Most of us have seen the car alarm “blinky” light through a vehicle’s window indicating that there’s an active (armed) alarm in use. In some cases (such as when someone didn’t want to spend the money on an actual alarm but still wanted to discourage potential thieves), car owners would install a fake blinky light.
The fake light is easy to differentiate from the real thing in the way it operates. The fake LED would blink constantly, while the “real thing” would blink only once every few seconds, leaving a long(-ish) pause between blinks, possibly to minimize draining the car battery over long periods of time where the car might not recharge the battery (like when it sits while the owner is on vacation).
If a car owner would like to make the flashing LED look more realistic without the actual expense of buying an alarm (user’s choice — I’m not advocating pro or con on an actual alarm), here’s an approach to make a fake alarm look more realistic.
Take a look at the schematic.
The 555 timer (U1) is set to run at 1 Hz. U2 (a 4040 binary counter), as shown, counts to three seconds at which point LED1 blinks for one second. A count of “4” resets the 4040, and again three seconds pass before the LED blinks again.
The timing interval mentioned in the previous paragraph is not engraved in stone; a four-input AND gate (U3; instead of the more common two-input variety) gives us the flexibility to choose a wider range of numbers (AND gates can also be combined to reach even higher numbers) for the pause time between LED blinks. (U3B can be used for the same reason on the RESET pulse as the 4040.)
One thing that needs to be pointed out is that the 4040 counter uses an active low instead of the more typical active high used by most ICs like the 555.
So, why not modify the blinky light mentioned initially instead of building an additional circuit? This is because the LEDs that flash automatically with no external circuitry have an approximate 50% duty cycle, flashing much too often for our application. Since the circuitry for this is enclosed in the LED itself, the rate can’t be modified.
This circuit can easily be powered from a 9V battery if the user doesn’t want to wire directly into the car’s electrical system. The circuitry itself is small enough to be hidden almost anywhere in the vehicle, although mounting the LED should be done in such a way that it looks like a professional install (so we don’t tip our hand), and should be mounted so that it’s readily seen by a potential car thief as they walk up to the driver’s side of the vehicle.
Disclaimer: This is not intended to replace an actual car alarm!
Always keep in mind that — just like anything else we add to our vehicle — safety first! For example, this circuit should not in any way interfere with safe operating.
Hopefully, this simple circuit can save you some time and frustration! NV