Other tools for electronics

A wire and cable stripper removes the insulation from the end of the connecting wire in a single action. It saves a lot of time. But most wire strippers are designed for use by electricians. They will strip the insulation from hefty mains cable, or TV antenna cables, but not from the thin wires such as we use in electronic hobby projects. Choose with care.

Wire cutters of the side cutter type trim the component leads short after they have been soldered to the board or terminal. They give a neat finish to the work and no other tool does the job.

Last on our list is a spot face cutter. It looks like a drill bit mounted in a plastic handle and it is used for cutting the circular breaks in the copper tracks of strip board.

A spot face cutter.

While on the subject of cutting the strips, ALWAYS examine each cut with a string magnifier. It is too easy to leave a microscopically thin hair of wire around the edge of the cut. It is surprising how much current can flow through such an invisible thread. Make this the first fault to look for (with that magnifier!) when a circuit fails to work.

If you are building one of the projects in this book or working to some other ready-made design, it is usual to buy just the components you need and no more. Even then it is wise to buy a few spare transistors and other items that you might damage though overheating or because of unintentional short circuits.

On the other hand, if you are designing your own circuits or modifying an existing circuit you need a small stock of the more commonly used components. Here are suggestions:

Resistors: A few of each of the E12 values from 100 Q up to 1 MO. The metal film type, with 1% tolerance, and rated at 0.5 W are the best. They are usually sold in packs of 10.

The easiest and most economical way to stock up with resistors is to buy a resistor pack that includes all the common values. Buy from a reliable source. So-called 'Bargain packs' from unknown sources may contain lots of the values that nobody wants.

E12 resistors

The values are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82. Next come multiples: x 10, x 100, x 1000, and so on, up to 10 MQ. There are also sub-multiples x0.1 and x0.01, but these are rarely needed.

Variable resistors: The miniature horizontal preset type are good for breadboard-ing. Sometimes called trimpots. Handy values are 470 Q, 1 kQ, 4.7 kQ, 10 kQ, 100 kQ, and 1 MQ.

Capacitors: Their main use in our circuits is for smoothing spikes and pulses out of the supply. Stock a few MKT polyester capacitors, value 100 nF. Transistors: Probably the most often needed is the BC548 for use in transistor switches, but other types can be used for this purpose. The table lists types to choose from.

Type no.

BC327

npn or pnp Max. current Current gain*

100-600

BC337

BC548

BC639

BC640

2N3904

npn npn npn pnp npn

1000

1000

100-600

110-800

* For a collector current of 10 mA. Transistors are often graded by gain; for example the BC548C has a higher gain than BC548A ad BC548B.

Viewed from below c - collector; b - base; e - emitter

Viewed from below c - collector; b - base; e - emitter

BC337 BC548

BC639 BC640

2N3904

Transistor pinouts.

MOSFETs (metal oxide silicon field effect transistors) are useful as switches and amplifiers when the source of the signal is unable to provide enough current to drive a BJT. As switches, they can control large currents and most types have a very low 'on' resistance. Their main disadvantage is that their threshold voltage, the gate voltage that turns the MOSFET on, varies appreciably from one transistor to another of the same type.

A MOSFET of interest for low power circuits is the VN10KM, for current up to 500 mA, an 'on' resistance of 5 and a transconductance of 0.2 S. A transconductance of 1 S means that a change in gate voltage of 1 V, results in a change of current of

1 A. The 2N7000 carries current up to 500 mA, with 'on' resistance 5Q, and transconductance 0.1 S. Its threshold voltage is in the range of 1 V to 2.5 V, typically

VN10KM

Pinouts of two low-power MOSFETs.

VN10KM

• Light emitting diodes: A few of the standard brightness 5 mm LEDs are a help when testing the output from logic circuits.

• Push-buttons: Buy two or three of these, solder short single-stranded wire leads to them and strip the ends of the wires. These provide digital input to breadboarded logic circuits.

This completes the list of components that often appear on the breadboard when designing circuits for input to and output from a microcontroller. As well as the above, build up a selection of sensors and actuators such as LDRs, photodiodes, infrared emitting diodes, a solid state buzzer, and a small loudspeaker. Exactly what you include depends on your interests. Where necessary, solder short leads to these so that they are ready for used on the breadboard.

A supplier's catalogue or two, or the equivalent on a CD, will usually give technical data, will help you know what is available, and may even suggest ideas that lead to successful new designs.

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