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
For example, if a circuit uses an LM386 IC, go to the first page of
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:
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.
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
Self Teaching Guide by Harry Kybett and Earl Boysen.