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Design Notes: Voltage Divider, Potentiometer and Light Sensor

Revision 1.0 · KiCad 10.0.x · Last updated: 2026-10-24

This document records every engineering decision behind the board: the electronics theory, the derivations, the calculations and tolerance analysis, the transfer curves, the KiCad configuration, the layout rules, the verification results, the manufacturing settings and the test procedure.

Contents

  1. Requirements
  2. Electronics background
  3. Calculations
  4. Transfer curves and response
  5. Schematic
  6. KiCad configuration
  7. Layout
  8. Verification
  9. Manufacturing outputs
  10. Bring-up and testing
  11. Lessons learned
  12. References

1. Requirements

# Requirement
R1 Powered from a 5 V DC source through a 2-pin header
R2 A fixed divider whose output is exactly half the supply, with a switchable load to show the loading effect
R3 A potentiometer divider that sweeps from 0 V to the full supply
R4 An LDR divider whose output rises with light, working across dim and bright rooms
R5 Every output measurable with a handheld multimeter, using test points and a common ground reference
R6 Total supply current below 2 mA
R7 Hand-solderable parts (through-hole and 0805 SMD)
R8 2-layer board that a low-cost fab can build without special options
R9 Zero ERC errors and zero DRC errors

2. Electronics background

2.1 The voltage divider

Two resistors in series across a source carry the same current. By Ohm’s law:

\[I = \frac{V_{in}}{R_1 + R_2} \qquad V_{out} = I \times R_2 = V_{in} \times \frac{R_2}{R_1 + R_2}\]

$V_{out}$ is the fraction of the input that falls across the lower resistor. It always lies between 0 V and $V_{in}$. If the two resistors are equal, the output is exactly half.

2.2 The Thevenin equivalent

Seen from its output, any divider behaves like a voltage source with a series resistance:

\[V_{th} = V_{in} \times \frac{R_2}{R_1 + R_2} \qquad R_{th} = R_1 \parallel R_2 = \frac{R_1 R_2}{R_1 + R_2}\]

$R_{th}$ is the divider’s output resistance. A small $R_{th}$ means a stiff output that resists loading, but it costs more supply current.

2.3 The loading effect

Connecting a load $R_L$ to the output puts it in parallel with $R_2$:

\[V_{out,loaded} = V_{in} \times \frac{R_2 \parallel R_L}{R_1 + (R_2 \parallel R_L)} = V_{th} \times \frac{R_L}{R_L + R_{th}}\]

The fractional error caused by the load is:

\[\text{error} = \frac{R_{th}}{R_{th} + R_L}\]
Load compared with $R_{th}$ Error
Equal to $R_{th}$ 50 %
10 times $R_{th}$ 9.1 %
100 times $R_{th}$ 1.0 %

Rule of thumb: the load should be at least 10 times (preferably 100 times) the divider’s output resistance. A multimeter is itself a load, so it must be much larger than $R_{th}$.

2.4 The potentiometer

A potentiometer is a resistive track with a sliding contact (the wiper).

Terminal Connection here
End 1 GND
End 3 +5V
Wiper (2) Output VB

With the wiper at fraction $x$ of the travel from end 1 to end 3:

\[V_B = V_{in} \times x \qquad R_{out} = x\,(1 - x)\,R_{total}\]

The output resistance is zero at the ends and largest at mid-travel, where it equals $R_{total}/4$. The current through the track is constant at $V_{in} / R_{total}$. A linear (type B) taper makes the voltage proportional to shaft angle. A load on the wiper makes the curve non-linear (see section 4).

Used with only two terminals (wiper tied to one end), the same part becomes a variable resistor.

2.5 The light-dependent resistor (LDR)

A CdS photoresistor is a semiconductor whose resistance falls when light hits it. It has no polarity. Over the working range, resistance follows a power law in illuminance $E$:

\[R(E) = R_{10}\left(\frac{E}{10\ \text{lux}}\right)^{-\gamma} \qquad \gamma = \log_{10}\!\left(\frac{R_{10}}{R_{100}}\right)\]

$R_{10}$ and $R_{100}$ are the resistances at 10 lux and 100 lux. A larger $\gamma$ means a steeper response.

