As the first board is a prototype, I decided to try and make the circuit board with minimal facilities and low cost. Materials included the single-sided copper clad fibreglass board, a Dremmel milling tool bit of 0.7mm for making the holes, an etch resist marker pen ( any permanent marker will do the job ), some copper etch powder and finally a special abrasive 'eraser' designed for cleaning and de-greasing the copper in one go. I began by printing out the circuit on an ordinary ink jet printer. I then tacked the diagram in place with some Araldite. I oiled the paper to make it transparent so that I could clearly see the where to drill the holes. The holes were drilled using the 0.7mm Dremmel bit mounted in an ordinary electric drill. 0.7mm is just the right size for the component wires and even the 'legs' of the DIP sockets. The only holes that I needed to widen slightly were the ones that took the 5V voltage regulator.  

Once the holes were drilled, I removed the paper, cleaned the board of residual Araldite, used the abrasive eraser to clean and de-grease the copper and then proceeded to 'join the dots'!. This was very painstaking and took a while to complete to my satisfaction. The technique to applying the marker pen is to apply it in 'blobs' or 'dots', letting this dry and then re-applying the marker pen. This ensures that the the ink is solid and there are no holes in the tracks. I deliberately designed the tracks to be as wide as practically possible to allow me the maximum area for soldering.

 Once the tracks were laid  it was ready to immerse into the etching solution. Before immersion, and as an extra precaution, I applied a little grease to each of the holes in the board to block them off. This was to ensure that the etching solution didn't undercut the holes. Immersion in the solution normally takes about 20 minutes to fully complete depending on the temperature. The above pictures show the results with a few of the components already mounted on the board.

This is the finished board. I had to make an adjustment with a couple of the tracks as they weren't correct. This involved cutting the tracks and soldering a couple of cross-over wires. This done, the board worked perfectly just like the bread-boarded setup.

Panel meter display. 5V-15V supply, and selectable two modes of operation 0V-2V and 0V-20V, cost around £10. Connected directly to the board for power and signal input. I could have used an ordinary digital voltmeter but purchased this with a view to making a standalone tach unit.

I needed a simple rotating body for initially testing the tachometer against. This is a 12V computer fan with blades removed, a nut epoxied to the hub and a small metal disk epoxied to the nut. The nut had two of the faces polished and the others blackened with a permanent marker. The metal disk was similarly marked.

Here the fan is rotating at around 10,000 rpm. The laser is aimed at the nut which produces two reflections per revolution. The tachometer has been jumpered to a divide by two setting. There are options for 1:1, 2:1, 6:1 and 8:1 depending on the type of rotating body that the tachometer is to be used with. The panel meter is currently displaying 99mV which indicates that the fan is rotating at 9900 rpm, +/- 100 rpm. Full scale reading is 1.500V which indicates an rpm of 150,000.

The bottom picture shows an improved tach testing device. Made from the turbine and shaft section of a small Nissan turbo. Shaft mounted on small ball races, oiled simply using Diesel for low friction. Operation involves attaching a vacuum cleaner to the welded tube on the turbine outlet which spins things up very nicely..!  Hope to get +60,000rpm which should be more than enough to test the sensor part of the tach.  

The tachometer circuit will be fitted into the metal box shown above for which the size of the circuit board was designed. I've decided that the whole package will be designed primarily for use with the PC interface with only two input/output cables. One will carry the 12V power supply and signal output wires and will connect to the  interface board via 3-way jack plugs. The other cable will be for the sensor, carrying laser power and photodiode signal wires and which will split into separate laser and photodiode 'heads'. The tach unit could easily be converted into a standalone device by splitting the tach/interface cable into seperate power and display cables to feed a panel meter such as the one used for testing. You could even make a handheld device, but this would required repackaging in a larger box with a suitably small 12V battery supply.

This method of producing circuit boards is fine if you are making small one off boards like this but it is not to be recommended for larger or multiple boards! There are many other more convenient ways for transferring a circuit design onto circuit board and I recommend you try these! Alternatively there are a number of companies out there that will make up a few small boards for minimal cost. Once the prototype has been proven to work I intend to use a manufacturer for creating duplicate boards.

