Chapter 2. Infrared Beacon

In this chapter, we will be building a flashing beacon. It's not just any old flashing light, though. This one will be invisible, unless you are wearing night vision goggles or viewing a black-and-white video camera.

What is a 555 timer and how does it work?

The integrated circuit that we will be using in this chapter has been around since the 1970s and is still going strong today. You can see the 555 timer circuit in the following diagram:

LM555 Timer pin out

The LM555 has two modes of operation. For this project, we will be using it in astable mode. The word astable means unstable, basically with respect to an oscillator. Now, in some cases we do not want the circuit to be astable, for example an amplifier. In this case, astable is a good mode.

The pins have various functions, depending on the mode in which we are using the device. The functions are as follows:

• Pins 8 and 1 are always power and ground respectively.

• Pin 3 is always the output pin that we will use to turn the LEDs ON and OFF.

• Pin 4 is the reset pin. Pulling this pin low (to ground) disables the oscillator in astable mode. Essentially the output goes low and stays low.

• Pin 5 is the control voltage pin. By applying a voltage to this pin, we can...

Our 555 timer circuit

In our circuit, when the output is high, it turns on Q1, which turns on D2. When the output is low, it sinks the current, which turns off Q1 and turns on D1.

Breadboard layout

Assembling our flasher PCB

The following figure is of the PCB design that is available for download from the Packt site (https://www.packtpub.com). This PCB is designed to fit inside a tube with a one-inch internal diameter. You will see the reason for this in a moment.

Flasher PCB

Assembly of the PCB is straightforward. I would suggest that you solder in the lowest parts first and work your way up to the higher ones. This way, the parts won't fall out when you turn the board over to solder them in. In this case, install the resistors first; then the LM555; and finally the capacitor and the transistor. You can also see how the LEDs have been bent at 90 degrees.

Finished PCB

Building and assembling the case

The flasher is designed to fit inside a piece of one-inch PVC plumbing pipe. The LEDs are bent at 90 degrees to the PCB and inserted through holes drilled in a one-inch PVC pipe cap. A #7 drill bit is used to drill the holes for the LEDs. The cap is then turned over and a quarter of an inch drill bit is used to enlarge the rear of the holes so that the LEDs will penetrate further into the cap. You should drill about half way through the cap with the quarter of an inch drill. A drill press with a depth gauge will come in handy here.

Cap showing drilled holes

One end of the case is sealed shut with another one-inch end cap, which is glued into place. The cap with the holes is not, so that you can change the batteries. Also, I did not include an on-off switch in the design for the sake of simplicity, and to make the case as moisture-proof as I could.

PCB assembly inserted in cap

The preceding image shows the completed PCB assembly installed in the end cap. These...

Going further

There are a number of modifications that you can make to this project. For example, it is possible to fit a miniature toggle switch into the top cap, alongside the LEDs.

It is also possible to fit the circuit into a smaller case by using a 3 V lithium coin cell. I chose PVC pipe because I find it easier to machine than other materials and through hole components for first-time builders.

The following image is of a much smaller device. Patents and non-disclosure agreements prevent me from showing you anymore; however it is an example of what can be achieved with a little ingenuity.

Small IR flasher

Summary

In this chapter, we learned how LM555 timers work in astable mode. We built and tested a simple but useful circuit to demonstrate this.

In the next chapter, we will be building another "secret agent" device: a covert motion detector.