One of my readers is always concerned about the availability of some components used in my circuits. Even though new makes are releasing continuously, the old makes are still in the market. Indian electronics market is rich in new as well as old components and the old one cannot be considered as obsolete. Substitutes are also easily available. We cannot argue if the component is not available in one country since the availability and use varies in different regions. A wise designer can easily substitute a component if the make shown in the circuit is not available.
Electronics circuit design is an intelligent act which requires theory as well as experimentation. Too much theory will always confuse because theoretical calculations and actual working of the circuit varies since most of the components are not 100% accurate in working. So experimentation is better than over dependence on the calculator. For example if we select 1K current dropping resistor through ohms law calculation, in practical use it will be 900 ohms or 1100 ohms. Theoretically there will be 0.7 volt drop in the base emitter junction of a transistor and 1.8 volt drop across the LED. Practically it can vary. For example if you measure the forward voltage of 10 LEDs purchased from the same pack, the forward voltage of the LEDs may vary from 1.5 volts to 2 volts. This is the real situation. Similarly, a leaky capacitor will not charge completely. This will upset the total calculation. Without blaming the design, the wise decision is to change the component or replacing the value. A person always stick on too much theory cannot do this but an experimenter can solve the problem within seconds. Experience and practical knowledge can do lot of helps in this matter. That is the intelligence in circuit construction. I am not depending on too much theory but considering only the average values of resistors and capacitors since + or – 1 % change will not affect the performance of the circuit if it is not a sensitive one. Only after constructing the prototype, it is possible to see the difference in the theoretical calculations and the working of the circuit. Following are the images of the so called obsolete components which are still available in most places.
2N 3055 High power transistor