All temperature measurement, e.g. exhaust gas, oil, fuel, etc, will be performed using thermocouple devices. This is to keep things as simple and consistent as possible so that less work is required. Examples are shown above with different length probes. A thermocouple is a special type of wire that generates a very small voltage in proportion to the amount of heat applied. It is possible to take a direct reading using a digital voltmeter but this will not result in a very accurate readings. Instead there is an IC that will accept the small voltage signal from a  thermocouple and amplify it into a corresponding voltage which can be read by a voltmeter or fed as input into an ADC as I will be doing. Thermocouple devices come in a variety of temperature ranges and packages such as probes for immersing into hot gases or liquids or clamps that will allow you to clamp the probe to a pipe for taking external readings. The thermocouple of choice is the K-type which is good for +1200 degrees centigrade.

Temperature module. Using K-Type thermocouple for measuring 0 to +1250 degrees centigrade.

Pressure measurement will be performed by dedicated IC's, a few examples of which are shown above from a number of manufacturers. These are designed to take pressure readings and convert these into temperature compensated  proportional voltages. Voltage ranges from 0V-5V full scale can be produced which are ideal for reading by a voltmeter or ADC.

Once again cost and suitable specifications were the criteria for deciding to design and build my own interface. The interface board will initially have 11, 12-bit resolution ( 1.2mV resolution over a 5V input range ) analogue-to-digital ( ADC ) inputs for the sensors and 8, 8-bit resolution ( 20mV resolution over a 5V output range) digital-to-analogue ( DAC ) outputs for driving various motors for control purposes. These IC's  use a specific interface protocol called SPI (Serial Peripheral Interface) that needs to be used to 'talk' to these IC's in order to control them.

PC interface circuit. I haven't built or tested this circuit yet, but based on a fair amount of research and expert advice it should work.. in theory..! ;oP I chose these particular IC's because they provided the best compromise in terms of packaging, specification and interface protocol (SPI) for which the PC parallel port lends itself very well. The ADC and DAC chips will be driven by a separate software module which will effectively constitute a standalone voltage reading/writing device for testing purposes. This module will then be bolted onto the main software at a later date.

Above shows the laptop to be used, which I got given free. It's an old Toshiba Portege 300CT,  Intel Pentium 133Mhz, 32Mb onboard memory expandable to 64Mb, 1.5Gb hard disk and a 10.4" screen with a maximum resolution of 1024x600. Below is a universal in-car laptop charger bought new and cheap through Ebay. It works off a 12V car battery, and has selectable output voltages, a perfect and simple solution to onboard power supply! ;o) The software is to be implemented using Visual Basic 6 and the graphics implemented using the simple Windows GDI interface. There are a few requirements for the software. One is to be able to read the voltages from the various sensors via the ADC and display them for easy readability. The software is to implement a simple read/write interface across all the input/output devices regardless of type. Another requirement is to allow for the logging of that information for later analysis. A third requirement is to provide some form of control interface that will allow the user to set parameters for the controlling of motors/actuators based on sensor inputs. This last requirement is a large task to implement in itself as it requires the control interface to be designed so that it makes it easy for the user to choose any permutations of input and to control any permutations of output.

I started to write the software from scratch primarily because I wanted a dedicated interface. There are software modules that you can purchase for the displaying of data using classic analogue 'needle' or 'dial' type gauges but these are relatively expensive and from what I've seen not fully customisable. I wanted at least a fully customisable needle gauge and also a 'bar' style percentage gauge for temperature readings for example.  Some examples of the implemented gauges are shown above. The current implementation allows for any style of needle to be designed based on simple drawing primitives and whether that needle is of the ordinary type or 'flood' type. The face of the dials can be any background graphic. The examples above are ones I pulled from the net and tweaked in Photoshop. Shown is the software running on a 1.8Ghz PC. I've transferred the software over to the laptop and despite it's relatively low spec it runs surprisingly well although a little slow on the 'Flood' type gauges at full scale deflection but certainly still usable. A memory upgrade to 64Mb may help a little, or possibly a faster laptop at a later date..

 ..To be completed..