How to Read Schematics for Beginners

How to read schematics for beginners – when starting to learn electronics, beginners need to learn how to read schematic diagrams. Schematic diagrams show the components and electrical connections of a circuit in schematic or diagram format. A schematic diagram is also known as a circuit diagram, or just schematic.

How to Read Schematics for Beginners

How to Read Schematics for Beginners

Schematic Symbols

Before being able to read a schematic, it is necessary to learn and recognize the symbol for each component in a circuit. Each symbol in a schematic represents a physical electrical or electronic component.

In the circuit above, the schematic is shown on the left and has two symbols representing two components. The symbol on the very left represents a battery. On the right of the schematic is the symbol that represents a light bulb.

On the right of the above image is the physical implementation of the schematic on the left. Here the actual light bulb and battery can be seen.

Electrical Connections

Electrical connections between the bulb and battery are represented by lines in the schematic. These lines are wires in the actual circuit.

How to Read Schematics

The article on how to read circuit diagrams for beginners on the Starting Electronics website shows the very basics of how to read schematics. This article uses the same light bulb and battery circuit to explain the basics of circuit diagrams. It shows how to recognize when two wires are connected, or whether they are just crossing each other.

After reading this article, a series of tutorials follows to get the reader to recognize electronic components and their schematic symbols. Each electronic circuit can be built on breadboard. A good way to learn how to read electronics schematics is to follow the tutorials, look at the schematic diagrams and build the circuits.

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LM3909 IC 1.5V LED Flasher Circuit

LM3909 IC 1.5V LED flasher circuit. A circuit that uses the now obsolete LM3909 IC to flash an LED from a single 1.5V cell. This IC and circuit is now a piece of history. I had one of these circuits running on a PCB for years, the circuit finally failed. My attempts to repair the circuit were unsuccessful. It appears that the IC finally failed. Read on for a look at some electronics history.

LM3909 1.5V LED Flasher Circuit Diagram

Below is the circuit diagram of an LM3909 LED flasher taken from an out of print electronics magazine. I built this circuit on a tiny PCB many years ago. The circuit operated from a single 1.5V cell, but could also operate from a single 1.2V rechargeable cell.

LM3909 1.5V LED Flasher Circuit Diagram

LM3909 1.5V LED Flasher Circuit Diagram

Flashing an LED from a Single Cell

I remember the LM3909 being expensive, costing many more times than a 555 IC. The problem with using a 555 is that it could not be used to flash an LED from a single 1.5V cell, but had to operate from a higher voltage. A 555 also drains a lot of current from a battery because of its internal voltage divider resistors.

When the LM3909 became available it was popular with hobbyists because an LED can not be lit up from a single 1.5V cell. Here was an IC that would flash an LED from a 1.5 or 1.2 volt cell, pretty impressive. The cell would last for a long time too.

LM3909 now Obsolete

Unfortunately the LM3909 is not available anymore. It was made obsolete several years ago and has no equivalent or replacement part. There are some transistor circuits available that will flash an LED from a single cell.

Attempting to Revive my LM3909 Circuit

The only LM3909 IC that I have was used in a LED flasher circuit built on a PCB. Fortunately I had used an 8-pin IC socket on the board, so could remove the IC to test it on breadboard. The PCB and breadboard test circuit are shown below. Notice that only two additional components are needed in the circuit – a capacitor and LED.

LM3909 PCB Circuit and Breadboard Circuit

LM3909 PCB Circuit and Breadboard Circuit

After many years of service, the LM3909 finally failed. After testing the PCB circuit with a new battery and then building the circuit on breadboard and testing it, the LM3909 was finally declared dead.