Introduction: Create the Schematic of Your Motion Detector Alarm

Picture of Create the Schematic of Your Motion Detector Alarm

The following information is a single lesson in a larger project. Find more great projects here.

Project Overview:

Need to protect your personal space? Signal if a sibling has been poking around in your room or keep out unwanted visitors by creating a working Motion Sensor Alarm!

The motion sensor alarm you will build is a circuit that uses a prefabricated sensor, called a PIR Sensor, to detect motion. If motion is detected, the circuit will trigger two different types of output, an LED and a Piezo Buzzer.

Before creating this circuit in real life, you will create a circuit schematic, which is like a circuit map. Engineers and designers use schematics to plan component and circuit layout.

Step 1: Starting the Circuit

Picture of Starting the Circuit

The most important part of this circuit is the PIR Sensor.

A PIR sensor varies the flow of electricity depending on the intensity of infrared light (it is called a passive sensor because it does not generate energy or electricity itself).

  1. Click the “Components” button.
  2. Select “All Components”, and type PIR in to the Search box.
  3. Locate the PIR Sensor schematic symbol and click “Add”.
  4. Drag the PIR symbol to the schematic workspace.

Step 2: Add an LED

Picture of Add an LED

First add an LED that will illuminate whenever the PIR Sensor is triggered.

A Light Emitting Diode (commonly called an LED) uses semiconductor technology to generate light. When electric current is applied to an LED, electrons fill electron holes within a pn-junction, which releases photons and causes the LED to illuminate. The LED will serve as the light source for your prototype circuit. LEDs are polar components, which mean they have a positive side, called anode, and negative side, called cathode. As a result, the LED can only be inserted into a circuit in one direction.

  1. Click the “Components” button.
  2. Select “All Components”, and type LED in the Search box.
  3. Locate the LED and click “Add”.
  4. Rotate the LED symbol three times using the rotate button on the upper left of the workspace. The small arrows should be facing down and to the right.
  5. Move the LED so that it aligns to the center right of the PIR sensor.
  6. Using the drop down menu in the blue label box, switch the LED color to green.
  7. Change the label to read 10mm.

Step 3: Add a Piezo Buzzer to Your Circuit

Picture of Add a Piezo Buzzer to Your Circuit

Add a Piezo Buzzer to your circuit. The Piezo Buzzer is a type of component that generates sound using the piezoelectric effect. The piezoelectric effect occurs when electricity passes through certain materials, such as crystals, which in turn vibrate at a frequency that results in sound.

  1. Click the “Components” button.
  2. Select “All Components”, and type Piezo in the Search box.
  3. Locate the Piezo Buzzer and click “Add”.
  4. Rotate the Piezo Buzzer symbol once using the rotate button on the upper left of the workspace.
  5. Move the Piezo Buzzer so that it hovers above and to the right of the PIR sensor.

Step 4: Add a Resistor to Your Circuit

Picture of Add a Resistor to Your Circuit

Add a Resistor to your circuit. A resistor reduces the current within a circuit, and lowers the voltage. Resistors do not have polarity, therefore, they do not have a positive and negative side. This resistor will protect your LED from receiving too much current.

  1. Click the “Components” button.
  2. Select “123D Components”.
  3. Select the Resistor symbol.
  4. Rotate the Resistor once using the rotate tool in the upper left corner of the workspace.
  5. Move the Resistor so that it horizontally aligns between the center of the PIR sensor and the 10mm LED.
  6. Using the blue box in the upper left hand corner, change the label to read “1.5k”. You can also indicate this value using the drop down menu at the bottom of the blue box.
  7. Hover your mouse over the center “OUT” terminal of the PIR sensor. When a small red box appears, click and drag the resulting blue line to connect with the near terminal of the 1.5k Resistor.
  8. Hover your mouse over the other, unattached terminal of the 1.5k Resistor. When a small red box appears, click and drag the resulting blue line to connect with the near terminal of the LED.

Step 5: Add a Battery

Picture of Add a Battery

Add a 9V battery to your circuit to power the components.

A battery contains chemicals which react and release electrons, creating a voltage potential (a difference between two voltages). Connecting a Battery to the circuit allows for electricity to flow through components.

  1. Click the “Components” button.
  2. Select “All Components”, and type Battery in the Search box.
  3. Locate the 9V Battery and click “Add”.
  4. Rotate the 9V Battery once using the rotate tool in the upper left corner of the workspace. The positive terminal should be facing towards the top of the workspace.
  5. Move the 9V Battery so that it aligns to the center left of the PIR Sensor.
  6. Using the blue box in the upper left hand corner, change the label to read “9V”.
  7. Hover your mouse over the top (positive) terminal of the 9V Battery. When a small red box appears, click and drag the resulting blue line to connect with the VCC pin (positive input) of the PIR Sensor.
  8. Hover your mouse over the bottom (negative) terminal of the 9V Battery. When a small red box appears, click and drag the resulting blue line to intersect with the GND pin (ground) of the PIR Sensor.

Step 6: Connect to Power

Picture of Connect to Power

Continue to hook up the additional components to power.

  1. Hover your mouse over the unattached (negative) terminal of the 10mm LED. When a small red box appears, click and drag the resulting blue line to intersect with the GND pin (ground) of the PIR Sensor.
  2. Hover your mouse over the bottom (negative) terminal of the Piezo buzzer. When a small red box appears, click and drag the resulting blue line to intersect with the GND trace coming from the PIR Sensor.
  3. Hover your mouse over the unattached top (positive) terminal of the Piezo buzzer. When a small red box appears, click and drag the resulting blue line to intersect with the OUT trace coming from the PIR Sensor. This current hookup allows for both the LED and the Piezo buzzer to be output sources for the PIR Sensor.

Congratulations, you have completed this project!

In the next lesson you will learn to build the actual alarm!

Next Lesson:Build a Working Prototype of the Motion Detector Alarm

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