Introduction: Make a Simple Ambient Light Sensor/Night Light Paper Circuit
This is part of a series of Instructables detailing free science kits and activities developed by the spectrUM Discovery Area in Missoula, MT. The kits and activities are for use at home, in the classroom, or in a distance learning setting with teachers.
For more on how our museum shares these activities with students and teachers, see this video on YouTube.
Making a simple battery-powered night light is a great way to learn about circuits and sensors. This sensor will detect when it is dark, and turn on a red LED. It would be great to put in your hallway or bedroom, and is powered by a small watch battery that should last a few weeks or more.
This circuit is a bit simpler than many of the others that you might find on Instructables or elsewhere - we won't be using an additional transistor to our light sensor, as we'll be using the properties of a specific phototransistor. It's easiest to build on a piece of paper, and helpful to print the included card to cut out and tape copper tape to connect the components. If you prefer to use wires and solder or a breadboard, those are options as well - I'll include a circuit diagram so as to help with those alternative methods of connecting the components.
This Instructable is entered into the Battery Powered Contest, so if you find it helpful or useful be sure to vote!
- Red LED (red is are better in this circuit because of the low power consumption). We like these 10mm ones from Evil Mad Scientist, they are easy to manipulate with smaller fingers.
- 680 Ohm resistor, 1/4 watt
- Ambient Light Sensor - it is important that you use this one from Radio Shack (part number 2760326, 5mm 3-24V) We've tried others from Digi-Key, Mouser and Arrow and this one is the only one that works without a transistor - if you find a better/cheaper alternative that works, put it in the comments!
- CR2032 3V coin cell battery
- 5mm Copper tape with conductive adhesive - I love this stuff from SparkFun
- Mini binder clip
- Printed card with circuit layout (it's nicer to print on cardstock or thick paper but copy paper works too)
Step 1: Print Your Circuit Card or Plan Alternative Layout
Use the attached files to print the circuit card. One is a single image that you can print, the other is a 8.5 x 11 sheet full of them that you can cut out with scissors or a paper cutter. I recommend cardstock or thicker paper, though regular copy paper can work too. Each should be about 2" wide by about 4" tall.
If you are wiring this up using solder and wires or breadboard, see the circuit diagram above for reference on how to assemble the circuit (I know the symbol is for a photoresistor vs phototransistor, but the program I was using didn't have a phototransistor symbol - be sure to use the components specified in the supplies step).
Step 2: Add the Resistor
Time to wire this puppy up! Take your copper tape and cut a small section off, then tape over the gray circle where the battery will go and off to the left to where the resistor will be. USE CAUTION: the edges of the copper tape can be somewhat sharp, so don't run your finger along the edges or it can cut you. This will be the positive side of our circuit, and the positive side of the battery will be face down on this piece of the copper tape.
Then, take the 680 Ohm resistor and lay it down where the image indicates on the card. Resistors do not care about the direction that electricity flows in the circuit - called polarity - so lay it down with either leg pointing towards the battery. Then take a piece of copper tape and secure it so it's connected to the previous section of tape that is going to be underneath the battery. Push down a bit on the tape so it makes a good connection with the resistor - this will be important to do for all the rest of the components as well.
Step 3: Add the Light Sensor and LED and Tape Down the Positive (+) Side
Now, we'll add the ambient light sensor and the LED to our circuit. These two components are sensitive to polarity, meaning that electricity has to flow through them in a certain way for them to work. Take a close look at each of them - notice that one leg is longer than the other leg? The longer leg on both of these is the positive (+) side of the component. The shorter leg is the negative (-) side of the component. In DC circuits, positive is usually color-coded in wires as red, and negative is color-coded as black. We've followed this same convention on the circuit card.
Bend the legs of each component so that they can lay flat on the paper as show in the images, making sure that you know which leg is the longer/positive one (it can be hard to tell the difference when they are not right next to each other). In this step, we want to tape down the positive (+) side of both so that they connect with our resistor. If you get done and your circuit is not working, it's highly likely that this is what is going on - neither of these components will work if wired in backwards.
