DIY Bi-Metallic Strip An introduction to the applications of thermal expansion and two-way switching. Written By: Mahaaveer BN 2018 Page 1 of 14
INTRODUCTION A bi-metallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The strips are joined together throughout their length, the expansion forces the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. TOOLS: Scissor (1) PARTS: Foam (1) 12 x 2 cm Copper Strip (1) 12 cm Aluminium Tape (1) 12 cm Safety Pin (2) Metal Rod (1) 6 cm Connecting Wire (4) 10 cm Battery Holder (1) With 2 x AA Batteries Matchbox (1) 2018 Page 2 of 14
Precautions Scissors have functional sharp edges. Contact may result in injury. Always keep blades away from fingers and body. Handle with care. Please realize that batteries might cause damages like leaking, fire or explode when misused or defective. Refrain from mechanically damaging them. Be careful not to short the terminals of the batteries. Step 1 - Securing the Batteries As you may already know, a battery has two terminals - a positive (+) and a negative (-). On a AA Battery, the positive is indicated by a nub on one side. While the other flat side is the negative. Stretch the elastic band over each battery from positive side to negative side. 2018 Page 3 of 14
Step 2 - Adding Contacts Insert a safety pin (head-first) in between the battery and the elastic band. This pin should be inserted near the positive side of the battery. Pull the safety pin along the battery till the head sits flat on the positive nub. Take care that the pin doesn't slip out from under the elastic band. You should end up with a battery with the safety pin sticking out from the positive end. Step 3 - Joining the Batteries Together Insert the battery assembly into the space between the elastic band and the other cell (in the elastic band). This time towards the negative end. Drag up the cell so that the safety pin is flat on the negative terminal as well. You should end up with two cells connected positive to negative by the safety pin and two free terminals. 2018 Page 4 of 14
Step 4 - Adding Main Terminals Now insert one of the safety pins in between the free positive terminal and the elastic band. Now insert a safety pin under the elastic band over the free negative terminal as well. You will end up with two safety pins sticking out in opposite directions, one being positive and the other negative. Now, use the scissors to cut the ice cream sticks into two lengths of 3.5-4 cms each. Step 5 - Finishing Up Insert one of the ice cream stick pieces in between the elastic bands and across both cells. Use the other piece to secure the other side. 2018 Page 5 of 14
Play! Our battery holder is now finished! You can test its functionality by attaching an LED across the safety pins. Troubleshooting Here is a circuit diagram of the holder. We are connecting the batteries in series for at total of 3 volts. If the battery holder refuses to light an led, check the contact between the pins and battery terminals. Also check the tension on the elastic bands and add spacers with ice cream stick pieces. Be careful not to short the terminals of the batteries. If the batteries heat up while in the holder, this cloud indicate an electrical short circuit. Remove the batteries and check your connections. 2018 Page 6 of 14
Precautions Scissors have functional sharp edges. Contact may result in injury. Always keep blades away from fingers and body. Handle with care. Beware of the edges of the metal tape strips, they can cut the skin if handled improperly. Do not heat the strip too much, the adhesive on the tape will start to burn. Step 1 - Joining the Metals Take both the strips and make sure they are more or less of equal length. If not trim the longer one with scissors. For easier electrical connection it is better if the copper strip is 2 mm shorter than the aluminium strip. Peel away the paper at the back of the aluminium strip to reveal the sticky side. Now carefully paste the copper strip along one edge of the aluminium. The copper strip should cover half the aluminium strip's width and most of its length, except for about 2 mm at one end. 2018 Page 7 of 14
Step 2 - Trimming the Strip Now cut off the rest of the aluminium strip, taking care not to cut the copper strip too. The final product should be a relatively smooth strip with copper on one side and aluminium on the other. Step 3 - Adding a Mount Point Now, peel back the aluminium slightly (about 0.5-1 cm ) from the side where we made a tab earlier. Insert the metal rod between the two halves of the strip. Now press the two metals back together behind the rod to lock it in place. You can use glue but make sure not to get any glue on the edge close to the rod. This is were we will be connecting wire to the strip. 2018 Page 8 of 14
Step 4 - Making the Base Poke the strip like a "flagpole" to one end of the long side of the base. The strip should be 4-5 cm above the base. Use a drop of glue to secure the point where the rod meets the base and let it dry. Step 5 - Connecting to the Strip Once the glue is dry, straighten the strip and make sure it runs along the base. Strip a piece of wire about 1.5 cm at both ends. Make a small hole (with scissors or a safety pin) on the strip on the edge closer to the rod, and tie one end of the wire to it. 2018 Page 9 of 14
Step 6 - Mounting the Contacts - I Make a small fold on the strip around the wire to make sure the exposed wire is connected to both sides of the strip. Make two holes with scissors on the other end of the base. These holes should be spaced a centimeter apart. Step 7 - Mounting the Contacts - II Insert a safety pin (tail first) into each of the holes made earlier. The two pins should be about a centimeter apart with the strip running between them. Slightly move the strip so that the aluminium side is touching the contact pin on the aluminium side. Remove insulation from the ends of two additional connecting wires. 2018 Page 10 of 14
Step 8 - Wiring the Contacts Connect one wire to each of the safety pin bodies. You can additionally push the wires into the holes made by the pins to secure them. The strip is now complete and should look something like the picture, with 3 free wire ends. Step 9 - LED Indicator We'll have to connect an LED to each of the switch's terminals. Start by identifying the polarity of the LEDs. The longer leg is positive. Now twist both negative terminals legs together. This "common" negative will be connected to the negative terminal of the battery. To connect the common negative terminal of the LEDs, strip an additional piece of wire. 2018 Page 11 of 14
Step 10 - Final Wiring Connect the negative terminal of the battery holder to one end of the wire. Now connect the wire from the rod side to the positive terminal of the battery holder. Connect one of the wires from the safety pins to each positive pin of the LEDs. The pin of the aluminium side connect to the green positive and the copper side pin gets connected to the red LED's positive pin. 2018 Page 12 of 14
Play! If everything is connected correctly, the green LED should be glowing when the strip is in its default position. Now carefully introduce a match under the center of the strip and watch it bend until the strip touches the other pin. The green LED turns off and the red LED turns on - indicating that the strip is in its 'heated' position. 2018 Page 13 of 14
Troubleshooting Here is a circuit diagram of this experiment. You can see that the strip acts like a two pole switch, routing the connection through one LED or another; depending on its temperature. Make sure that the strip is as straight as possible and doesn't have creases. This will help it bend in the right direction when heated. The strip bends toward the copper side when heated, so make sure that the red LED is attached to that side. If the LEDs don't glow at all, check the polarity of the LEDs they will not work if reversed. If the strip does not turn on the red LED when heated, try reducing the distance between the two safety pins. 2018 Page 14 of 14