Datasheets: The Key to Reading Schematics

You've found the schematic online for a cool electronic project that you're eager to build. However, not all project information you find online is created equal. Sometimes you'll get great descriptions of what the project does and how the circuit functions, and sometimes you just get a schematic with component values that leaves you scratching your head over how everything comes together. If a schematic doesn't come with a clear explanation of how the circuit works, you can use datasheets to fill in the missing information.

Going to the Source

Go to the manufacturer's site and open the datasheets for the most complicated components; for example the ICs and sensors. First, check out the summary of what the manufacturer intends each component to be used for.

For example, if a circuit uses an LM386 IC, go to the first page of  National Semiconductors' LM386 datasheet. The summary indicates that this is an amplifier that can be powered by batteries with a voltage as low as 4 volts, intended for use in applications such as sound systems.

Next check out the section covering typical applications. For example, on page 5 of the PDF version of the LM386 datasheet, there are schematics showing what components should be connected between pins 1 and 8 of the IC, as well as the other necessary connections to produce an amplifier with a gain of either 20, 50, or 200. By comparing sample schematics provided here to the schematic provided for your project, you can figure out what gain the circuit is designed to provide.

You may be surprised to discover that the schematic you're puzzling over looks very similar to one of the schematics in the application section of the datasheet. The only changes might be the addition of a few components, such as a microphone and op amp, and a few changes to component values. That's because circuit designers don't reinvent the wheel; instead they generally start with an application schematic recommended by the manufacturer (whose engineers should be knowledgeable about the best way to use that particular component) and tweak the circuit to optimize it for their project.

Pinning Down Pins

Another piece of information that can help you understand how a circuit works comes in the form of the pinout drawing of each component. Pinout drawings identify the function of each pin, such as an input pin, ground pin, output pin, etc. In the LM386 datasheet the pinout drawing is at the bottom of page 1 and tells you that pin 1 should be connected to ground, pin 5 is the output, pins 1 and 8 are used to control the gain, pin 6 should be connected to the supply voltage, pin 2 is an inverting input, pin 3 is a non-invertering input and pin 7 is called a bypass pin. As you can see in the application schematics a capacitor is connected between pin 7 and ground when high gain is desired, this acts to prevent the amplifier becoming an oscillator.

Pinout drawings are also valuable when you are assembling the circuit. For example it's very easy to put a transistor in backwards. Checking the pinout drawing in the datasheet for that transistor is an easy way to find out which lead is which.

At this point you should have a good feel for how the circuit works. The next step would be to check key values in the section of the datasheet that lists electrical characteristics to make sure the components are matched properly. I can't cover all the parameters you might find listed, but here are a couple of examples: 

     Supply voltage maximum and minimum. For example, if you are building a circuit that uses an LM386 (4 volt minimum supply voltage) to amplify the output of a ZN416 (1.6 volt maximum supply voltage) you need a one battery pack to power the ZN416 and another battery pack to power the rest of the circuit. This prevents the ZN416 IC from being destroyed by the 4.5 or 6 volts being used to power the rest of the circuit.

     Output power and load resistance (RL) for amplifiers. On page 2 of the LM386N-1 datasheet it says if the supply voltage is 6 volts and the load resistance is 8 ohms, then the typical output power is 325 milliwatts. This tells you that you should use an 8 ohm speaker rated at half a watt or higher to listen to the sound the LM386 amplifies. If you use a speaker with a lower power rating than the amplifier you may damage the speaker.

When you get to the point where you want to design your own circuits you'll need to understand many of the parameters listed in the electrical characteristics section of datasheets. At that point I'd suggest reading, and keeping around as a reference, my book called Complete Electronics Self Teaching Guide by Harry Kybett and Earl Boysen.