Schematics, Breadboards and Multimeters

A schematic is a map of a circuit. Instead of drawing what the parts look like, it uses simple symbols, so the same diagram can be understood anywhere.

• Battery: a long line and a short line. The long line is +.
• Resistor: a zigzag line. (Some diagrams use a small rectangle instead. Both mean the same thing.)
• LED: a triangle pointing in the direction current flows, with a bar at the tip and two little arrows for light.
• Switch: a line that can swing open, like a gate.
• Wires: straight lines. A dot where wires cross means they’re connected; no dot means they just cross over.

A breadboard is a plastic board full of holes that lets you build circuits just by pushing parts and wires in. Inside, metal strips connect certain holes together:

• In the middle area, each short row of five holes is connected. Parts plugged into the same five-hole row are joined.
• A gap down the centre separates the two halves. Rows on opposite sides of the gap are not connected.
• Long rails run along the edges, usually marked red (+) and blue (−), for battery power. On some boards the rails are split halfway along, so check yours.

The most common breadboard mistake: plugging both legs of a part into the same five-hole row. That connects the part to itself, so no current flows through it.

A multimeter measures voltage, resistance and continuity (whether two points are connected). Inexpensive digital ones are fine for learning.

• Voltage (DC volts, shown as a V with a straight line): touch the red probe to + and the black probe to −. Check a fresh AA battery: you should see a little over 1.5 V (rechargeable NiMH cells read about 1.2 to 1.4 V).
• Continuity (the beep setting): with the circuit’s battery disconnected, touch both probes to two points. A beep means they’re connected. Great for finding broken wires.
• Resistance (Ω): with the battery disconnected, measure a resistor by touching a probe to each leg. A 220 Ω, 5% resistor should read between 209 and 231 Ω. Don’t hold both metal probe tips with your fingers: your body adds another path and spoils the reading on large resistors.

Voltage is measured across a part (in parallel): put the probes on either side of the LED to see its forward voltage, or across the resistor to see the rest of the battery’s voltage. The two should add up to roughly the battery voltage (Kirchhoff’s voltage law from Lesson 3).

Then use Ohm’s law to work out the current without measuring it directly: I = V_resistor ÷ R.

To measure current, the meter must be placed in series, so the current flows through it, usually after moving the red probe to a separate socket. In that mode the meter has very low resistance. If you then touch the probes across a battery, you’ve made a short circuit through the meter, which can blow its internal fuse or damage it. For Stage 1, measure voltages and calculate current with Ohm’s law instead. Always return the meter to the voltage setting, with the red probe in the V socket, when you’re done.

The longer line.

The resistor is bypassed, because both legs share one metal strip. Depending on the wiring, either nothing works, or the LED gets the full battery current with no protection and can burn out.

A resistance measurement uses the meter’s own small battery. Outside voltage gives a wrong reading and can damage the meter.