Proximity-Sensing LEDs

This project of mine started because I wanted to learn how to layout my own printed circuit board (PCB). I needed a simple and easy-to-solder circuit, so I chose this one because who doesn't love interactive LEDs?

In this Instructable I will only be showing the implementation of my circuit on a breadboard. In my next Instructable (now available here), I will demonstrate my process of designing and laying out the PCB.

As I mentioned I wanted a simple project and this one is just that! Students, hobbyists, and anyone else of all skill levels will be able to easily put this together. Let's get started!

Step 1: Introduction to the Circuit

Proximity-Sensing LEDs
Proximity-Sensing LEDs

This step is the "How it Works" section. If you prefer to get right into making the circuit, skip to the next step.

If you're still with me, I'm going to start with a brief introduction of some of the components I used in this circuit. (An exact list of materials is in the next step.)

  • The component that looks like a black LED is not actually an LED at all. It is a photo-transistor. How does a photo-transistor work? When the photo-transistor receives a certain wavelength of light, it "turns on" and allows current to flow through it. When the photo-transistor is not receiving that wavelength of light, it is "off". That being said, the photo-transistor is essentially acting as a switch in our circuit. Note: The photo-transistor I used is made to respond best to light with a wavelength of 880nm.
  • The pink LED in the image above is an infrared (IR) LED which does exactly what it sounds like it would do. Rather than emitting light that our eyes can see, it emits light in the infrared range of the electromagnetic spectrum. It is in series with a 220Ω current-limiting resistor to protect it from burning out. Note: The IR LED I used is made to emit light at a wavelength of 880nm. Sound familiar? I'll get back to this in a bit.
  • The blue LED is just that, a blue LED. It is also connected to a 220Ω current-limiting resistor.
  • The only other components I used were resistors and wires.

    So how does this all work? What makes it proximity-sensing? Remember in the explanation above that the photo-transistor acts like a switch. So when the photo-transistor is off, no current is flowing across it to our blue LED and the LED is off as well. Now look at the other side of our circuit. That's where the IR LED is connected, and it is connected such that it is always on and emitting 880nm infrared waves. Remember that I also mentioned the photo-transistor is set to respond best to wavelengths of 880nm? That's how the proximity-sensing works! When an object (such as your hand) goes over this little "cluster", IR light of 880nm is emitted from the IR LED. This light reflects off of your hand and back to the circuit. When the photo-transistor picks it up, it turns on allowing current to flow through from the source to our blue LED lighting it up!

    Note: The light we're dealing with doesn't have to specifically be 880nm to for this to work. The important thing is just that the photo-transistor responds best to the wavelength of light that the IR LED emits.

    Step 2: Gather the Materials

    Proximity-Sensing LEDs

    This circuit consists of "clusters" that are in parallel. Since the clusters are in parallel, this means you can add as many as you want without the LEDs getting any dimmer! You could have 1000 clusters if you wanted to and every LED would be just as bright! (Your battery wouldn't last very long though.) For my implementation I used 24.

    For each cluster you will need:

  • Photo-transistor
  • IR LED
  • LED of any color
  • 2 x 220Ω resistors
  • 47kΩ resistor
  • A couple small wires

    Note: Photo-transistors and IR LEDs are available at different wavelengths. You don't have to use 880nm as I mentioned in the previous step. For best results though, use photo-transistors that are made to respond best to the wavelength that your IR LEDs emit.

    For the rest of the circuit you will also need:

  • A breadboard (I'm using 3. Use as many as you like!)
  • A power source and connector (not pictured)

    For a power source I'm using a 9V battery because I had one at my desk already. You have a lot of other options here though such as a 6V lantern battery or 4 AA batteries.

    Step 3: Connect Power Rails

    Proximity-Sensing LEDs

    I like to start by getting all of my power and ground rails connected. Just as you can see in the picture above I connected all of my positive (red) rails and negative (blue) rails. I also plugged in my battery connector, but I'm leaving the battery out until the end so there's no current through the circuit while I'm building it.

    Step 4: Build the first cluster!

    Proximity-Sensing LEDs
    Proximity-Sensing LEDs
    Proximity-Sensing LEDs
    Proximity-Sensing LEDs

    I prefer to start with building one cluster to test my design.

    Note: Remember that the photo-transistor is not an LED. However, since it looks like an LED I will refer to its pins as anode (+) and cathode (-) for simplicity. I also included an image above that shows how to determine which pin is the anode and which is the cathode. I also included the circuit schematic, an animated breadboard image, and a photo of my circuit for reference.

  • Connect the "anode" of the photo-transistor to the positive power rail.
  • Connect the 47kΩ resistor from the cathode of the photo-transistor to ground. This resistor acts as what's called a pull-down resistor. It helps direct the current to where we want it to go.
  • Also connect the cathode of the photo-transistor to the anode of the blue LED.
  • Connect the cathode of the blue LED to ground with a 220Ω resistor.
  • On the other side of the "valley" of the breadboard, connect the anode of the IR LED to the positive rail.
  • Connect the cathode of the IR LED to ground with a 220Ω resistor.

    If you would like, go ahead and connect your power source and test it out!

    (If it's not working, see the final step for troubleshooting procedures.)

    Step 5: Finish the other clusters

    Proximity-Sensing LEDs
    Proximity-Sensing LEDs

    Now that you (hopefully) have a working cluster, add as many more as you would like! For me personally, I go faster if I go one component at a time e.g. add all the IR LEDs, then add all the photo-transistors etc. That's just my personal preference. Do what works best for you though. The design for each cluster is the same as how it was covered in the previous step.

    I included some pictures of my progress above.

    Step 6: Try it out!

    Proximity-Sensing LEDs

    Here's a GIF of my circuit in action. Try yours out! You can use just about any object: hands, rulers, books, etc.

    Hopefully yours is working at this point, but if not I included some troubleshooting procedures in the next step.

    If yours is working though, awesome! Post a GIF of it in the comments below! And feel free to post any comments/questions/suggestions.

    One last thing, I mentioned that I originally built this circuit to eventually move it to a PCB so I could learn PCB design/layout. The PCB I made was a success and I'll be making the Instructable documenting my design process very soon! Thanks for checking this out!

    Step 7: Troubleshooting

    Proximity-Sensing LEDs

    Hopefully you won't need this step, but here it is just in case!

  • If your circuit isn't working, one thing you can check is if the IR LEDs are actually on. Because if we can't see light in the infrared range, how do we know the IR LEDs are actually on? There's a simple way to check this. Simply just connect power and then look at the IR LEDs through a camera (phone cameras work fine). Through a camera they'll look like regular LEDs lighting up. You can see in the picture above that it looks like a regular pink LED, but that's one of my IR LEDs.
  • It's also possible that you're not getting enough voltage at the anode of the LED. This would result from there being too high of a voltage drop across the photo-transistor. To minimize the voltage drop across the photo-transistor, increase the value of the pull-down resistor. By increasing the value of the pull-down resistor, the voltage-drop across the photo-transistor decreases because the internal resistance of the photo-transistor is smaller relative to the larger pull-down resistor. Increase it gradually though because if you put in too large of a resistor, the LED will always be on.

    These are the most common issues you would run into with this circuit. If you come across other obstacles though, please comment and I'll get back to you quickly with a solution.

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