Building a Basic Door Alarm: A fun and educational project for kids (and adults!)

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When my oldest watched Home Alone for the first time, she was inspired to set a series of traps all around the house to trap “bad guys” just like Macaulay Culkin did in the movies.

Today’s experiment is for the kids who love setting up DIY traps around the house!

This experiment combines building a working electrical circuit to make a buzzer and LED turn on and off, depending on if the door is opened or closed. When the door is closed, the reed switch opens the circuit, and when the door is closed, the circuit is closed and all components are functioning.

Grab your aftershave and best Culkin scream (if you’ve seen Home Alone, that is), and let’s build a door alarm trap!

How to make the Basic Door Alarm experiment

Supplies you will need

For this experiment, you’ll need:

Supplies needed for the Basic Door Alarm experiment

Before you start

We are dealing with electrical components, so please watch your child when conducting this experiment.

Instructions

Here is how to do this experiment with your child:

Step 1:

First, add two AA batteries to the battery holder and ensure the tab is open, so no power is running.

Two AA batteries in the battery holder with the tab open

Step 2: Connect to the reed switch

In this step, we are connecting the positive wire from the battery holder and an alligator clip (preferably red, indicating positive power flow) to the reed switch.

The reed switch recommended in the Supplies section has three spots for connection, called terminals: “COM” (Common), “N.C.” (Normally Closed), and “N.O.” (Normally Open).

The positive (red) wire from the battery holder will be connected to the “COM” terminal.

Using your screwdriver, loosen the screw in the “COM” terminal. Wedge the positive, exposed wire from the battery holder behind the plate in “COM”. Tighten the screw back down to hold the wire in place.

Next, grab a red alligator clip and connect it to the “N.O.” terminal.

Quick aside to explain “Normally Open”: when the magnet is in contact with the reed switch, our circuit is open, hence the “Normally Open” connotation. When the magnet breaks contact with the reed switch, the circuit closes. See the notes in the “The STEM behind the Basic Door Alarm experiment” below for a more thorough explanation.

“COM”, or Common, terminal in the reed switch

Using a screwdriver to loosen the screw in the Common terminal

Positive power flow wire from the battery holder behind the Common terminal plate

Red alligator clip on the Normally Open terminal on the reed switch

Step 3: Connect the positive power flow to LED and buzzer

We’re going to connect the positive power from the reed switch (that red alligator clip from the previous step) to the LED and buzzer in this step.

Locate the positive and negative legs on the LED by finding the longer leg (anode, positive) and the shorter leg (cathode, negative).

Connect the red alligator clip from the reed switch to the anode of the LED and the exposed, positive wire from the buzzer. The anode and the red wire from the buzzer will be connected to the same clip.

Annotated image showing the anode and cathode of the LED

The anode of the LED and exposed red wire from the buzzer connected to one alligator clip

Step 4: Connecting the negative power flow

Use your black alligator clip and connect it to the exposed black wire from the battery holder (the negative power flow).

Finally, connect the other end of the black alligator clip to the cathode of the LED and the exposed black wire from the buzzer.

If you have similar LEDs to what we use, the red LED will not light up when you close the circuit. We had to switch to a blue LED.

Black alligator clip connected to the exposed black wire from the battery holder

The exposed black wire from the buzzer and the cathode of the LED connected to the black alligator clip

Step 5: Test out the alarm!

Close the tab on the battery holder and move the magnet away from the reed switch. The buzzer should sound and the LED should light up.

Moving the magnet away from the reed switch to close the circuit

The STEM behind the Basic Door Alarm experiment

This experiment teaches:

Circuits
Magnetism
Cause and effect

How it works

This experiment works by building a circuit that opens and closes to make a buzzer sound and an LED light up.

The main component in this experiment working properly is the reed switch. Reed switches in a circuit allow the circuit to essentially function on its own without a person having to open and close the circuit.

When the door is closed and the reed switch is in contact with the magnet, the circuit is open and nothing happens. When the door is opened and the reed switch breaks contact with the magnet, the circuit is closed, and our buzzer sounds and LED lights up.

Circuits

This experiment is a great way to showcase how circuits must be closed in order to function properly.

When the door is closed, the buzzer remains off and nothing is happening. While that may seem like the circuit isn’t functioning properly, it’s operating just the way it’s supposed to!

The reason is that we chose a reed switch to complete our circuit. A reed switch is a way to open and close a circuit without having to manually do it yourself.

There are two options on our reed switch: Normally Closed (“N.C.”) and Normally Open (“N.O.”). If we choose “N.C.”, the circuit will be closed (and operating normally) if the reed switch is in contact with the magnet. If we choose “N.O.”, the circuit will be open (and not operating) if the reed switch is in contact with the magnet.

Here’s how it works: inside of the reed switch, there are two blades parallel to one another, and in the case of a Normally Open switch, those blades are not touching. The magnetic field from the contact with the magnet is keeping these blades from touching. When we separate the magnet from the reed switch, the blades close, completing the circuit and allowing the flow of electricity through all of the components.

In other words, when the magnet and the reed switch are in contact, the circuit is open. When they are separated, the circuit is closed.

Magnetism

The experiment highlights how magnets can influence electrical circuits through the reed switch.

As mentioned above, the reed switch is controlled only by contact with a magnet. This is because there are two blades in the reed switch that react to a magnetic field.

Depending on the type of reed switch used, the magnet can either make the blades start in the open or closed position, then change to the opposite position when contact with the magnet is broken.

Cause and effect

This experiment is great for showcasing cause and effect! When the door is closed, the circuit is open and nothing is happening.

When we open the door and close the circuit, the LED lights up and the buzzer goes off.