Lesson 3 · Weeks 4-6

The voltage divider is the single most useful circuit in this unit. Every resistive sensor you use is a divider with one leg that changes.

1. Measure the supply first. Put the multimeter across 5V and GND. It will not read 5.00V — USB power arrives through a fuse and a cable, and both drop a little. Use the number you measured in every calculation below, not 5.00.
2. Calculate. Choose two resistors. Work out what the voltage at the junction should be. Write it down before you build anything.
3. Build it on the breadboard.
4. Measure it across the lower resistor.
5. Compare calculated against measured.
6. Rank the causes of the gap, largest first:
   • The supply. If you used 5.00V instead of your measured value, this is most of your error on its own.
   • The meter. A cheap multimeter is typically accurate to about ±0.5% of reading plus a digit or two.
   • The resistors. These are 1% metal film, so two of them bound the divider ratio error at roughly ±2% worst case, and usually far less.
7. Now measure resistance directly. Switch the meter to ohms and measure each resistor out of circuit. How close is each to its marked value?
8. Repeat with three different pairs, including one where the two resistors are very different in value, and describe what that does to the output.
9. Then try them in parallel. Put two resistors in parallel and calculate the total using 1/Rt = 1/R1 + 1/R2. Predict, measure, compare. Note that the total is always smaller than either resistor — if your answer is bigger, you have used the series formula.

An LED is not a resistor. Connected straight across a supply it draws as much current as it can and destroys itself. The resistor is what makes it a circuit.

1. Find the numbers. Look up the forward voltage and maximum current for your LED colour in the datasheet.
2. Calculate the resistor you need to limit current to a safe value.
3. Build and measure the current, and compare it with your target.
4. Change the resistor to a much larger value and measure again. Record what happens to both current and brightness.
5. Compare two colours. A red and a blue LED with the same resistor draw different currents. Explain why using their forward voltages.
6. Calculate the power. Power is P = V x I. Work out how much power the resistor is dissipating, and how much the LED is. Compare the resistor's figure against its rating — these are quarter-watt parts, so anything approaching 0.25W is a problem.
7. Then calculate energy. Energy is power x time. If your LED ran continuously for a day, how much energy would it use? Do the same sum for an old 60W incandescent lamp and compare. This is the number behind every efficiency claim you will make later in the unit.

Do not skip step 1. A resistor value copied from the internet works by accident, and this unit is about not working by accident.