Medium or High. Since this is a portable application, designs for a voltage regulator for the PIC chip, along with indicators for battery level will also have to be designed and added.

Idea 3

For our final idea, we have decided to use a 555 timer to produce our frequency with a variable resistor to alter the frequency. The gas will be controlled by a simple regulator.

Input:

 The input to our circuit will either be a variable resistor for varying the frequency of the 555 timer. Another option would be to use a rotary switch switch as a single pole 5 throw with a predetermined resistor. The advantage of this would mean that a set frequency could be achieved. Decals or labels could be used on the device to tell the user the frequency.

Rotary Switch

Control:

For the control, we have used a 555 timer in astable configuration to provide our frequency.
The frequency, of the output pulses is determined by the values of two resistors, R1 and R2 and by the timing capacitor, C.
The design formula for the frequency of the pulses is:

 




The period, t, of the pulses is given by:



The HIGH and LOW times of each pulse can be calculated from:

The duty cycle of the waveform, usually expressed as a percentage, is given by:




Fro, this we calculated the values of our resistor and capacitor so that the frquency can vary between 2 and 8KHz. Below shows a simple schematic of our circuit

555 in Astable mode

Output:

The gas output of this idea takes a more mechanical approach. The gas will be controlled by a built in regulator such as this one below

Mini CO2 regulator

This regulator will be used in conjuction with a helium canister or 'charger' like this. The problem with this concept is that the chargers fro helium are rather expensive, however it is the easiest route to go.

Idea 2

Control:


For our second idea I decided on a simple microcontroller to output a high frequency. After much research and keeping the cost in line with my other ideas, I settled on the PICAXE series of microcontrollers, which are based on the range of Microchip PICs. Not only are these chips available in DIL packages, (A stipulation for our project) but also have inexpensive start up costs. The programmer being a simple Serial to 3.5mm connector
Below shows a 14 pin Pic called the PICAXE 14m. The advantage with this over the the Mosfets is that the frequency can be varied.

Output: 

Idea 1

Control:

For our first idea we decided to use a voltage source inverter using Mosfets to give our frequency

Voltage source inverter

I began modelling a simplified circuit on Livewire which is based on that from
http://escholarship.lib.okayama-u.ac.jp/electrical engineering/2

The idea for this was to feed into a third order resonance circuit then onto a step up transformer

A simple on/off switch would be used to power this part of the circuit.

Output:


For the control of the gas I decided to use a small canister of helium. These were found to be relatively cheap. Another option would be to use Nitrogen, another inert gas, however industry does not seem to produce them in such small canisters at the helium

The helium would be controlled by a solenoid, which acts in a normally closed state. This adds a safety aspect to the project, in the case that if power was cut from the device, the gas would cease to escape.
The solenoid would be controlled via a simple push to make switch from a separate 12 volt supply

N/C Solenoid with PTM switch

Ideas for Electronics

Since the labs in the University of Liverpool are So lovingly organised in such a way. That parts for a project that takes place in February for four weeks, Has to have a parts sheet listing all the components you are going to use, and Somehow expect you to have a near enough completed circuit diagram to pull outta your head in Mid-December.....
I do apologise if no research is at the section, as I unfortunately have to rush into circuit ideas to make a completed parts list for Monday. Absolutely ridiculous!

Please organise the lab projects better, this goes for Year 1 as well!!

Anywho, from Dr Taylor and the brief research Barry and I collated, we found that a circuit that generated a pulse at 2-8KHz that fed into a hand built transformer to generate around 2KV that would excite an inert gas like helium into a plasma. Dr Taylor also added that a display of the frequency would be a nice addition to the circuit. From this Barry and Me drew up a simple specification.

Spec:

• Device has to be portable, so battery powered
• High frequency pulse generator circuit
• Variable frequency (if possible)
• Way to display frequency (if possible)
• Ideal way to control the flow of gas 
• Addition of safety features 

From this I would use my research in addition to my own material to draw up three ideas. I have deceided the best way to go about this is by a modular approach. By splitting up the input, control and output, this allows not only my circuit designs to be easier, but also fault finding when it comes to prototyping. In addition if one part fails on inspection day, at least we can demonstrate that other parts of the circuit function well.

Brief introduction

For our second year project myself, Michael John Lynch and my wonderfully brilliant lab partner, Barry Smith have chosen to design and build a micro plasma generator

After a short brief with our tutor, Dr Taylor (Thursday 24th November 2011) and some initial research. We found three core elements to the project, the electrical, electronic and mechanical side. We decided that Barry should take the electrical side, and I the electronic side, and we would work together on the mechanical aspect.