Make a Simple Ambient Light Sensor/Night Light Paper Circuit

EDIT: We've added a 2nd version of this activity that improves the circuit a bit in terms of function and reliability based on input from the amazing Instructables community (see comments at the end of this Instructable). If you are coming here from our printed kit instructions, skip directly to Step 7 for the improved Version 2.0.

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.

Version 1.0 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 sort of hacking the properties of a specific phototransistor. In Version 2.0, we'll have a transistor power the LED when the lights are not on. 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 for Version 1.0 (part number 2760326, 5mm 3-24V)
• CR2032 3V coin cell battery
• For Version 2.0, you'll need the above plus a NPN 2222A transistor like this one from Mouser
• 5mm Copper tape with conductive adhesive - I love this stuff from SparkFun
• Scissors
• Mini binder clip
• Printed card with circuit layout (it's nicer to print on cardstock or thick paper but copy paper works too)

Other Light Sensor and Resistor Combos to use with transistor version 2.0

• This Everlight Sensor from Mouser and a 30K Ohm resistor
• The Vishay TEPT5600 sensor and a 20K Ohm resistor
• The Vishay TEPT5700 sensor and a 348K Ohm resistor

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. Version 2.0 is better and includes a transistor in the build and also requires a bit more copper tape. I recommend cardstock or thicker paper, though regular copy paper can work too. Version 1.0 should be about 2" wide by about 4" tall, Version 2.0 should be 2.5" wide and 4.25" 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. Included in the images above is a schematic for both versions.

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.

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.

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.

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!

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!

Based on input from the Instructables community (see comment threads below), we've added a version that includes a transistor in the build and works for longer and a bit more reliable. For this version, we'll add some copper strips of tape down on the card first, then tape the components over top of these traces. We're doing this because while we are using copper tape with conductive adhesive, sometimes that adhesive just is not reliably conductive to make our circuit work reliably. Adding the copper traces first gives us a bit of "belt and suspenders" redundancy for how the components are all connected together. With the exception of the transistor, all other components are the same. You'll need 12-15 inches of copper tape because of the redundancy we are building in here.

On the printed card there are dotted boxes along where we'll want to tape the underlying copper traces. See the images above. You can tear the tape with your hands, it just helps to cut it with scissors for precision. USE CAUTION - the edges of copper tape can be sharp if you run your fingers along them.

Fill in the dotted boxes with copper tape - you'll need 5 total strips of it. Where the transistor will go near the top, make sure that the traces do not come in contact with each other or you'll create a short circuit there and it won't work!

Once you've created your traces, start adding components to your card. I like to start with the resistor and light sensor. Lay each over where the picture of the component is indicated, then tape in place. The ambient light sensor has a longer leg and a shorter leg - the longer leg must connect on the left-side where the resistor is, or your circuit won't work! The resistor and light sensor legs can overlap each other for good connection. Make sure the light sensor leg is not touching the longest strip of copper tape over on the left that goes to the transistor.

Now we'll add the transistor as indicated in the upper left-hand corner. It is important you place the resistor with the flat side down! Splay the three legs of the transistor out a bit so they will lay down on the traces you taped earlier, then tape down the left and middle legs (collector and base legs) on to the traces. Leave the one on the right - the emitter - until the next step. It's tricky to get the tape to not contact other pieces here, but we need to make sure that there is separation between the legs of the transistor or it will short-circuit. If your circuit is not working, this is a good place to check - make sure the tape isn't touching two of the legs at the same time.

All transistors have a base, collector, and emitter. If the flat side is down on our transistor, the collector gets power from the battery directly (the long copper trace along the lefthand side of the card), the base is triggered by the resistor and light sensor (middle leg), and the emitter sends power along to the LED (the leg of the transistor connected to the LED).

Now we'll add the LED - almost there! Tape it down where indicated and make sure that the longer leg of the LED - which is positive - is touching the leg of the transistor as shown.

You'll have a bit of extra leg leftover on the light sensor and LED - I like to bend them so they follow our copper trace that goes along the negative side, then tape over them both with a single piece of copper tape. This makes for a secure and reliable connection here.

Now add the CR2032 watch battery to the circuit. It needs to go with the flat side down, which is positive. Usually this is indicated with a + on positive side, and the negative side has some bumps or grid pattern on it. Fold the negative side over as shown, then clip with the binder clip. Your circuit is ready to test!

Turn off the lights or cover the sensor as shown above. What happens in dim light vs darkness? In the photo above, I covered the sensor with my finger but also had to block the light from the window to get the LED to light up - these sensors can get light from the side as well as the top.

If your circuit is not working, see Step 6 above for some troubleshooting tips - it's about the same between Version 1.0 and 2.0. Push down connections, make sure there is not a short up by the transistor, and that all components have been added in the correct orientation with longer legs on the positive side and the transistor with flat side down.

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. In Version 1.0, the sensor is shorting the circuit to the LED when light is falling on the sensor. In 2.0, the sensor is not triggering the transistor when light is falling on it. When it is dark, the sensor allows the current to flow to the LED in 1.0, and triggers the transistor in 2.0. 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.

To learn more about what electricians do, see this video from one of spectrUM's role models who's a licensed electrician.