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Opamp Design and Test Board

Opamp Design and Test Board

Opamps are wonderful little building blocks, but quickly building a test circuit is a real pain. Most people will use a breadboard, one of those plastic blocks with holes and connections, but I have found them to be a complete pain in the backside.
Description

For this project, two low-cost dual opamps are used. I suggest that nothing too exotic be used, because the really good, fast opamps will only oscillate because of the lead lengths. This board is for testing ideas, not final circuits, but is tremendously useful.

The layout of the top of the board is shown in Figure 1, and the opamp symbols are simply drawn on the top (non copper side) using a fine felt-tip pen. Mark out where all the holes go for the wire loops these are shown as heavy lines) that provide connection points to each opamp, earth, and the power supply rails. Make the holes the right size for the wire you are going to use so the loops don’t fall out every time you solder something to them.

Now that you have a board with a bunch of holes in it, you need to separate the various pads that are used for everything other than earth connections. There is no need to be too particular about this, and rather than try to etch the printed circuit board, use a small section of hacksaw blade, and simply cut the copper away from the board to leave each of the terminals isolated. The remainder of the board serves an an earth (ground plane), and will help prevent noise pickup.

Although it looks like there are way too many supply connections provided, you will probably kick yourself later if you leave them out. At the very least, leave the ones up the middle of the board.

To be a success, the hacksaw blade may need to be snapped off so you have teeth right at the end where they are needed. To use the cutter, place a metal straightedge along the line you wish to cut, and draw the cutter towards you using enough pressure to cut the copper. Note the orientation of the teeth in Figure 2. Use a blade with large teeth, as small ones will take far too long to do the job. If you get the right blade, most cuts in the copper should take only one or two cuts.

Figure 3 shows how the copper should be cut – this is looking from the copper side of the board, so you should be able to make a series of straight line cuts as shown. The opamp layout is shown in light grey and the loops are again shown as heavy lines so you can relate to where everything goes. If you have an engraving tool or mini-drill with a suitable cutter, use this instead of a hacksaw blade. The separate +ve and -ve sections are simply joined together with insulated wire.

When you are done, make sure that you double check that each “land” is isolated from the main board with a multimeter. A close visual check with a powerful magnifying glass will ensure that you don’t have an accident waiting to happen. Install all the wire loops first – these should be about 5mm (1/4″) high, and installed so they can’t fall in or out of the board. Small kinks in the leads can help here.

The opamps should be mounted on their backs, and glued down to the board with insulation below the pins – these should be carefully bent out as shown in Figure 4. Do not bend the leads at the body of the opamp, as they may either break, or crack the case. When gluing the opamps down, use hot-melt glue, or something else that is not too permanent. The power supply can simply be “skyhooked”, with the larger components also glued down and acting as mounting points (filter caps and voltage pot for example). If you want to, you can also make some isolated pads for the power supply, but this is not really necessary.

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