Notebook #2

Introduction

In the first experiment I introduced the breadboard and how to connect components without soldering. In this experiment I will build my first complete working electrical circuit.

A circuit is simply a continuous path through which electric current can flow. If the path is broken anywhere, the circuit stops working.

The circuit I will build uses four basic components:

  • A battery to provide electrical energy
  • A momentary contact switch to control the flow of electricity
  • A resistor (1,000 ohms) to limit the current
  • An LED (light emitting diode) that produces light when current flows through it

This experiment also introduces the digital multimeter (DMM), which is one of the most important tools in electronics. I will use the DMM to measure voltage at different points in the circuit and observe what happens when the switch is pressed.

Circuit Diagram

Figure 1:The symbols for the components and how they are connected. The points A, B and C are where we will make some measurements.

Notes:

  • The LED symbol includes an arrow-like diode symbol. The pointed end indicates the direction current normally flows.
  • The long lead of the LED is the positive side (anode).
  • The short lead is the negative side (cathode).

Materials

  • Breadboard
  • 9-volt battery (or battery pack)
  • Momentary pushbutton switch
  • 1 kΩ resistor
  • LED
  • Jumper wires
  • Digital multimeter (DMM)

Procedure

Preliminaries

Before we do any else, try out the DMM. This device measures voltage, current and resistance. Set the meter to read voltage, and place the leads on the 9 V battery. You should read approximately, usually not exactly, 9V. Switch the DMM to read resistance, usually labelled Ω. You will likely read something close to, but not exactly 1000 Ω. The LED doesn’t behave like a resistor, so at this point there is not really a useful measurement to make with the DMM.

1. Build the circuit on the breadboard

  1. Connect the positive terminal of the battery to one row of the breadboard.
  2. Insert the momentary switch so that pressing the button will connect two rows together.
  3. Connect the 1 kΩ resistor to the output side of the switch.
  4. Connect the resistor to the anode (long lead) of the LED.
  5. Connect the cathode (short lead) of the LED back to the negative terminal of the battery.

At this point the circuit should form a complete loop from the battery, through the components, and back to the battery.

The completed circuit. The red wire that goes off to the side is the + side of the battery and the yellow lead goes to the – side.

2. Test the circuit

If everything is connected correctly:

  • Press the switch. The LED should light
  • Releasing the switch should turn the LED off

This confirms that current flows only when the circuit path is completed.

3. Measure voltage and current with the digital multimeter

Set the DMM to DC volts. Use the black probe on the negative battery terminal as a reference point. Touch the red probe to the following points while the switch is pressed:

  1. Battery positive terminal (A)
  2. The wire leaving the switch (B)
  3. The point between the resistor and LED (C)
  4. Measure the voltage from (B) to (C)

Record the voltage at each point.

Set the DMM to measure current (mA). Disconnect the cathode end of the LED from the ground or return to the battery negative side. Use the black probe on the negative battery terminal as a reference point. Touch the red probe to the cathode end of the LED. This puts the DMM within the circuit path.

Press the switch and record the current.

Results

Example observations:

  • The battery provides about 9 volts.
  • The voltage near the battery is close to the full battery voltage.
  • The voltage decreases as you move through the components in the circuit.
  • The LED lights only when the switch closes the circuit path.
  • When the switch is released, the LED turns off and the voltages disappear because the path is broken.

What You Learned

This experiment demonstrates several fundamental ideas in electronics.

  1. A circuit must form a continuous loop for current to flow.
  2. A switch controls whether the path is open or closed.
  3. A resistor is placed in the circuit to control the flow of electricity.
  4. An LED converts electrical energy into light.
  5. A digital multimeter allows measurement of voltage and current at different points in a circuit.
  6. Voltage exists between two points, which is why the DMM always uses two probes.

In the next experiment I will explore the relationship between voltage, resistance, and current, which leads to one of the most important ideas in electronics.

My Measurements

This is what I measured

Battery voltage = 9.29 Volts
Resistor resistance = 988 Ω
Voltage at (A) = 9.28 Volts
Voltage at (B) = 9.22 Volts
Voltage at (C) = 2.82 Volts
Voltage from (B) to (C) = 6.28 Volts
Current = 6.3 mA

As a preview into the next experiment, notice that the sum of the voltages 2.82 + 6.28 is about 9.22. And also 6.22 (the voltage across the resistor) is approximately 0.0063 (current) x 988 (resistance). There is likely some amount of resistance with poor breadboard joints and the switch, otherwise, these computed numbers would be exact.