How to Create a Fake Car Alarm Using a 555 Timer

This project shows how to make a flashing LED light with a five second delay using an NE555. This could serve as a fake car alarm, as it mimics a car alarm system with it's bright red flashing LED.

Difficulty Level

The circuit itself is not difficult to construct, so this is a project that beginners should be able to replicate with some background knowledge on the basics of electronics. To actually understand how the circuit itself works is more complicated, and would require more than a basic knowledge of electronics.

Motivation

On April 27, 2020, my beautiful, 2016, heavily hail damaged Hyundai Elantra was stolen out of our driveway and totaled the next day after being involved in a police chase. I can only imagine that if I had a fake car alarm circuit such as this in my car at the time, the thief would have certainly been deterred and I would still have my perfect vehicle.

- A 555 timer, I used an NE555 in this circuit, but there are other 555 timers that will function similarly

- 3 resistors, 1k Ohms, 10k Ohms, and 680k Ohms

- One capacitor, 10uF

- An LED light, red to better mimic a car alarm system

- A 9 V battery and a 9 V battery clip to connect the battery to the circuit

- Lots of wires!

- A circuit board to make connecting the wires easier

Understanding the schematic for the circuit is essential to to being able to build it correctly. If you don't build the circuit correctly, you will know right away, because it won't work! That being said, if you mess up the circuit initially (I did once or twice making this), it's not the end of the world, you can always rebuild it and try to be more systematic about how you construct it. I used LTSpice to create one schematic, you can either go off mine, or make your own so that it makes sense to you. Your own schematic makes a lot more sense to you than anyone else's!

The LTSpice schematic shows the NE555 with all of the pins in order as they would be on the actual part, which on the picture are matched up like this:

1 ________ 8

2 ________ 7

3 ________ 6

4 ________ 5

This results in some overlap of wires on the schematic, so I have also included a drawing of the schematic where the wires do not overlap, but the pins are not how they actually appear on the NE555. This is just so that the connections can be seen more clearly.

When constructing circuits that have multiple branches from the power source, I find it easiest to fully construct one branch at a time to minimize mistakes. The branch with resistors 1 and 2 and the capacitor is the most complicated, so it makes sense to start with that one so that you can clearly see every connection without other stuff getting in the way.

If you are using a breadboard like mine, then you can simply hook up the power source to the positive column on the left of the board so that you can have your three connections branch off from any point of the column, and space out the circuit a bit more so it doesn't get jumbled and confusing. For the same reason, you can hook of the negative side of the battery to the negative column on the other side of the board to space things out a bit more. This is not a necessity, these positive and negative columns on the board are merely suggestions that help with organizing your circuit, but they can be useful.

I used a couple more wires than necessary in my circuit, again just so that I could make it more clear to myself what every connection was and make sure that I had constructed each branch correctly. I also tried to keep everything from this first branch at the top of the board so that I can have plenty of room at the bottom for the other two branches, and I can continue to keep each branch kind of in it's own section on the board. I even color coded each branch using similar colored wires.

The branch that doesn't have any components and connects to terminals 4 and 8 of the NE555 is fairly straightforward, and you shouldn't have any problems hooking it up. I put this branch in the middle section of my board, and used dark colored wires. I also connected the ground terminal of the NE555 to the ground column on the board.

The only part of the circuit we haven't constructed yet is the actual output of the circuit, with the LED. This is connected to the output terminal (terminal 3) of the NE555 and provides the light with a current for a split second every five seconds as the capacitor charges and then rapidly discharges.

This circuit works by using the properties of the NE555 timer to switch voltage from the output terminal (terminal 3) to LOW for the bright flash, and HIGH for no light at all. When the output pin has a HIGH voltage, the voltage difference between the branch containing the LED and the timer is effectively the same, so there is no current flowing through the LED, and no light. When the output pin is on LOW, the voltage difference is much greater on the other side of the LED, so there is current flowing through the LED which produces the light. The capacitor takes much longer to charge than to discharge because of how the 555 timer works to attach the capacitor with the different resistors. It takes much longer to charge because when the voltage is less than 2/3 of the V_supply (Vcc) terminal, the capacitor is being charged from the battery through the large 680k Ohm resistor. Once the Voltage in the capacitor gets above the 2/3 threshold, the timer switches the output terminal to LOW and connects the capacitor to the discharge terminal and to ground, which begins the discharge of the capacitor. The discharge is much quicker because now the capacitor is only connected to the 10k Ohm resistor, which is why the light only flashes briefly. Once the capacitor discharges to less than 1/3 of the supply voltage, the output switches back to HIGH, turning the light off and the charging of the capacitor begins again, thus repeating the cycle.

For more information on what is inside of a 555 timer and how each terminal functions, check out this link below.

https://www.electronics-tutorials.ws/waveforms/555...

Now your car is completely safe from theft! Thieves will think twice when they see the powerful red flash emanating from the inside of your car. Also, since this is a very low current circuit, a 9 V battery should last anywhere between 6 months and a year, so you won't have to replace it very often!

Disclaimer: I am not responsible for any cars stolen that have this foolproof defense employed.