Building and validating prototypes is an essential step in developing electronic devices and circuits. It is extremely rare to go directly from paper to finished product and have it work perfectly first time. There are many ways to prototype your design and a given project may have several prototypes for different purposes. I’m going to focus on prototyping circuits, where the emphasis is on validating circuit topologies and component selection, and maybe validating basic firmware functionality.
In my mind, circuit prototyping methods fall into three broad categories – solderless breadboard (Figure 1), soldered breadboards, and printed circuit boards. Each of this has a place (although I don’t use solderless breadboards for reasons I discuss below).
A typical solderless breadboard. These are suitable for through-hole components and useful to lash up a quick test circuit. They are not really suitable for prototypes that you expect to have around for any length of time as they are fairly fragile.
Solderless breadboards are great for quickly lashing up a simple sub-circuit to validate some part of your design. They are also good for experimenting as its very easy to make changes to your circuit which explains why they are ideal for beginners and often used in engineering courses. They are really only designed for through-hole components, so if a device you are using is only available in some kind of SMT package, you will need to find a ready-made break-out board for it or use an adaptor like those shown in Figure 2. This means you may have to resort to soldering anyway.
If you want to use a surface-mount part on a breadboard designed for through-hole components, you may need to use an adapter board like these. The SMT pins are broken out to pin headers on a 2.54 mm pitch.
My biggest gripe with solderless breadboards is their lack of robustness. The spring contacts wear out with use and eventually become unreliable. It’s also just too easy to bump something and have it become disconnected while you are probing the circuit. There is nothing more frustrating than spending time chasing a problem that turns out to be due to an unreliable prototype.
That’s why I prefer to use soldered breadboards or go straight to a PCB for prototypes. I use both methods, depending on my level of confidence in the circuit and the mix of components. If I feel there may be a fair bit of change in a circuit, or I want to try something quickly, I will use a solderless breadboard. I prefer the type with a matrix of plated-through holes like that shown in Figure 3. These also require an adaptor board for many surface-mount parts, as you can see, but with care you can also solder some surface mount components down directly. You can see some 0805 surface mount components and even a SOT223 regulator in the photo.
This is a close-up of a soldered breadboard showing a LQFP SMT package on a break-out board and various smt and through-hole parts soldered directly to the board. This is my preferred method for prototyping if I am not going to use a custom PCB.
I use fine single-core wire to make the connections on the back of the board as shown in Figure 4. This results in a very robust and stable prototype which can be modified where necessary. Note the labels on the bottom of the board to help with wiring and the rubber feet which make sure the prototype is stable and not sitting on the wiring when on the bench.
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This figure shows the bottom of the board of Figure 3. The interconnections are made using fine tinned copper wire or wire-wrap wire. The labels help avoid wiring errors. Note the rubber feet to ensure the wiring is clear of the bench.
I do however find myself moving to a prototype printed circuit board more often these days since the cost is so low and the turnaround time so fast. It is also more or less mandatory for high-speed circuits or those using high pin-count packages such as BGAs. A PCB will obviously accommodate any package you care to use and will be extremely robust.
The key is to remember that this is a prototype and to design it accordingly. In my case this means making the prototype PCB large enough to work on comfortably and including test points to access critical parts of the circuit. I sometimes also add zero-ohm resistors to allow me to isolate sections of the circuit for testing. It’s also helpful to be able to isolate the power supplies to various parts of the circuit, and to facilitate the measurement of power supply current. Figure 5 shows a recent prototype with an array of test points around the edges of the board. A set of test probes are attached to some of these.
The most robust form of prototype is a custom PCB such as this. Note the liberal use of test points around the perimeter of the board. Those on the right are connected to the test set-up via miniature grabbers which are held in place with double-sided foam tape. The eagle-eyed will see the “bodge wires” and the “dead-bug” style wiring to the chip to the left of the QFN package.
If you do think you may have to modify a PCB prototype it helps if you can stick to two layers. With care, you can make modifications – cutting tracks with a sharp blade and adding jumper wires. Figure 5 also shows an example of a shows a close-up of a modification on one of my prototypes. The chip just to the left of the QFN package is wired in “dead bug style” because I did not have it in the right footprint. I use very fine enamelled copper wire and stick it down with nail polish so it won’t move.
I think the most important things to bear in mind with any prototype is to make sure it is fit for purpose. If you think you will need to live with the prototype for weeks or months, make sure it is suitably robust. Make sure you can access all the signals that you need in an appropriate way. Probing is fine for momentary connections, but if you need to stay connected for any length of time consider a test point or connector. Make sure your prototype is large enough (or can be fixed to an appropriate base as per Figure 5) so that it is not dragged off your bench by the weight of the cables connected to it. Ask me how I learned this!
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
