Ohm’s Law

Every calculation in this project comes back to one equation: V = I x R (voltage equals current times resistance). This page is the refresher plus the beacon-specific uses.

The triangle

  V
 I R

Cover the value you want: V = I x R, I = V / R, R = V / I.

Worked example: the LED resistor

A beacon LED runs from 3.3 V and needs 10 mA; the LED drops 2 V. The resistor must drop the rest:

R = (3.3 - 2.0) / 0.01 = 130 ohms

Use 150 ohms (standard value); the LED sees 8.7 mA, well within spec and kinder to the battery.

Worked example: the battery monitor divider

The ADC reads up to 3.3 V but the battery reaches 4.2 V. A divider of R1 = 100k and R2 = 100k halves the voltage:

V_adc = 4.2 x (100 / (100 + 100)) = 2.1 V

The Battery Monitor Details page gives the exact values used.

Worked example: buzzer limiting

A buzzer rated for 5 V on a 3.3 V rail usually runs fine without a resistor, but if it is too loud, add series resistance:

R = (3.3 - 1.5) / 0.02 = 90 ohms

The three mistakes

Related pages

• Electrical Specifications for the project’s values
• LED Wiring for the LED circuit
• Battery Monitor Details for the divider