Audience

People who haven't thought about circuits since school

e.g. Me.

What is it?

Energy resulting from the flow of electrons around a circuit

AC

DC

Ohm's Law

v = i r

• Voltage, volts - v
• Current, amperes - i
• Resistance, ohms - r

Voltage

Potential difference in a circuit

Amps

• A measurement of current
• Current is charge moving over time
• Charge is measured in columbs
• 1 columb per second
• 1 columb = 6.241×10¹⁸ * charge of one proton

Amps

• Columbs are c in SI units
• Amps are A in SI units
• Ohm refers to Amps as i
• Thanks for nothing SI

Resistors

Resistors

• Limits the flow of current
• Used to reduce voltage in a circuit
• Can come in many forms

How do I use resistors?

How do I use resistors?

• Circuit at 9v
• Examine spec sheet!
• LED needs 20mA
• r = v / i
• r = 9 / 0.02
• r = 450Ω

Ohm's Law

Resistors in series

Resistors in series

R_total = R₁ + R₂ + R₃ + ...

Resistors in parallel

Resistors in parallel

1 / R_total = (1 / R₁) + (1 / R₂) + (1 / R₃) + ...

Enough theory!

LEDs in series

LEDs in series

Light detector

• Light Dependent Resistor
• Variable resistor
• With light - low resistance (100s of Ω)
• In darkness - high resistance (MΩs)

Light detector

Transistor

Transistor

• Can amplify or switch circuits
• As base resistance varies, current in other circuit varies
• An Arduino can vary voltage using PWM

Enough theory again!

Darkness detector

Recap

• Components require different voltages
• Use resistors to reduce voltage
• Ohm's law v = i r
• Read spec sheets!

Next time at NodeBots of London

• Components
• Hardware debugging
• Communications protocols