1. Resistor and LED: the first circuit
Step 1 of the automatic light: B1 (9 V) → R1 470 Ω → LED1 → back to B1. I = (9 − 2) / 470 ≈ 15 mA: bright, and well inside the LED’s 20 mA. Component 1 of 14: the resistor. A resistor turns a voltage into a definite current: I = V / R. It is the most common part on any board, and its job here is the one you will meet again and again: limiting the current through an LED.
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Parts list
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- LED current
- Voltage across R1
How it works
The resistor. A resistor turns a voltage into a definite current: I = V / R. It is the most common part on any board, and its job here is the one you will meet again and again: limiting the current through an LED.
Reading it. Value by colour bands (yellow-violet-brown = 470 Ω), tolerance band (gold 5 %, brown 1 %), power by size (a small ¼ W body is the usual). The LED: the long leg is the anode (+), the flat side of the rim is the cathode (−); red needs about 2 V, white/blue about 3 V.
Testing it. Out of circuit, the meter on Ω reads the value within tolerance. In circuit, measure the voltage across it and divide by R: that is the current. An LED tests with the meter’s diode range: it glows faintly and reads 1.6–3 V one way, nothing the other.
In the build. B1 (9 V) → R1 470 Ω → LED1 → back to B1. I = (9 − 2) / 470 ≈ 15 mA: bright, and well inside the LED’s 20 mA.
Next: lesson 2, switch.
Things to try
- Change R1 to 1 k: dimmer, about 7 mA.
- Change R1 to 47 Ω and watch the LED burn out (Undo restores it). That is why the resistor is there.
Background
One circuit, built up one part at a time: by lesson 14 it is a solar-powered, dusk-sensing, motion-triggered timed lamp with an alarm.