Electronic Circuit Construction:

Transcription

1 Electronic Circuit Construction: Various methods are used for building electronic circuits. The method that you choose depends on a number of factors, including the resources available to you and whether you are building a one-off prototype or a large number of identical circuits. Point-to-point wiring: With the advent of miniature components, printed circuit boards and integrated circuits, point-to-point wiring construction is a construction technique that is nowadays considered obsolete. Unless you are dealing with a very small number of components or have a particular desire to use tag strips and group boards, point-to-point wiring is not a particularly attractive construction method these days! Breadboard construction: Breadboard construction is often used for assembling and testing simple circuit prior to production of a more permanent circuit using a stripboard or printed circuit board. The advantage of this technique is that changes can be quickly and easily made to a circuit and all of the components can be re-used. The obvious disadvantages of breadboard construction are that it is unsuitable for permanent use and also unsuitable for complex circuits (i.e. circuits with more than half a dozen, or so, active devices or integrated circuits). Matrix board (Vero-Board) construction: This low-cost technique avoids the need for a printed circuit but is generally only suitable for one-off prototypes. A matrix board consists of an insulated board into which a matrix of holes are drilled with copper tracks arranged as strips on the reverse side of the board. Component leads are inserted through the holes and soldered into place. Strips (or tracks) are linked together with short length of tinned copper wire (inserted through holes in the board and soldered into place on the underside of the board). Tracks can be broken at various points as appropriate. The advantage of this technique is that it avoids the need for a printed circuit board (which may be Page 1 of 13

2 relatively expensive and may take some time to design). Disadvantages of matrix board construction are that it is usually only suitable for one-off production and the end result is invariably less compact than a printed circuit board. Printed circuit boards: Printed circuit board construction technique is ideal for volume manufacture of electronic circuits where speed and repeatability of production are important. Depending on the complexity of a circuit, various types of printed circuit board are possible. The most basic form of printed circuit (and one which is suitable for home construction) has copper tracks on one side and components mounted on the other. More complex printed circuit boards have tracks on both sides (they are referred to as doublesided ) whilst boards with up to four layers are used for some of the most sophisticated and densely packed electronic equipment (for example, computer motherboards). Surface mounting: This technique is suitable for subminiature leadless components. These are designed for automated soldering directly to pads on the surface of a printed circuit board. This technique makes it possible to pack in the largest number of components into the smallest space but, since the components require specialized handling and soldering equipment, it is not suitable for home construction nor is it suitable for hand-built prototypes. Using matrix boards and stripboards (Vero-boards): Matrix boards and stripboards are ideal for simple prototype and one-off electronic circuit construction. The distinction between matrix boards and stripboards is simply that the former has no copper tracks and the user has to make extensive use of press-fit terminal pins which are used for component connection. Extensive inter-wiring is then necessary to link terminal pins together. This may be carried out using sleeved tinned copper wire (of appropriate gauge) or short lengths of PVC-insulated hook-up or equipment wire. Like their matrix board counterparts, stripboards are also pierced with a matrix of holes which, again, are almost invariably placed on a 0.1 in pitch. The important difference, however, is that stripboards have copper strips bounded to one surface which link together rows of holes along the complete length of the board. The result, therefore, is something of a compromise between a naked matrix board and a true Page 2 of 13

