Electronics Projects: How to Build a Switched Lamp Circuit

Building a switched lamp circuit presents a simple electronics construction project that lets you explore the use of a simple on/off switch to control a lamp.

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This project uses a DPDT knife switch from RadioShack. It's unlikely that you'd use a knife switch in an actual electronic circuit. However, a knife switch like this is a perfect tool for learning the ins and outs of working with switches.

For one thing, it is entirely exposed, so you can see how it works. Additionally, since they come on their own base and have screw terminals, connecting them in temporary circuits is simple because you don’t have to do any soldering.

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As you can see, the knife switch is a double pole, double throw (DPDT) switch, which means it operates like two SPDT switches that are mechanically linked.

The six terminals on the switch are labeled 1X, 1A, 1B, 2X, 2A, and 2B. The 1 and 2 designate which of the two circuits is being switched. The X terminals are the input terminals in the center of the switch, and the A and B terminals are for the two possible outputs.

Thus, when the switch is flipped one way, 1X is connected to 1A and 2X is connected to 2A. When the switch is flipped the other way, 1X is connected to 1B and 2X is connected to 2B.

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If you want to experiment with different variations of this project, try the following:

  • Try moving the switch from the positive side of the circuit to the negative side. In other words, connect the red lead from the battery holder to the lamp and connect the black lead to the 1X terminal on the switch.

    The circuit will function the same. This shows that the location of a switch in a circuit often doesn't matter. If the circuit is broken anywhere, current cannot flow. Thus, whether the switch is before or after the lamp doesn’t matter.

  • Cut a second 6″ piece of wire and strip the insulation from both ends. Then, wire the circuit so that the red battery lead goes to switch terminal 1X, the black lead goes to switch terminal 2X, one of the wires goes from switch terminal 1A to one of the lamp terminals, and the other wire goes from switch terminal 2A to one of the lamp terminals.

    Now you've created the circuit. In this circuit, the knife switch is used as a DPST (double pole, single throw) switch to interrupt the circuit on both the negative and the positive side of the lamp.

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