Required Materials

Goals

  • Using a bread board
  • Understanding “voltage”, “current” and “resistance”
  • Ohm’s and Kirchhoff’s laws
  • setting up a simple circuit with an LED
  • using a Multi-Meter

Your first Circuit

A “Breadboard” allows you to patch together electronic circuits quickly. You can simply plug-in electronic components and wires. In order to help you, some of the holes are already internally connected. You will notice that along each of its sides there is a “+” red line next to several holes, and a “-” blue line, these holes are all connected together. As for the holes in the center, they are conveniently laid out in a grid pattern, 1, A through 63, J. Holes {1, A} – {1, E} are connected along with {1, F} – {1, J}. The picture below shows this quite well:

A typical breadboard (left) and schematics of internal connections (right).

With this information in mind, we can now start placing components on our breadboard. Our goal will be to set up the following circuit:

From left to right: schematic view, breadboard, USB power supply

This circuit consists of a voltage source (5V), a resistor with value 1 kOhm and an LED. This representation is known as a circuit diagram. The USB cable (actually not a standard USB cable), but the TTL-232R-5V-WE from FTDIChip, has 6 wires coming out of it. The black wire is ground and the red wire is +5V. Make sure you do not short any of the other wires. It is best to just bend them away and fix/isolate them with Scotch tape. We will later use the Orange and the Yellow wires.

Your computer’s USB port is rated for 5V and 100mA continuous current usage. If you use too much current, for example when you attach the cable to a short-circuit, your computer will yell at you and shut off the power to the USB port. You will then need to pull the plug and plug it back in. Be careful, you might break your computer!

You can think about electric current as the amount of water flowing in a pipe and about voltage as its pressure. If you want to drive a mill, you need both enough pressure as enough current. (A high-pressured drop would just shoot a hole through the blades, whereas lots of water does not help either, if it does not flow.)

Calculating the current flow through a LED

In order not to burn a LED you have to limit the maximum current that will flow through it. This is usually done by putting it in series with a resistor. Have a look at the datasheet of a typical LED. Important information you are looking for are the maximum forward current and the forward voltage. Look them up now.


Find out how many volts are dropped over the diode (forward voltage) and how much current it consumes (forward current)

The forward voltage for this LED is given as 2V. As the voltage across both the LED and the resistor is 5V, you know that you have to drop 3V over the resistor. This is due to Kirchhoff’s Voltage law. It says, that the sum of voltages over any closed circuit is zero. Look at the circuit diagram: there are 5V over the voltage source (left half of the circuit), so there must be 5V over the resistor and LED combined (right half of the circuit).

You also know from the data sheet that the maximum forward current is 20mA (0.02A). Kirchhoff’s Current law states that the sum of all currents flowing into a node are the same as the sum of all currents flowing out of a node. Therefore, if 20mA flow into the LED, 20mA must flow out of the resistor. We now know the current through the resistor and the voltage that must be dropped over it. These quantities are related to its resistance via Ohm’s Law. It says that V=R*I, or Voltage equals Resistance times Current.

Using the equation V/I=R you can calculate that you require a 150 Ohm resistor to drop 3V at 20mA. Notice that 20mA is a maximum rating and any resistor below 150 Ohm is likely to break the LED. Likewise, if you want the LED to shine less bright, larger resistors will reduce the current that pass through the LED further. In fact, if you use a larger resistor, you can calculate the current that will run through the resistor by I=V/R. Here, U is given by the difference of the supply voltage and the voltage dropped over the LED. As there is less current flowing in the circuit, the LED will shine darker.


Setup a simple circuit with an LED and a 1kOhm (1000 Ohms) resistor in series on the breadboard, hook it up to 5V, and measure the voltage across the resistor as well as the LED. Now measure also the current that runs through the LED. For this, you have to put the multimeter in series with the resistor. Verify that voltage drop and current drop are as expected.

Further reading:

 

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