Circuit Notebook 85 - Using Stripboard (Veroboard) for Video Projects

In the past, when the relevant Printed Circuit Boards were readily available from BATC Member's Services, I was an avid builder of BATC projects. Unfortunately, because of low demand it is no longer economical to provide them. I do not have access to PCB facilities so I have to consider alternatives. For very short term projects I have resorted to using a 'Plugblock' (Maplin FD31J) and fixing the completed circuit and 'plugblock' in a suitable box. For a more permanent arrangement I use stripboard, for example the Video Detector in Circuit Notebook 83.

Stripboard

Stripboard is made of PC board material (e.g. SRBP) 1.6mm thick and is punched with 1 mm holes on a 0.1 inch grid. One face is plain, where the components are mounted, (the 'component side') the other face (the 'copper side') has copper strips running the length of the board at 0.1 inch centres. Stripboard is available in various lengths and widths (from Maplin as SRBP Matrix Board e.g. JP49D) and can be readily cut to size. The purpose is to make use of the copper strip as a conductor along one direction while using components and wire links, at right angles to the copper strips, to form the complete circuit. The copper strip can be cut at appropriate positions to isolate a particular section. Stripboard users have varying methods for arriving at a completed board. The method I use is described below.

The Circuit Diagram

For the purposes of explaining the method I am using the circuit diagram of an Audio/Video switcher, shown in Fig. 1. This will eventually be used our ATV Repeater GB3TM. Its purpose is to sequentially look at several video inputs (controlled by an onboard PIC) and hold when a valid signal has been located. I have already patched up the circuit on a Plugblock to check that it works. It contains audio, video and logic signals. It is not meant to be a constructional project but a real circuit with which to show the various stages leading to a finished unit.

First draw or redraw the circuit diagram in pencil on squared paper (school exercise book). Drawing the circuit in a tidy way will be helpful in laying out the stripboard later. The ICs are drawn in their physical form and orientation, with the pins numbered and in the correct positions, as viewed from above. The connections between the components are drawn with either horizontal or vertical lines and right angle bends. If the lines become too close together or something needs repositioning, then rub out and redraw. This may seem tedious but it is the first stage of a process which hopefully leads to the most suitable arrangement for finally laying out the components on the stripboard. A circuit drafting programme on your PC could be used, but only if the IC pin layout agrees with the physical positions. It leads to confusion if it doesn't!

The Stripboard Layout

After manipulating the circuit into an accurate and tidy diagram, the information is transferred into the layout drawing, as shown in Fig. 2. It is helpful to have the circuit pinned up in front of you when drawing the layout. It is also helpful to make the drawing to a larger scale than the 0.1 inch grid of the stripboard. I find that 0.25 inch or 5mm squared paper to be about right. I have printed out on my PC some A4 sheets with lines and small circles to look like stripboard, but squared paper is fine.

Only one layout drawing is made, this is of the component side. The copper strips are shown as viewed through the board (X-ray fashion). For each soldered joint a black blob is penciled in at the hole position, for each cut in the copper track an 'X' is marked on the hole. Tracks are only cut at the position of a hole.

I normally start laying out the board with the most signal-critical part of the circuit first which, in this case, is IC1 and IC2. Keep in mind where the signal and address lines might go and how the supply lines may link together. Draw in wire links to connect together, say, all the earth pins. Put 'X's to indicate breaks in the tracks. If a clash occurs somewhere, then rub out and try again. Resistors can be fitted flat on the board or stood up 'Japanese style' taking up less space.

When all is complete and every connection has been checked, then connecting pins and fixing holes are added. You can now see the overall size of the board. The copper tracks are marked alphabetically with track 'A' at the bottom and the holes are numbered horizontally with row '1' at the extreme left. This gives each hole a unique address e.g. N13 (which is a cut in the track under IC2).

Stripboard Preparation

A piece of stripboard is cut to size, in this case 24strips wide by 38 holes long. It must be possibly to identify any hole from both the component side and the copper side. A strip of masking tape is stuck on the component side along the bottom edge, just below the lowest row of holes. Another piece of masking tape is stuck at the extreme left edge, but not covering any holes. See Fig. 3. Turn the board over and place strips of tape back-to-back with the previous strips of tape. See Fig. 4.

To avoid confusion please note that the piece of stripboard shown in the photographs had unusual white stripes already printed on the plain side which unfortunately makes it look like the copper side! Use a fibre tip pen to identify each copper strip by letter and each row of holes by number. The component and copper sides with letters and numbers are shown in Figs. 3 & 4 respectively.

The copper tracks can be cut using either a Spot Face Cutter (Maplin FL25C) or a 3.5mm drill held in a pin vice. Working from the left side of the layout, identify a break in a track at, for example, B3. On the copper side locate hole B3 and cut the track. Continue until all the breaks have been cut. It is easier to cut tracks before any soldering has been done. See Fig. 5.

Fitting Links and Components

It is best to start with wire links followed by resistors, small components, IC sockets, connecting pins, leaving any large components until last. For links I use 22swg tinned copper wire which has been pre-stretched to make it really straight. I bend the wires and component leads to match the hole spacing and after inserting the component I slightly splay the leads so that the wire or component stays in place during soldering. The completed unit is shown in Fig. 6.

Testing

Testing follows the usual procedure, a critical visual inspection for correct components, dry joints, solder blobs etc. Common faults are missing wire links or the omission of a cut in a track. If the ICs are on sockets then remove them and make an ohmmeter check for possible short circuits. If all ok, insert ICs, apply power and perform static voltage checks followed by a functional test.

How long will all this take?

Starting with a circuit similar to Fig. 1, which is a tested circuit, I would guess the project could be completed in about two evenings.

Stray capacitance

The measured total stray capacitance between one track and the two tracks either side is 3.5pF/inch (1.4pF /cm). At first sight this may seem a lot. In the layout discussed, the most critical components, as far a stray capacitance is concerned, are R7 and R8, which set the gain of the video amplifier in IC2. Stray capacitance here could affect the video frequency response.

The stray capacitance across R7 is 3.5pF/inch x 0.4 inch (length of adjacent track) x 0.5 (one side of track only) = 0.7pF. The stray capacitance across R8 is the same. In this instance the attenuator R7 and R8 is actually frequency compensated by the equal stray capacitances across the equal value resistors (how lucky can you get?). The measured frequency response of the stripboard video switch circuit is flat to 6MHz (the limit of my multiburst generator).

Reference

Making Circuits (Plugboards & Stripboard), Construction and Workshop Practice, Radio Communication Handbook p.16.17, Sixth Edition, Radio Society of Great Britain

Figures

Fig.1. Circuit Diagram

Fig.2. Stripboard Layout

Fig.3. Stripboard Component Side

Fig.4. Stripboard Copper Side

Fig.5. Stripboard cut tracks

Fig.6. Completed A/V Switcher