GL5528 values from the manufacturer’s datasheet:

Parameter Value
Light resistance at 10 lux 10 kΩ to 20 kΩ
Dark resistance 1 MΩ (minimum), measured 10 s after removing the 10 lux light
γ (between 10 and 100 lux) 0.6 (some resellers list 0.7)
Response time about 20 ms rising, 30 ms falling
Maximum voltage 150 V DC
Maximum power 100 mW
Operating temperature −30 °C to +70 °C
Spectral peak 540 nm (green)

Practical limitations:

2.6 Measuring a divider with a multimeter

A handheld multimeter on its DC volts range typically has an input resistance of about 10 MΩ (check your meter). The measurement error is:

\[\text{error} = \frac{R_{th}}{R_{th} + 10\ \text{M}\Omega}\]

For the fixed divider ($R_{th}$ = 5 kΩ) this is 0.05 %. For the potentiometer at mid-travel ($R_{th}$ = 2.5 kΩ) it is 0.025 %. The meter is therefore a negligible load on all three sections.

3. Calculations

3.1 Inputs

Symbol Meaning Value Source
$V_{in}$ Supply voltage 5.0 V (4.75 V to 5.25 V) Requirement R1
$R_1, R_2, R_3, R_4$ Fixed resistors 10 kΩ, 1 % Standard E12 value, see 3.9
$R_{RV1}$ Potentiometer track 10 kΩ linear Standard value
$R_{10}$ LDR resistance at 10 lux 15 kΩ typical (10 kΩ to 20 kΩ) GL5528 datasheet
$\gamma$ LDR slope 0.6 GL5528 datasheet
$R_{dark}$ LDR dark resistance 1 MΩ minimum GL5528 datasheet

3.2 Section A: fixed divider

\[V_A = 5.0\ \text{V} \times \frac{10\ \text{k}\Omega}{10\ \text{k}\Omega + 10\ \text{k}\Omega} = 2.50\ \text{V}\] \[R_{th} = 10\ \text{k}\Omega \parallel 10\ \text{k}\Omega = 5\ \text{k}\Omega \qquad I = \frac{5.0\ \text{V}}{20\ \text{k}\Omega} = 0.25\ \text{mA} \qquad P_{R1} = P_{R2} = I^2 R = 0.625\ \text{mW}\]

Tolerance (resistors 1 %):

Case R1 R2 VA at 5.00 V VA with supply ±5 %
Nominal 10.00 kΩ 10.00 kΩ 2.500 V  
Lowest 10.10 kΩ 9.90 kΩ 2.475 V 2.351 V at 4.75 V
Highest 9.90 kΩ 10.10 kΩ 2.525 V 2.651 V at 5.25 V

The output tracks the supply (it is a ratio), so supply error appears directly in VA. Always compare against the measured supply at TP1, not 5.000 V.

3.3 Section A with the load fitted (JP1)

\[R_2 \parallel R_3 = 10\ \text{k}\Omega \parallel 10\ \text{k}\Omega = 5\ \text{k}\Omega \qquad V_{A,loaded} = 5.0\ \text{V} \times \frac{5\ \text{k}\Omega}{10\ \text{k}\Omega + 5\ \text{k}\Omega} = 1.667\ \text{V}\]
Quantity Value
Error caused by the load −0.833 V (−33.3 %), equal to $R_{th} / (R_{th} + R_L) = 5/15$
Current through R1 (5.0 − 1.667) V / 10 kΩ = 0.333 mA
Current through R2 and through R3 0.167 mA each
Power in R1 1.11 mW
Power in R2 and in R3 0.278 mW each
Tolerance range (resistors 1 %, 5.00 V) 1.645 V to 1.689 V

Loading table (fixed divider, $R_{th}$ = 5 kΩ, 5.00 V supply):

Load $R_L$ VA Error
5 kΩ 1.250 V 50.0 %
10 kΩ (R3, this board) 1.667 V 33.3 %
50 kΩ 2.273 V 9.1 %
500 kΩ 2.475 V 1.0 %
10 MΩ (multimeter) 2.499 V 0.05 %

3.4 Section B: potentiometer

$V_B = 5.0\ \text{V} \times x$. The track current is $5.0\ \text{V} / 10\ \text{k}\Omega = 0.5\ \text{mA}$ and its power is 2.5 mW, which is far below any trimmer’s rating.

