Reading Schematics

CHAPTER 13 Reading Schematics

A circuit schematic is a drawing of the electrical connections needed to make the circuit function. Such a diagram uses a series of standard electronic symbols to represent the various components, such as resistors and capacitors, that are used in the project. All of the electrical information is given in the schematic. Translating the schematic into the actual physical connection of components requires knowledge of the components themselves and how they should be connected together. In the circuit schematics associated with the construction projects described later, I have made sure that these have been uniformly drawn so that they can be easily read by the beginning hobbyist.

You should know some simple guidelines for reading a schematic. Start by looking at the top half of the schematic for the common rail running across from left to right. That rail will most commonly be the positive power line rail. At the bottom of the schematic, also running from left to right, will be the ground rail, which is generally connected to the negative power supply line. If this schematic has an input (such as an amplifier), then an input line will be at the left-hand side of the schematic. Finally, there is almost always some form of output, which is the line situated on the right-hand side of the schematic.

So, now you know that there are typically four basic sides to a schematic and generally what these lines should correspond to. Let’s add some detail. Along the upper top rail, which we now know to be the positive power rail, there will be several standard components. First of all, the power switch is sit- uated in the positive power rail. With all of the circuits shown here, the switch is always positioned along the right-hand side. Incidentally, the battery, which is of course needed to power these circuits, is always positioned at the extreme right-hand edge. These conventions are purely my own, but following them consistently makes reading schematics much easier.

Next to the power switch there is sometimes a light-emitting diode (LED) indicator light, which is always positioned on the side of the switch that is farthest from the battery. If you have an LED, then there must always be a resistor to limit the current. Next, as an aid for good circuit stability, two capac-

32 BEGINNING ANALOG ELECTRONICS THROUGH PROJECTS itors, an electrolytic and a disc ceramic, are positioned on the same side as the

LED and run directly across the positive and negative rails. Thus, you now know that in general a circuit schematic will almost always contain a battery, switch, LED, resistor, and two capacitors. Consult the 10 projects diagrammed later in Chapter 17 and you will see this is true. These are standard circuit com- ponents, and regardless of the type of circuit, whether it is for an amplifier or an oscillator or for something else, you will find these components.

Another two components can also be added to our list. If the circuit schematic is for a simple amplifier, it is likely that the amplifier is an ac type, in which case there will be a capacitor at the input side and another capacitor at the output side. So, here are two more easily recognized components.

Finally, simple battery-powered circuits using op-amps such as the LM 741 require that the integrated circuit (IC) is fed with a split power supply, that is, a dual positive and negative supply. One way to do this is to use two bat- teries, but this is a cumbersome solution where it is not critically needed. Instead, a common arrangement is to use three components to generate the split supply. Two equal-value resistors are taken from the positive and negative rails, and the junction is tied together. If we have a 9-volt battery supply, then the midpoint will be half the voltage, or 4.5 volts. A capacitor is the third com- ponent we can use to provide a split power supply, as it is placed across the junction and ground to provide smoothing.

In summary, we have accounted for a total of 11 components so far that are almost always present in basic electronics projects—if you were building an ac amplifier, for example. That is a significant number of components that can be classified as common components, thus making the task of interpreting schematics for the beginner much easier. When you come across a new circuit, start looking for these common components. After you’ve accounted for them, the circuit will look less daunting.