LessonThe voltage divider · 17 min

Battery monitors, level shifting and bias

Dividers do far more than read sensors. This lecture follows three jobs you will meet in nearly every product: measuring a battery, connecting a 5 V chip to a 3.3 V one, and setting the working point of a transistor or regulator.

1. The battery monitor

1. Find the worst-case input. A "12 V" lead-acid battery is 12.7 V full, 14.4 V while charging and maybe 15 V if a charger fails. Design for 15 V.
2. Choose the ratio so the worst case stays under the ADC's limit (5 V here): ratio ≤ 5 / 15 = 0.333. 22 k / 10 k gives 0.3125: 15 V → 4.69 V.
3. Choose the size: 10 k - 100 k range. Small enough for the ADC (see the loading lecture), big enough not to drain the battery: 12.6 V / 32 kΩ = 0.39 mA.
4. Convert back in software.

2. Level shifting: 5 V into 3.3 V

A 3.3 V chip such as the ESP32 or a Raspberry Pi can be damaged by 5 V on its pins. For a signal going one way, from the 5 V side to the 3.3 V side, a divider is the cheapest fix: 1 k / 2 k, 10 k / 20 k or 1.8 k / 3.3 k all give about 3.3 V. The other direction usually needs nothing: 3.3 V is high enough to count as a HIGH for most 5 V inputs.

3. Bias and feedback: a divider sets a working point

• Transistor amplifiers use a divider (like R1 / R2 above) to hold the base at a steady voltage.
• Adjustable regulators: an LM317 holds 1.25 V across its feedback resistor, so a divider sets the output: V_out = 1.25 × (1 + R2 / R1). The little trimmer on a cheap LM2596 buck module is the bottom half of such a feedback divider.
• Thresholds: a comparator switching a low-battery alarm or a street light compares the sensor's voltage with a reference made by another divider.