Progress Using Switches and Lamps

This page depicts wiring up batteries, resistors, switches, and lamps. This is not a tutorial, you may need some help talking through this. In Solder and Circuits class, we can do all this for real, if you like.

The first several images set the foundation, then the questions start.

The switch is a double-throw switch, it can conduct current on both positions. In the following image, notice how the wire goes to the left terminal, and the LED lights up.

In the following image, the switch is right but the wire is left. No current flows.

It is your turn. Sketch this out or print it out and pencil in where the wires would go. If you need help thinking about this, get some help.

Most people need to work through all of these wiring scenarios, there are some surprises in here.

You don't have to kow this material if you are just going to make amplifiers, but when you need to use transistors to turn on and off things, you need all of these circuits in your bag of tricks.

The switch so far is SPDT, single pole double throw. The other common types are SPST, DPDT, DPST, and SPDT center off.

The following circuit shows up in most homes. There is a real trick to wiring this. It is easy to wire it so that either switch turns off the lamp, but the hard thing is to wire it so you never have to go to the other side of the room to turn on or off the lamp.

The next one uses a new switch, a DPDT. Double pole double throw, it is two switches stacked next to each other.

The next one isn't practical, you rarely need it. But it can be wired, and it works in an ingenious way.

Let's get into the simplest logic. These are pushbuttons.

Let's do some algebra. First, the easy things: series batteries just add up, and series resistors just add up. The current = V/R is called Ohm's Law. Use your calculator and you get the current, .000531 amps.

Now we get into parallel currents. Though each resistor has 1.5 volts, they have different currents because they have different ohms.

Let's continue with the image above. The currents coming out of the resistors toward the right add up, .00351 amps. This is the current that comes on up through the battery.

Here is a hard assignment. Use algebra to find the formula for the equivalent resistance of two resistors in parallel. You can call the two resistors R1 and R2. You must use Ohm's Law and some of the facts above. Most people can't do this, but if you try and get the right answer, you are doing well. Hint: your answer will not have 1.5V in it, it will be a function of only R1 and R2.

The formula for parallel resistors is one of the most-used formulas in electricity.

Here is another hard assignment. You often need some particular resistance in a circuit, like 97 ohms, and you don't have that, but you do have 100 ohms. Find the formula for the resistor in parallel so that the equivalent is a particular value, Req.

These two assignments are hard. Here is a little story. When Mr. E was in seventh-grade science, Mr. Whiteside told us to go home and find the formula for degrees Fahrenheit, given degrees Centigrade. We knew that 32F = 0C and 212F = 100 C.

I didn't make any progress on this at all. One boy got the formula, though, Randy Humphrey. F = 9/5 C +32. He drew a graph and used y = mx+b. What a brain. He went on to be a star football player and senior class president.