Tips and Tricks (Part18)

Previous Tip & Trick

I recently participated in the newly created Parth’Lab initiative which was a fine time for sharing information of all natures. Their approach is very progressive, so that any participant feels comfortable at plugin an arduino board, wrapping a couple of electronic components on a bread board and ultimately running a traffic lights application.

This workshop was a good opportunity to remind to advanced users that once upon a time they were beginners too and that it takes some time and some resources before getting into the complex projects. Next tip and tricks will be very much oriented in the direction of these basic mandatory information required to step in successfully in the world of electronic makers, starting with a new release of the Arduinoos pin-out sheet.

Electronics pinout sheet rev2

Electronics pinout sheet rev2

Then is a suggestion for the preparation of “pre-wired” components which will help you to shorten your wiring efforts and, above all, prevent short circuits. Most presentations of use of bread boards show this type of wiring were component legs are left bare and subject to short circuits between them.

resistors

Messy isn’t it?

The alternative that I suggest is very simple, cheap and proved to be efficient along many years of experience. It consists in shortening each resistor legs to 1 cm and soldering a piece of copper wire on one end, as per the example below.

resistor2

One immediate question to answer to is the length of this piece of wire. Well, it depends! It depends mainly on the type of circuit you are developing. If your design involves small signals, high frequency or even RF (radio frequency), keep the wire as short as possible such as 5 cm (2 inches). Next picture illustrates a (perfectly working) charge amplifier which feature a high density of components and very short wire lengths.

proto_charge_amplifier

This other picture illustrate an auto-ranging audio pre-amplifier with “standard” wire length

proto_amplifier

For other signals, you can use wires of 10 or more cm. Ideally you will prepare several length of wires in order to cover most of your needs. The other pending question is related to the values of these pre-wired resistors. Which leads to some explanations about resistors.

Imagine that you are designing a circuit and after long boring calculations it happens that you need a 0.99 kOhm resistor (so as to say 990 Ohm). It is very likely that you will have trouble to find this value at your next door hobby shop or even at your Internet supplier. The reason is that it is almost impossible to make and store resistors of almost any value ranging from 0.001 Ohm to 10 MOhm!

Instead, the Electronic Industries Association (EIA), proposed to use “preferred values”. These values are parsed in so called “series” which relate to the tolerances of manufacturing of components. And this is why: taking a 100 ohm resistor which is the first value in the E12 series, it makes no sense to produce a 105 ohm resistor since 105 ohms falls within the expected 10% tolerance. So that the next value in the E12 series is 120 ohms, followed by 150, 180, etc.

The higher the number next the “E” the better the tolerance and the smaller the steps between values. As an example, in the E24 series (5% tolerance), the values start with 100, 110, 120, 130. In the E48 series (2% tolerance) , the values start with 100, 105, 107, 110. In the E96 series (1% tolerance) , the values start with 100,  102, 105, 106, 107, etc. That’s far more than what we need to develop prototypes.

My suggestion is to start with the E12 series and learn how to deal with resistors in series or in parallel if a very specific value is required. The values from the E12 table correspond to the mantissa of the E12 series: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7,  5.6, 6.8 and 8.2. Apply the appropriate exponent to this mantissa to build the resistor value: e.g. a 330 resistor results from 3.3 x 10^2.

resistor

Back to our need for a 990 resistor, which any way does not even exist in the E192 series, we can play a little bit around with formula and try to find the best arrangement:

2 resistors in series: Rres = R1 + R2.

g3515

Taking R1 = 820 and R2 = 150 gives Rres = 970 ohm which results in a 2% error versus 990 ohm

2 resistors in parallel : 1/Rres = 1/R1 + 1/R2.

g3492

Taking R1 = 8200 and R2 = 100 gives Rres = 98.8 ohm which results in a 0.2% error versus 990 ohm

This simple example demonstrates that combining only two resistors can bring us very close to the targeted value.

Ultimately, this is a suggested list of resistors for a basic starter kit: 47 ohm x 5, 100 ohm x 10, 150 ohm x 5, 220 ohm x 10, 470 ohm x 10, 560 ohm x 5, 680 ohm x 5, 820 ohm x 5, 1 kohm x 10, 1.5 kohm x 5, 2.2 kohm x 10, 4.7 kohm x 10, 5.6 kohm x 5, 6.8 kohm x 5, 8.2 kohm x 5, 10 kohm x 10, 15 kohm x 5, 22 kohm x 10, 33 kohm x 5, 47 kohm x 10, 100 kohm x 10, 220 kohm x 5, 470 kohm x 5, 1 Mohm x 5. These 24 values should cover 80% of your needs. All in 125 mW radial packages. Although these values are available in 10% tolerance, you may get them in 1% tolerance as the price difference is minor between the two types of resistors. The number of resistors per value depends very much on the complexity of your project.

Not to mention the use of trimmers which will deliver an infinite range of solutions!

trimmer

HTH

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