Defend Your Gift This Holiday Season - How to Set Up Your Own Analog Alarm

One of the reasons why Christmas is so fun is that it's full of surprises. However, sometimes people diminish the effects of the excitement by taking a peek inside their gift to satisfy their curiosity. Here's how you can preserve the surprise using clever holiday circuits that trigger alarms when someone starts to peak at their gift before Christmas.

Setting Up Your Holiday Alarm

The components for your holiday alarm include a speaker, 3 resistors (1kO, 33kO, and10kO), an NPN transistor, a diode, and 2 capacitors (0.1 µF ceramic capacitor and 68 µF electrolytic capacitor). You'll also need a 9-volt battery to power the device and copper strips or tape to create an alarm switch. The alarm is built on an Integrated Circuit (IC) amp - such as the Texas Instruments LM386. The unit does not need a processor or any other special hardware.

An alarm signal is generated by the 1 kHz square wave oscillator circuit, and an audio signal is sent to the speaker when the gift is lifted from two conductive strips. This movement activates the switch that makes it possible for the alarm to sound. The on/off switch of the oscillator is controlled by a resistor, NPN transistor, and an optional diode.

This experiment can be constructed in different ways, as alarm sounds may range in volume. Ideally, the speaker delivers sound when the gift item is removed from the conductive strips. You may find that the alarm has the most desirable sound when a diode is placed between the transistor base and switch. In this model, the cathode terminal should face the switch so that when the voltage drops, a small amount of current is generated through the Bipolar Junction Transistor (BJT).

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When the BJT is shut off, the amplifier seeks an alternate path for current. The IC will lead to saturation at either the positive or negative rail. For the circuit to oscillate, the signal must shift back and forth between the two rails. A transistor is strategically placed in the circuit to interrupt the current flow for switching oscillation on or off.

Create an Oscillation Switch

An oscillation switch can be created with three conductive strips that create a reliable connection. You can also use copper tape to make the switch. Two other metal strips are needed: one connected with the transistor's base through the diode and another used for a ground connection. A third metal strip is placed at the bottom of the gift on top of the other two conductive strips.

When the gift sits on top of the metal strips, the switch is closed. However, as soon as the gift is lifted, it opens the switch, which triggers the alarm. Again, there are multiple ways to design your holiday circuit to set off an alarm.

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