Wiper position VB (no load) Output resistance VB with a 10 kΩ load on the wiper
0 % 0.00 V 0 Ω 0.00 V
25 % 1.25 V 1.875 kΩ 1.05 V
50 % 2.50 V 2.500 kΩ 2.00 V
75 % 3.75 V 1.875 kΩ 3.16 V
100 % 5.00 V 0 Ω 5.00 V

3.5 Section C: LDR divider

\[V_C = V_{in} \times \frac{R_4}{R_4 + R_{LDR}} \qquad R_{LDR} = R_4 \times \frac{V_{in} - V_C}{V_C}\]

The second form lets you calculate the LDR’s resistance from a measured VC.

Typical values (R₁₀ = 15 kΩ, γ = 0.6; extrapolated outside 10 lux to 100 lux):

Illuminance R_LDR VC typical VC range for R₁₀ = 10 kΩ to 20 kΩ VC typical with γ = 0.7
Dark (0 lux) 1 MΩ or more 0.05 V or less    
1 lux 59.7 kΩ 0.72 V 0.56 V to 1.00 V 0.59 V
10 lux 15.0 kΩ 2.00 V 1.67 V to 2.50 V 2.00 V
100 lux 3.77 kΩ 3.63 V 3.33 V to 4.00 V 3.85 V
300 lux 1.95 kΩ 4.18 V 3.97 V to 4.43 V 4.39 V
1000 lux 0.95 kΩ 4.57 V 4.44 V to 4.70 V 4.72 V

Why $R_4$ = 10 kΩ. The divider is most sensitive to changes in LDR resistance when $R_4 \approx R_{LDR}$, because the slope $dV_C / d\ln R_{LDR} = -V_{in} R_4 R_{LDR} / (R_4 + R_{LDR})^2$ peaks at $R_{LDR} = R_4$. With the typical LDR, $R_{LDR}$ = 10 kΩ occurs at:

\[E = 10\ \text{lux} \times \left(\frac{15\ \text{k}\Omega}{10\ \text{k}\Omega}\right)^{1/0.6} \approx 20\ \text{lux}\]

which is a dim-room level, a good centre for a night-light threshold (Project 8). 10 kΩ is also a standard value shared with the other sections.

Dark level. With $R_{LDR}$ = 1 MΩ: $V_C = 5.0 \times 10\ \text{k} / 1.01\ \text{M} = 0.0495$ V. A real part is usually much darker-resistive, so VC approaches 0 V.

3.6 Power and ratings

Part Worst-case power Rating Result
R1 to R4 (0805) 2.1 mW (R4 at 1000 lux) about 125 mW under 2 % utilised
LDR1 $V_{in}^2 / (4 R_4)$ = 0.625 mW (maximum, at $R_{LDR} = R_4$) 100 mW well within
LDR1 voltage 4.95 V (dark) 150 V well within
RV1 2.5 mW trimmer rating, hundreds of mW well within

3.7 Supply current budget

Branch Typical Maximum
A: fixed divider 0.250 mA (JP1 open) 0.333 mA (JP1 fitted)
B: potentiometer 0.500 mA 0.500 mA
C: LDR divider 0.200 mA (10 lux) 0.457 mA (1000 lux)
Total 0.95 mA 1.29 mA
Power at 5 V 4.75 mW 6.5 mW

Total current stays below 2 mA, meeting requirement R6.

3.8 Why 10 kΩ everywhere

Reason Detail
Low supply current 0.25 mA to 0.5 mA per section
Low enough to resist loading $R_{th}$ of 2.5 kΩ to 5 kΩ is 2000 times smaller than a 10 MΩ multimeter
High enough to waste little power Milliwatts per section
Matches the LDR The LDR is 10 kΩ to 20 kΩ at 10 lux
Standard value Every common resistor series includes 10 kΩ, and one value simplifies purchasing