3 printed circuit. Compared with the matrix board, the stripboard has the advantage that relatively few wire links are required and that components can be mounted and soldered directly to the copper strips without the need for terminal pins. Conventional types of stripboard (those with parallel runs of strips throughout the entire board surface) are generally unsuitable for relatively complex circuitry of the type associated. Stripboard layout techniques: The following steps are required when laying out a circuit for stripboard construction: 1. Carefully examine a copy of the circuit diagram. 2. Mark all components to be mounted off-board and identify (using appropriate letters and/or numbers, e.g. SK, pin-2) all points at which an off-board connection is to be made. 3. Identify any multiple connections required between integrated circuits or between integrated circuits and connectors. Arrange such components in physical proximity and with such orientation that will effectively minimize the number of links required. 4. Identify components that require special attention (such as those which require heatsinks or have special screening requirements). Ensure that such components are positioned sensibly bearing in mind their particular needs. 5. Keep inputs and outputs at opposite ends of the stripboard. This not only helps maintain a logical circuit layout (progressing from input to output) but, in high gain circuits, it may also be instrumental in preventing instability due to unwanted feedback. 6. Use standard sizes of stripboard wherever possible. Where boards have to be cut to size, it is usually more efficient to align the strips along the major axis of the board. 7. Consider the means of mounting the stripboard. If it is to be secured using bolts and threaded spacers (or equivalent) it will be necessary to allow adequate clearance around the mounting holes. 8. Produce a rough layout for the stripboard first using paper ruled with squares, the corners of the squares representing the holes in the stripboard. This process can be carried out actual size using 0.1 in. graph paper or suitably enlarged by means of an appropriate choice of paper. For preference, it is wise to choose paper with a feint blue or green grid as this will subsequently disappear after photocopying leaving you with a clean layout. 9. Identify all conductors that will be handling high currents (i.e. those in excess of 1 A) and use adjacent strips connected in parallel at various points along the length of the board. 10. Identify the strips that will be used to convey the supply rails: as far as possible these should be continuous from one end of the board to the other. It is often convenient to use adjacent strips for supply and 0 V (or ground ) since decoupling capacitors can easily be distributed at strategic points. Ideally, such capacitors should be positioned in close proximity to the positive supply input pin to all integrated circuits which are likely to demand sudden transient currents (e.g. 555 timers, comparators, IC power amplifiers). 11. Minimize, as far as possible, the number of links required. These should be made on the upper (component) side of the stripboard. Only in exceptional cases should links be made on the underside (foil side) of the board. 12. Experiment with positioning of integrated circuits (it is good practice, though not essential, to align them all in the same direction). In some cases, logic gates may be exchanged from package Page 3 of 13

4 to package (or within the same package) in order to minimize strip usage and links. (If you have to resort to this dodge, do not forget to amend the circuit diagram!) When the stripboard layout is complete, it is important to carefully check it against the circuit diagram. Not only can this save considerable frustration at a later stage but it can be instrumental in preventing some costly mistakes. In particular, one should follow the positive supply and 0 V (or ground ) strips and check that all chips and other devices have supplies. A good technique involves the use of coloured pencils that are used to trace the circuit and stripboard layout; associating each line in the circuit diagram with a physical interconnection on the stripboard. Colours are used as follows: Positive supply rails: Red; Negative supply rails: Black; Common 0 V rail: Green; Analogue signals: Yellow/Pink; Digital signals: White/Grey; Off-board connections: Orange/Violet; Mains wiring: Brown and blue Assembly of the stripboard is happily a quite straightforward process. The sequence used for stripboard assembly will normally involve mounting IC sockets first followed by transistors, diodes, resistors, capacitors and other passive components. Finally, terminal pins and links should be fitted before making the track breaks. Track breaks could be easily done using a knife or better by using a 3mm (or larger) twist drill. The drill is placed on the hole where the track is to be breaked, and then turned by hand until all the copper is removed so that the break is created. On completion, the board should be carefully checked, paying particular attention to all polarized components (e.g. diodes, transistors and electrolytic capacitors). Next are some diagrams of how circuit construction should be designed when working on veroboards. The (x) marks on the layout represent the locations where the copper tracks require to be broken. (a) Fig. 1: (a). The schematic diagram of the circuit to be constructed.. The parts required to construct the circuit. The blue tool is used to create the breaks in the copper tracks Could be replaced by a twist drill. Page 4 of 13

5 (a) Fig. 2: (a). The vero-board viewed from bottom. Note the copper strips joining the holes and separeted by insulating material of the board.. The vero-board layout. Note the (x) marks to indicate where the tracks require to be broken. (a) Fig. 3: The finished product. (a). The component layout and. The bottom view with the components soldered to tracks and the copper track breaks done where necessary. Page 5 of 13