Electricity needs a road that goes all the way around. Leave a gap anywhere, and nothing happens.
Key idea
A circuit is a complete loop: from the battery, through the parts, and back to the battery. A switch opens and closes the loop. An LED lights only when current flows through it the right way, and it needs a resistor to keep the current small.
Why it works
A robot's brain, sensors and motors are all circuits. Every robot builder, from age eight to eighty, checks the same thing first when something does not work: is the loop complete?
Let's think it through
Worked example: Why is it dark?
Your LED will not light. The battery is new. What could be wrong?
- Is the loop complete, with no loose wire?
- Is the switch closed?
- An LED has a long leg (+) and a short leg (−). Is it in backward?
Answer: Check for a gap, the switch, and which way the LED faces.
Common mistake
Thinking a part is broken when it is just in backward. LEDs work only one way around. Flip it and try again.
Hands-on project: a paper circuit card
Parts (about \$10 to \$15 for enough to make several cards)
- Copper tape with conductive glue
- A 3-volt coin cell (CR2032)
- A few small LEDs
- A binder clip, card stock and markers
- Draw two lines on the card for the copper tape roads: one from the + side of the battery to the LED's long leg, one from the LED's short leg back to the − side.
- Stick down the tape. Bend the LED's legs flat and tape them onto the roads.
- Fold a corner of the card over the battery and hold it with the binder clip. When the fold presses down, the loop closes and the LED lights. You built a switch.
Safety.
- Use only batteries: coin cells, AA or AAA. Never use a wall outlet for projects.
- Coin cells are dangerous if swallowed. A grown-up keeps them, hands them out one at a time, and puts them away. Keep them far from little brothers and sisters.
- Do not connect a battery's + and − sides with just a wire. That is a short circuit; the wire and battery get hot.