Choosing The Right Components

Introduction

There's no point in designing circuits with rare or exotic parts that no one can find. My circuits use a very limited range of basic general purpose components. This is deliberate. The components were chosen because they're cheap and widely available. But - if you can't find the right part - or you simply want to substitute something from your junk box - this page will help you select a suitable alternative.


Fixed Value Resistors

Unless otherwise stated - all fixed value resistors are 0.25 Watt carbon film. I specified quarter-watt carbon-film resistors because they're the cheapest and most widely available resistors on the market. However - you don't have to use carbon-film resistors. And they don't have to be quarter-watt. You can use any type of resistor - including those with a higher wattage. The main drawbacks are - they are likely to be more expensive. And they may not fit as comfortably - into the stripboard layout.

Here and there - the value of a resistor is important. But very few resistor values are critical. If you have to use a substitute resistor - choose one with a higher value. For example - you can safely replace a 2k2 resistor with say a 2k7 - or a 4k7 etc. The higher value resistor may not always work entirely satisfactorily. But - it won't do any harm either.

If you want to reduce the size of your circuit board - you can use 0.125 Watt resistors instead. As a rough guide - if your supply is 12-volt or lower - and the value of the resistor is 2k2 or higher - it's safe to use 0.125 Watt resistors. The formula is: W = V² ÷ R = 12² ÷ 2k2 = 65mW

I use the British Standards Institute system of notation. It can be very easy to miss the decimal point - and mistake a 4.7k resistor for a 47k resistor - especially if the printer or handwriting is poor. So - in BSI notation - the letter replaces the decimal point. And - since very few keyboards have an Omega key - a capital "R" replaces the Greek letter Omega.

Examples:
Variable Resistors

The Variable Resistors can be any type you like. You just have to make sure that the pins on your pots fit the 0.1 inch (2.54 mm) matrix of the stripboard. The pots that lie flat on the board - are the easiest to adjust. But you can use upright pots if you prefer. And multi-turn pots will let you make much finer adjustments.

I often specify 4M7 pots. But 5M pots will do just as well. If you can't find such high value pots - connect a 1 Meg pot in series with a fixed value resistor. Choose the value of the fixed resistor to take you close to your desired setting. Then use the 1 Meg pot to make the fine adjustments.


Ceramic Capacitors

I specify 50v ceramic capacitors - because they're the cheapest and most widely available capacitors on the market. However - you don't have to use ceramic capacitors - and they don't have to be 50v. Any type of capacitor will do. And anything up to about 100v should be fine. Over 100v - and they may be too big to fit comfortably into the stripboard layout.

Practically all of the ceramic capacitors are used to suppress high frequency interference signals. I generally specify 100nF capacitors - because they're widely available. But you'll probably find that any value capacitor - from 10nF upwards - will work equally as well.

Nanofarads: Electrolytic Capacitors

Electrolytic capacitors come in two styles - axial and radial. Axial capacitors have one lead at each end. And radial capacitors have both leads at the same end. The negative lead of a polarized axial capacitor is generally connected to its metal case. And the negative lead of a polarized radial capacitor is generally identified by a stripe running down one side of the can.

It's essential that you connect your electrolytic capacitors the right way round. If they are connected the wrong way round - they will conduct DC current. And - if that current is large enough - the capacitors will heat up - and explode.

I generally specify 25v polarized radial electrolytic capacitors. These are probably the most widely available electrolytic capacitors on the market. You can usually use an axial capacitor - in place of a radial capacitor - by standing it on one end. And - in place of a 25v capacitor - you can always use a capacitor with a higher voltage rating. However - in both cases - you may find that the substitute component will not fit comfortably into the stripboard layout.

The values of the electrolytic capacitors are generally fairly important - because they're usually used as timers. However - you can always try a substitute value. It may not work satisfactorily - but it won't do any harm either.



NPN Transistors

I generally specify the BC547 - because they're among the cheapest and most widely available transistors on the market. They have an absolute maximum collector current capacity (Ic_max) of 100mA. This is more than adequate for the demands I make upon them. On the rare occasions where the collector current will exceed about 50mA - I specify the BC337. It has an (Ic_max) of 500mA.

Some BC547s have the suffix A, B or C. The letter gives a rough indication of the transistor's gain. My circuits are designed to minimize the importance of gain. So - if your transistor has an additional letter - you can safely ignore it. Any BC547 will work fine.

