Simple DC Circuits

One of the merit badge requirements is to build a single pole double throw (SPDT) switch and show that it works. Another requirement is to hook up a battery to a light or a buzzer. Building a switch is easy. You can either get an eraser, three metal thumb tacks and a an a paper clip, or you can build one using a block of wood, a metal strip, three screws, and some washers. It is important that none of the terminals are touching each other.

I'm not going to go into any detail here on how to build a SPDT switch as it is covered in your merit badge book, and you can see one in the pictures here.

The lights that I use here were taken from a damaged Christmas tree light strand.

Here is the circuit diagram for the circuit we're about to build (The "3V" means "3 volts," but we could use a 6V or 9V battery if we wanted):

DC circuit diagram, with a battery, SPDT switch,
and two lamps connected to either SPDT terminals

The lines in the circuit represent wires, the weird thing on the left is a battery symbol, and the two circular things represent lamps (the circle represents a glass bulb, while the loop represents the filament in the bulb that glows).

When the switch is in this position, only the red light will turn on. When the switch is placed in the second position, only the green light will turn on. Here is what this circuit looks like when it is all connected:

Connected circuit with red light on
Arrows show direction of current flow

You can connect stripped wire by twisting the stripped (bare metal) portions to each other. You could also crimp on connectors too.

Now, if we change the switch position like so:

DC circuit diagram, with a battery, SPDT switch,
and two lamps connected to either SPDT terminals

The green light would be on instead (you only need one circuit diagram, you don't need to draw a new diagram for each possible switch position). Here is what the circuit would look like in action:

Connected circuit with green light on
Arrows show direction of current flow

If you build your switch, and built this circuit using your switch, you completed the following requirements for the Electricity merit badge: 11

If you would like the parts to build this switch, you can get them from your local hardware store (you'll also need tin snips to cut the metal strip for the switch arm -- wearing safety goggles while cutting is a very good idea), and a screw driver. The metal strip can be found in the plumbing section (it's called "plumber's tape").

The other option is you can buy an Electricity Merit Badge Kit from me that has all the stuff you need to complete the electricity merit badge (batteries, battery holder, parts to build the switch above, electromagnet wire, and a bolt). I've given out similar kits in the past at merit badge midways.