Tape over the top part of the resistor leg, the long leg of the ambient light sensor and the long leg of the red LED all in one piece. Push around where this tape touches each of these component leads, using your fingernail is a good way to get good contact.
Step 4: Tape Down the Negative/Ground Side of the Circuit
Now, we just need one more piece of copper tape that is a bit longer than either of the other two sections. Tape down the negative (-) leg of the LED and ambient light sensor (the shorter leg), and run the tape all the way down to the bottom of the paper card to the very edge as shown. It's important that it runs all the way to the end so that you can fold it over the negative (-) side of the battery in the next step. If it's too long you can bend it over the edge of the card to the back or cut it off.
In other circuits, you may see negative referred to as ground or earth, and it's usually color-coded as either black (in DC circuits) or green (in AC circuits), though there are other conventions as well.
Step 5: Add the Battery and Binder Clip Switch
We are ready to power up our circuit. Take your battery and lay it positive (+) side down. These usually have a big plus sign on the positive side, and the negative side typically has a grid or dotted pattern on it. If you can't tell, another way to tell is that the positive side wraps around the edges of the battery, whereas the negative side is just the "bottom". This is hard to explain, but if you look closely at your battery it should be obvious. This is how these types of batteries can sit flush inside of their holders in many other devices, making them easy to pop out and replace.
Simply lay your battery down, then fold the edge of the card with the tape running down the negative side over the top. Use the binder clip to secure this together. What happens? You can use the binder clip as the switch for your circuit - whenever you want to turn it off, just remove the binder clip and the battery!
Step 6: Test and Troubleshoot Your Circuit
If all is connected properly, your circuit will turn on when the ambient light sensor is covered (I just used my finger to cover it in the image above) or if it's in a dark room. Try testing out different light levels to see how bright the LED is when in bright, dim, or dark light.
Circuit not working? That's okay!! Creating electronic circuits almost always takes a bit of troubleshooting, and copper tape is no exception.
Here are some things you should investigate if your circuit is not working as expected:
- Try pushing down the tape a bit, especially where it meets the component legs. If there is a loose connection here it can prevent the flow of electricity in our circuit. Use your fingernail to push it very close to the legs of each component.
- Did you ensure that the longer/positive leg of the light sensor and LED are on the proper side? See step 3.
- Is the positive side of the battery facing down?
- Sometimes your battery can short circuit if the copper tape is touching the sides of the battery in addition to the bottom/negative side. Remove the clip and try to bow the folded part out a bit or move the battery away from it slightly to prevent this.
- Cover the light sensor and push down at various joints to see if the light lights up - sometimes you really need to push these down to ensure that there's a good connection!
Step 7: Extensions and Explanations
This sensor circuit works very much like a night light that you may already have in your house. When light falls on the ambient light sensor component, it interrupts electrical current to the LED. When it is dark, the sensor allows the current to flow to the LED. The sensor is a phototransistor, meaning that it changes how the circuit operates based on the amount of light shining on it.
This type of light sensing circuit is used in quite a variety of applications! Night lights are certainly useful, but it is also used in street lights to sense when it is dark in the evening so that they can turn on (and turn off when it gets light in the morning). Ever notice that your cell phone dims its screen when close to your face when talking, but brightens when you move it away from your face? It's using the same sensor as we use here to tell when your ear is covering the speaker.
Scientists often measure the amount of light falling on an area in nature to determine things like the effects of light pollution on nocturnal animals, how much light plants are receiving throughout the day, and so much more! They use a more sophisticated way to measure the light than our sensor could detect, but the concept is very similar.
Here are some ideas to ponder:
- Can you think of other applications of a circuit like this one in everyday life?
- What about a sensor that works the opposite of ours - it turns a light on when exposed to light, and off in the dark?
- What happens when you shine a bright light on the circuit when it's in a dimly lit room?
- If you're curious about why LEDs are sensitive to polarity, look up diodes or photodiodes on the internet - they are quite fascinating and a revolutionary technology in lighting. Many different circuit components are polarity-dependent.
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Battery Powered Contest