Prototype Construction

Prototype Construction
The main options for building a prototype circuit are:

1. A breadboard
2. Electrical screw connector strips.
3. Stripboard
4. A printed circuit board.

1. The typical breadboard unit consists of a matrix of clip holes wired in strips, into which component leads can be pushed to make a circuit. In my opinion, they are best avoided as it takes quite some effort to implement any significant circuit using them, some components do not fit well in the sockets which are small enough to take DIL IC packages, and when you do get a circuit working well on the breadboard, there is no guarantee that it will work well when you attempt to move it to a permanent soldered board:


While a plastic board of this type looks as if it should be quick and easy to use, I have never found it to be so ever since the boards were scaled down in size to take the closely-spaced pins of integrated circuits (“chips”). It is generally difficult to lay the components out in the same pattern as the circuit diagram, and if they are not, then it becomes slow to follow the circuit through on the breadboard layout.

2. The local hardware shop has cheap screw connectors which can be very effective. These come in several sizes and the smaller ones are very convenient for constructing transistor circuitry. They look like this:


Circuits can be assembled very easily, using these connectors and an example might be one of the John Bedini battery pulsing circuits which might have a layout like this:


I have built this circuit using this style of construction and it was very successful indeed, being very quick and easy to construct and it proved to be very tough and effective over a long period of use. The plastic strip has a hole between each connector strip and that allows you to bolt the strip to a base board on which you mount other components, in this case, the pulsing coil and the rotor with the magnets attached. Each connection block can take two or three wires. The wires need to have the insulation removed and the wires scraped clean and shiny if they are not already in that state. If more than one multi-strand wire is being put into one side of a connector, then it is usually best to twist the wires together before tightening the clamping screw. If you want, you can give the twisted wires a thin coat of solder, but this has to be done neatly to avoid producing a joint which is too large to fit into the connector. One connector can be cut out of the strip quite easily, using a pair of scissors or a craft knife. Single connectors can join two wires very effectively without the need to solder them.

3. Stripboard, usually called ‘Veroboard’ even if it is not made by Vero, is a quick and satisfactory method, although you have to make very tiny solder joints. Please be aware that the fumes from the burning resin when soldering are most definitely not good for your health and should be avoided by making sure that the ventilation is adequate.

4. A printed circuit board is feasible for a one-off prototype and making one will increase your production skills, so it is also a reasonable option if you have the etching and drilling equipment to hand. Buying all of the necessary equipment if you do not have any, can cost a fair amount, but the skills gained are significant and the finished boards looks very professional.

There are several other methods of construction, and many varieties of construction board and stripboard. Simple stripboard will be used in the following descriptions, although the method does apply to many different styles of construction.

The first step is to produce a layout for the components on the board. When designing the layout provision should be made for drilling holes to allow the completed board to be bolted to its case using bolts and insulating pillars to keep the soldered joints clear of all other surfaces.

The circuit diagram of the circuit to be built is the starting point.   You might wish to draw a light grid of lines to represent the matrix of holes in the strip board.   This helps to visualise the run of the copper strips and the sketch can be made to show the exact number of holes available on the piece of strip board to be used.   The strip board looks like this:


So you might wish to produce a layout sketch re-usable drawing like this:


where the horizontal strips are numbered and the vertical lines of holes are also numbered.   In this sketch, where the lines cross, represents a hole in the board.   The sketch of a possible physical layout can then be prepared and it might look like this when seen from the top although the copper strips on the underside of the board are shown in the sketch:


It is very important when producing a sketch like this, that the copper strips making up the circuit are not accidentally used to connect components further along the board, without breaking the copper strip between the two sections of the board.   It helps to mark a copy of the circuit diagram when you are sketching a possible physical layout on the strip board.   It might be done like this:


Here, the components just below the diode are ringed to show that they have been marked on the layout sketch and, if necessary, the copper strip broken to isolate the components.   A component worth mentioning in passing, is the capacitor marked with red in the circuit diagram.   This is a decoupling capacitor, fed from the 12V battery via a resistor and a diode (a diode is not normally used in this part of the circuit).

