This has to be one of the most simple electronic breadboard projects that there is. It only contains two components, a resistor and a light emitting diode or LED as we’ll now refer to it. This circuit simply lights the LED. Unlike a light bulb you can’t just wire it across a power supply. As an LED is a diode it needs to be connected the correct way round. Even if you did connect it the right way round it would just burn out because 9V is too high a voltage and too much current would flow through it. The resistor is there to drop the voltage down and limit the current flowing.
If you’d have stayed awake in your schools physics classes you may have learned some theory that may have been useful now. That theory is ohms law and I’ll confess to not listening or learning it while I was at school. I have read and learned since as it has become more relevant.
Ohms law is the relationship between voltage, current and resistance. If we take the standard LED current to be around 20mA and 2.5V we can work out the value of the resistor depending on the voltage that we choose.
Ohms law states V=IR which is voltage = current x resistance and so resistance = voltage divided by current.
First subtract the LED voltage (2.5V) from the supply voltage (9V) to give you the voltage that needs to be dropped by the resistor 9 - 2.5 = 6.5V
Taking the current to be the afore mentioned 20mA that is 0.02A, divide 6.5 by 0.02 to get the value of the resistor 325 ohms. You would use a 390 ohm resistor as the next available value. The value of 20mA would be considered a maximum and 15mA would be considered a more realistic value. Using these figures would be 6.5 divided by 0.015 which comes to 433.33 so a resistor of 470 ohms would be a typical value for 9V.
The first of our simple electronic breadboard projects is the transistor touch switch. Transistors come in two different varieties NPN and PNP. You can’t simply use either because they have to be wired up differently. The circuit diagram below shows an NPN transistor. You can tell this because the arrow on the emitter points out. The symbol for a PNP transistor is similar except the arrow points the other way round.
The transistor is off. As you touch the two wires your finger acts as a resistor and allows current to flow into the base of the transistor and turn it on allowing current to flow from the collector and through the LED to illuminate it. If you press your finger harder the resistance of your finger will lessen allowing more current to flow to the base. This will allow more current to flow from the collector and the LED will glow brighter.You can plug a couple of drawing pins into the breadboard and wire these up to become the touch terminals.
The breadboard layout.
The next of the simple electronic breadboard projects is called the flip-flop or more technically an astable multivibrator. Look at the circuit diagram below.
By deconstructing it you can see it’s basically two of the touch switch circuits. In the touch switch circuits your finger acted as a resistor to turn the transistor on and so light the led. In this circuit there is a 10K resistor in the same position so you can see that it will light the LED however as well as lighting the LED there is another connection from it that goes back to the base of the other transistor via a capacitor. This turns the transistor off. As one of the capacitors charges current flows in the base of the transistor turning it on and lighting the LED, when the capacitor becomes fully charged it stops the current flowing and so turns off the transistor. Then the other capacitor charges causing current to flow and turn that transistor on and thus light the other LED.
This continues so one LED lights then goes off as the other one lights then goes off. Larger value capacitors take longer to charge so the flashing speed is slower. By using lower value capacitors the speed of the flashing will be faster.
The breadboard layout.
Once you start to build simple electronic breadboard projects you will start to get more experienced and hopefully these two simple examples will get you on your way. Once you start to get a grasp of circuit diagrams you will be able to take small circuits and then turn them into breadboard projects yourself. If you want to progress to this the next level How to build a circuit on a breadboard has more tips to help you.