The BC547 can be replaced by any small NPN transistor - with a DC gain (h_FE) of at least 100 - and an (Ic_max) of at least 100mA. I think practically every small NPN transistor satisfies these requirements. The main thing to watch is the transistor's pin configuration. Even if it looks like a BC547 - don't assume that its pins are arranged in the same order.

Learn How To Identify A Transistor's Pin Configuration.


PNP Transistors

I generally specify the BC557 - because they're among the cheapest and most widely available transistors on the market. They have an absolute maximum collector current capacity (Ic_max) of 100mA. This is more than adequate for the demands I make upon them. On the rare occasions where the collector current will exceed about 50mA - I specify the BC327. It has an (Ic_max) of 500mA.

Some BC557s have the suffix A, B or C. The letter gives a rough indication of the transistor's gain. My circuits are designed to minimize the importance of gain. So - if your transistor has an additional letter - you can safely ignore it. Any BC557 will work fine.

The BC557 can be replaced by any small PNP transistor - with a DC gain (h_FE) of at least 100 - and an (Ic_max) of at least 100mA. I think practically every small PNP transistor satisfies these requirements. The main thing to watch is the transistor's pin configuration. Even if it looks like a BC557 - don't assume that its pins are arranged in the same order.

Learn How To Identify A Transistor's Pin Configuration.


Diodes

The diodes are usually used either to create one-way paths - or to protect delicate components from the harmful reverse voltage spikes given off by relay coils etc. I generally specify the 1N4148 - because they're among the cheapest and most widely available diodes on the market. However - almost any small diodes will work fine.

If a diode has to pass a current of more than a few milliamps - I generally specify the more robust 1N4001. It's rated at 50v/1amp. And it's available everywhere. However - you can safely substitute any diode with a rating of 50v/1amp - or better.

It's ALWAYS important to connect diodes the right way round. The small arrowhead in the symbol - indicates the direction of conventional current flow - through the diode. And the bar represents the line that circles the body of the diode - at the cathode (k) end. Current through the diode - will only flow in the direction of the arrowhead. .... But Be Careful .... If you're going to try an unknown diode - check that it's NOT a zener. Zener diodes conduct in both directions.



Cmos ICs

The Cmos ICs in my circuits all come from the "4000 Series". Among some amateurs - Cmos ICs have a wholly unjustified reputation for unreliability, unpredictability and fragility. All I can say is that I have Cmos circuits that have been working perfectly for decades.

"4000 Series" Cmos ICs are produced by a number of manufacturers. The different manufacturers use different prefixes and suffixes e.g. CD, HEF, MC1, BC, BE etc. As far as my designs are concerned - these letters can be ignored. As long as the main 4000 number is correct - the IC will work fine.

I'm sometimes asked if it's possible to substitute the equivalent 74 Series TTL devices. The truth is - I don't know. I've never tried. Certainly - the early TTL devices were unsuitable. They were made with bipolar transistors. So they couldn't match the input impedance of the MOSFET based Cmos ICs.

Today - this and some of the other shortcomings of TTL have been addressed. It's possible that the modern versions can be substituted for Cmos. But I don't know of any advantage to be gained from making the substitution. If you're tempted to have a go - note that the pin configuration of TTL devices is often different from that of their Cmos counterparts. You can't simply unplug a Cmos IC - and replace it with TTL.



Relays

In my prototypes - I use the 12v Omron G2E - or one of its many clones. These relays have a single set of change-over contacts - and a coil resistance of about 330Ω. The standard 12v G2E will work satisfactorily from about 8 Volts to 14 Volts. But the so called sensitive version - with its higher coil resistance - needs a minimum of 9 to 10 Volts.

You can substitute almost any relay you like for the G2E. But you'll have to identify its pin configuration - and make sure that its coil doesn't draw more than about 50mA. If the current through your coil is substantially greater than 50mA - replace the BC547 with a BC337 - or replace the BC557 with a BC327 - as the case may be.

How To Identify A Relay's Pin Configuration.


Thyristors/SCRs

I generally specify the TIC106D - or the smaller TICP106D - because they're among the cheapest and most widely available SCRs on the market. But - provided the current and voltage ratings are adequate - you'll probably find that any SCR will work fine.

You'll need to identify the (k) Cathode - the (a) Anode - and the (g) Gate pins of your SCR. Then the construction guides - for the individual circuits - will tell you which pin should be connected where.

You should also get away with using a triac - such as the TIC206D. Triacs are like two SCRs that are connected together - but in opposite directions. The Anode of each SCR is connected to the Cathode of the other. And both SCRs are controlled by the same Gate pin. This means that no matter which way round it's connected - if the Gate pin is in the right place - the triac will behave like an SCR.