The decoupling is to provide the 555 chip and drivers with a supply which is reasonably isolated from the heavy current-draw circuit not shown in this small section of the circuit diagram.   The pulsating heavy current draw of the rest of the circuit is capable of pulling the battery voltage down slightly many times per second.   This creates a voltage ripple on the positive supply line from the battery and to smother the ripple, the resistor and diode are used to feed a large reservoir capacitor which smoothes out the ripple.

The circuit itself is not beyond criticism.   Transistor ‘TR2’ and its associated components are redundant since pin 3 of the 555 chip already supplies the required signal (and with higher drive capacity) so the second output line should be taken directly from pin 3 of the 555 chip.   This snippet of circuit is only shown here as an example of marking up a circuit diagram when making a components layout sketch.

As the layout sketch is produced, the circuit diagram should be marked off with a highlighting pen to make sure that every part of the circuit diagram has been successfully copied to the sketch.   In the example below, not all of the highlighted strip is shown, since it runs off the small section of the board being shown here:


Many electronic components can be damaged by the high temperatures they are subjected to when being soldered in place.   I personally prefer to use a pair of long-nosed pliers to grip the component leads on the upper side of the board while making the solder joint on the underside of the board.   The heat running up the component lead then gets diverted into the large volume of metal in the pair of pliers and the component is protected from excessive heat.   On the same principle, I always use a DIL socket when soldering a circuit board, that way, the heat has dissipated fully before the IC is plugged into the socket.   It also has the advantage that the IC can be replaced without any difficulty should it become damaged.

If you are using CMOS integrated circuits in any construction, you need to avoid static electricity.   Very high levels of voltage build up on your clothes through brushing against objects.   This voltage is in the thousands of volts range.   It can supply so little current that it does not bother you and you probably do not notice it.   CMOS devices operate on such low amounts of current that they can very easily be damaged by your static electricity.   Computer hardware professionals wear an earthing lead strapped to their wrists when handling CMOS circuitry.   There is no need for you to go that far.   CMOS devices are supplied with their leads embedded in a conducting material.   Leave them in the material until you are ready to plug them into the circuit and then only hold the plastic body of the case and do not touch any of the pins.   Once in place in the circuit, the circuit components will prevent the build up of static charges on the chip.

Soldering is an easily-acquired skill.   Multi-cored solder is used for electronic circuit soldering.   This solder wire has flux resin contained within it and when melted on a metal surface, the flux removes the oxide layer on the metal, allowing a proper electrical joint to be made.   Consequently, it is important that the solder is placed on the joint area and the soldering iron placed on it when it is already in position.   If this is done, the flux can clean the joint area and the joint will be good.   If the solder is placed on the soldering iron and then the iron moved to the joint, the flux will have burnt away before the joint area is reached and the resulting joint will not be good.

A good solder joint will have a smooth shiny surface and pulling any wire going into the joint will have no effect as the wire is now solidly incorporated into the joint.   Making a good solder joint takes about half a second and certainly not more than one second.   You want to remove the soldering iron from the joint before an excessive amount of heat is run into the joint.   It is recommended that a good mechanical joint be made before soldering when connecting a wire to some form of terminal (this is often not possible).

The technique which I use is to stand the solder up on the workbench and bend the end so that it is sloping downwards towards me.   The lead of the component to be soldered is placed in the hole in the strip board and gripped just above the board with long-nosed pliers.   The board is turned upside down and the left thumb used to clamp the board against the pliers.   The board and pliers are then moved underneath the solder and positioned so that the solder lies on the copper strip, touching the component lead.   The right hand is now used to place the soldering iron briefly on the solder.   This melts the solder on the joint, allowing the flux to clean the area and producing a good joint.   After the joint is made, the board is still held with the pliers until the joint has cooled down.

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