3.9 Component purposes

Ref Value Purpose
J1 2-pin header Brings in 5 V and ground
R1 10 kΩ Upper half of the fixed divider
R2 10 kΩ Lower half of the fixed divider
R3 10 kΩ Load resistor, connected across R2 by JP1
JP1 2-pin header and shunt Switches the load in and out without soldering
RV1 10 kΩ linear Adjustable divider
LDR1 GL5528 Light-dependent upper half of the LDR divider
R4 10 kΩ Fixed lower half of the LDR divider
TP1 to TP5 Test points Probe points for +5V, GND, VA, VB, VC

4. Transfer curves and response

4.1 Transfer curves

Transfer curves

Plot Reading
A $V_A$ against load resistance on a log axis. The curve approaches 2.5 V for light loads and falls steeply as the load drops below about 10 kΩ. The marker shows JP1 fitted: 1.67 V
B $V_B$ against wiper position. Solid: a straight line (no load). Dashed: with a 10 kΩ load on the wiper, the curve sags below the line (2.0 V at 50 %)
C $V_C$ against illuminance on a log axis. The solid curve is the typical LDR, and the shaded band covers the 10 kΩ to 20 kΩ spread at 10 lux

4.2 Response in time

LDR response

Time Event VC
0 to 50 ms Dark 0.05 V
50 ms Light switches to 100 lux Starts to rise
50 ms to about 200 ms LDR resistance falls, VC rises with a time constant of about 20 ms Reaches 3.63 V
200 ms Light switches off Starts to fall
200 ms onwards LDR resistance recovers, VC falls with a time constant of about 30 ms Heads back towards 0.05 V

The diagram is an illustration: it treats the datasheet response times (about 20 ms rise and 30 ms decay) as first-order time constants. The real part also has a long memory of recent light, so after bright light it needs seconds to settle to its dark resistance. At a multimeter’s reading rate (a few readings per second) these delays are invisible.

5. Schematic

5.1 Symbols and footprints

Ref Library symbol Value Footprint
J1 Connector:Conn_01x02_Pin Conn_01x02_Pin Connector_PinHeader_2.54mm:PinHeader_1x02_P2.54mm_Vertical
R1 Device:R 10k Resistor_SMD:R_0805_2012Metric
R2 Device:R 10k Resistor_SMD:R_0805_2012Metric
R3 Device:R 10k Resistor_SMD:R_0805_2012Metric
R4 Device:R 10k Resistor_SMD:R_0805_2012Metric
JP1 Jumper:Jumper_2_Open Jumper_2_Open Connector_PinHeader_2.54mm:PinHeader_1x02_P2.54mm_Vertical
RV1 Device:R_Potentiometer 10k Potentiometer_THT:Potentiometer_Bourns_3296W_Vertical
LDR1 Device:R_Photo GL5528 OptoDevice:R_LDR_5.1x4.3mm_P3.4mm_Vertical
TP1 to TP5 Connector:TestPoint TP1 to TP5 TestPoint:TestPoint_THTPad_D2.0mm_Drill1.0mm
Power power:+5V, power:GND, power:PWR_FLAG ×2 n/a n/a

Check the footprints against the datasheets of the parts you buy: lead spacing of the LDR and pin order of the potentiometer. If they differ, choose the matching footprint from the OptoDevice and Potentiometer_THT libraries.

5.2 Nets

Net Connections
+5V J1-1, R1-1, RV1-3, LDR1-1, TP1
GND J1-2, R2-2, R3-2, RV1-1, R4-2, TP2
VA R1-2, R2-1, JP1-1, TP3
LOAD JP1-2, R3-1
VB RV1-2, TP4
VC LDR1-2, R4-1, TP5

5.3 Design decisions

5.4 Schematic conventions

6. KiCad configuration

6.1 Project creation

  1. Create the folder 03-voltage-divider inside the repository.
  2. In KiCad, choose File → New Project, browse to that folder, name the project 03_voltage_divider, and untick “Create a new folder for the project”.

6.2 Board setup

Import the settings from the previous board: in the PCB editor, open File → Board Setup, click Import Settings from Another Board…, select 02_555_blinker.kicad_pcb, tick the stackup, design rules, net classes and text defaults, and confirm.

