Revisiting Some Archived Articles that Have Not Been Lost, but May Have Been Forgotten and Are Worth a Fresh Read
Original Post Date: July 18, 2012
Part 1: The Simplest Circuit
In our present digital age, kids are in love with anything involving electricity. They are amazed by their magical merriment devices and the idea of knowing even a little bit of the secret sorcery behind them can keep kids engaged for hours. Today’s post will help you demonstrate a simple circuit, the basics of electricity, and how circuits are represented with schematics. A parts list is available at the end of the article (Do not worry—you only need four parts!).
Some parents worry about the safety of electronic projects. However, as long as the projects are powered by small batteries, there is very little that can go wrong. Mostly, if you do something wrong, you will just damage the individual parts, which are all very inexpensive. The one major safety precaution is to make sure you do not directly connect the positive to the negative on the battery without any components in between. This is known as a short-circuit and could physically damage the battery, which contains dangerous chemicals. If you notice the battery (or any other part) warming up, disconnect it. The power supply you are using has a plastic case and an on/off switch, so you can turn the switch off if you feel it getting warm.
The basic principle of electronics is this: batteries supply electric current to the circuit. If there is a complete circle connecting the power from the positive to the negative, electricity will flow. If there is not a complete circle, electricity will not flow.
One very confusing part of electronics is current flow. Every textbook about electricity tells us that current is the flow of electrons from the negative to the positive. However, every drawing of an electronic circuit shows electricity flowing from the positive to the negative! The reason for this is that the standards for circuit drawing were developed before anyone knew which way the current was flowing. In fact, the circuit would work whether positive energy was flowing to the negative terminal or negative energy to the positive terminal, so the early scientists just had to guess which way it was flowing.
From here on out (and you can blame Benjamin Franklin for this) we will talk about current flowing from the positive to the negative. This is known as the “conventional flow” (since it is based on drawing conventions) while the negative to the positive is known as the “electron flow” (since it is based on the actual movement of electrons).
So, when circuits are drawn, they are drawn using a schematic diagram. A schematic diagram is simply a line drawing representation of the circuit. It identifies all of the components that are needed and how they are connected. Below is the schematic for the circuit that we will make:
Since this is our first schematic, I have labeled all of the parts. The lines just mean that the parts are connected to each other in some way. Note that a schematic defines the parts and the connections. The physical configuration is entirely up to you. In other words, there is no reason to make a nice looking box with your wires like the schematic. They only show that the components are connected. How they are laid out is entirely up to you. With only three parts, there are not that many possibilities, but when we do more complicated circuits, it is important to keep this in mind.
There are three parts to this schematic: a power supply, an LED, and a resistor. The power supply is a 6-volt battery pack (4 AA batteries) and supplies electricity for the circuit. In electronics, there are two main measurements of electricity—voltage and current. Current is the raw rate of electricity flowing through your circuit and is measured in amps (or, in small electronic systems like these, milliamps, which are 1/1000th of an amp). Voltage is like water pressure; it is the amount of push that the electricity is giving. It can also be compared to some degree to height. We are using 6-volt power supply, so the distance from the positive to the negative is six volts. The negative side of the battery is called “the ground,” so the positive side is six volts above the ground. Various components in your system will use different amounts of voltage and therefore bring down the amount of voltage which remains in the rest of the circuit. The red wire coming out of the power supply is the positive side and the black is the negative side.
The next part of the circuit is the LED. LED stands for Light-Emitting Diode. A diode is a device that only allows current to flow one direction, and drops the voltage (most diodes which hobbyists use drop the voltage about 2-3 volts). A light-emitting diode, when current flows through it, will emit light. We will know our circuit is successful when the LED lights up. Note that since the LED only allows current to flow in one direction, it is very important that the LED be inserted in the correct direction. The longer pin (the anode) should be connected to the positive side of the circuit and the shorter pin (the cathode) should be connected to the negative side.
The last part of the circuit is the resistor. Resistors basically slow down the flow of electricity. The resistance of a resistor is measured in ohms, but we will not worry too much about that here. Why do we need the resistor? In electronics, many of the parts you use are there to prevent damage to other components. In the case of diodes, most hobbyist diodes can only take so much current. Without a resistor, the diode will flash and burn out as soon as you connect the power. Therefore, if you add a low-resistance resistor, it will limit the current flowing through the circuit and will keep the diode functioning properly. In addition, because it limits the flow of electricity from the battery, it also makes your battery power last longer. You can actually connect the resistor to either side of the diode, but I always place mine on the negative side, because on more complicated diode parts (like LED numeric displays or multicolor LEDs) this usually winds up saving components. For the project we have here, any resistor between 150 ohms and 500 ohms will work fine.
Now, to connect these pieces, just make sure each wire touches the next wire in the series. You can hold them together or twist them together or do whatever you want, as long as they are all touching. After connecting everything together (do not forget to connect the power supply to the two ends of the circuit) the LED will light up! Well, almost. If you use the battery pack we recommended, you also need to set the switch on the battery pack to on! If the light does not come on, then make sure the LED is put in the right direction.
So, as you can see, for a few dollars and a few minutes of time, you can help your kids begin learning about electricity and electronics. Below is the parts list including Radio Shack part numbers, so you can take the list to your nearest Radio Shack and they can get you hooked up in no time.
• 6-volt power supply (Enclosed 4 “AA” Battery Holder – Catalog #270-409)
• 4 AA batteries
• LED (NTE 3020 – Catalog #55050657 – many others will work fine)
• Resistor (NTE EW133 – Catalog #55049114 – anything between 150 and 500 ohms will work fine)