LED blink outside area


I want a LED to start blinking if it is outside some area (image attached).

What would be the best (and cheaper) solution for this? Could I do this with 2 proximity sensors?

Thank you.


It depends.
How big is the circle.
If the item with the LED is close to the ground, then you could use a white line and a reflection sensor.
If you're walking with some device, an "invisable dog fence" or "hearing aid loop" could solve this.


We need the whole story.
What is your application.
We need to know the environment you will have your project working in.
How big is the circle?

Tom..... :slight_smile:

Here are two interesting thoughts that can be really inexpensive, but it depends a little on the stability of your environment.

  1. A cheap OOK transmitter and receiver. The receiver antenna is located at the center of your circle, the transmitter is in the object being moved . OOK is just on/off keying, which means that when you apply a high signal to one pin on the TX, it transmits a pure sine wave. The receiver, when detecting the sine wave, asserts a high on a pin. That's it: single pin interface.

You have the transmitter transmit periodically, and look for a signal at the arduino within the specified time period. What you do next is to either improve or attenuate the signal strength to get the desired maximum range. The antennas must be omnidirectional, and the one on the transmitter will have to have a nearly uniform pattern spherically.

The reason this requires a stable environment is that signals don't only go straight from Tx to Rx, they also bounce off nearby objects, which can either enhance or weaken the signal at the receive antenna. So if this application lives in an unchanging nearby environment, this may work.

On Ebay, you can buy these little radios for about $4 a pair or less.

  1. Second choice, same idea, but use ultrasonic transducer and mic. In his case, you must be careful that your pickup and transmitter are omnidirectional, as for the RF case. On the other hand, because the wavelengths of sounds are much greater than those of radio waves, the reflection problems won't occur except for larger objects. Generally, an object less than a half-wavelength in extent is almost invisible to a wave, sound or RF.