Setting Value
Copper layers 2, board thickness 1.6 mm, 1 oz copper
Minimum clearance 0.2 mm
Minimum track width 0.2 mm
Minimum connection width 0.2 mm
Minimum annular width 0.15 mm
Minimum via diameter 0.6 mm
Copper to hole clearance 0.3 mm
Copper to edge clearance 0.5 mm
Minimum drill size 0.3 mm
Hole to hole clearance 0.5 mm
Silkscreen: minimum clearance / text height / text thickness 0.15 mm / 1.0 mm / 0.15 mm
Net class Clearance Track width Via diameter / drill Nets
Default 0.2 mm 0.25 mm 0.6 mm / 0.3 mm all others
Power 0.2 mm 0.5 mm 0.8 mm / 0.4 mm +5V, GND

These are the same values used for Projects 1 and 2. They are based on typical low-cost 2-layer fab capabilities (see References). Re-check your fab’s current limits before ordering.

6.3 Grid

PCB grid: 0.5 mm for placement, 0.25 mm or 0.1 mm for routing. For the test point row use 5.08 mm.

7. Layout

7.1 Outline

A rectangle on Edge.Cuts, 70 × 40 mm, with 3 mm corner radius. The outline is a single closed shape with no gaps.

7.2 Placement

Rule Reason
Arrange the three sections side by side, A on the left, B in the middle, C on the right Matches the schematic and the silkscreen boxes
J1 at the left edge Easy connection of the supply
Test points TP1 to TP5 in a row along the bottom edge on a 5.08 mm pitch, in the order +5V, GND, VA, VB, VC Spaced for multimeter probes and hook clips, and GND sits next to the signals
RV1 near the top edge, with its adjustment screw facing up Screwdriver access
LDR1 at the right edge, with at least 10 mm of clear space around it Nothing may shade the sensor
Keep tall parts (RV1, about 10 mm) away from LDR1 and never place them on its light side A tall neighbour casts a shadow and corrupts the reading
R1, R2, R3 and JP1 grouped together Short connections in section A
R4 close to LDR1 Short VC node

7.3 Routing

Rule Value
Signal track width (VA, VB, VC, LOAD) 0.25 mm
Power track width (+5V, GND) 0.5 mm
Corner angles 45° or curved, never 90°
Copper to board edge at least 0.5 mm
Via size 0.6 mm pad / 0.3 mm drill

Keep the measured nodes VA, VB and VC short and away from the +5V track. They carry no current worth speaking of, so they are easy to disturb. After soldering, clean flux residue with isopropyl alcohol, because dirty boards leak and the LDR1 dark-state impedance is high.

7.4 Ground pour and stitching vias

7.5 Silkscreen

8. Verification

Check How Result
ERC Schematic editor → Inspect → Electrical Rules Checker 0 errors
DRC PCB editor → Inspect → Design Rules Checker 0 errors, 0 unrouted
Board statistics PCB editor status bar 22 pads, 6 nets
3D view PCB editor → View → 3D Viewer (Alt+3) LDR1 and RV1 orientation correct, nothing shading LDR1
Gerber check Third-party viewer (KiCad Gerber Viewer, Gerbv, tracespace) Outline closed, holes aligned, silkscreen clear of pads

Saved reports are in reports/.

9. Manufacturing outputs

9.1 Gerber plot settings (File → Fabrication Outputs → Gerbers)

Setting Value
Output directory fabrication/gerbers
Layers F.Cu, B.Cu, F.Paste, B.Paste, F.Silkscreen, B.Silkscreen, F.Mask, B.Mask, Edge.Cuts
Use Protel filename extensions On
Subtract soldermask from silkscreen On
Check zone fills before plotting On
Coordinate format 4.6, unit mm
Use extended X2 format On
Include netlist attributes On
Disable aperture macros Off
Use drill/place file origin Off
Plot drawing sheet Off

9.2 Drill file settings

Setting Value
Drill file format Excellon
Oval holes drill mode Use alternate drill mode
Drill origin Absolute
Drill units Millimetres
Zeros format Decimal format
Map file format (optional) Gerber

9.3 Suggested fab order options

Option Value
Layers 2
Thickness 1.6 mm
Outer copper weight 1 oz
Surface finish HASL (lead-free if available)
Quantity Fab minimum

Verify the generated files in a Gerber viewer, zip the gerbers folder, and upload the zip.

10. Bring-up and testing

10.1 Before powering

  1. Solder the 0805 resistors first, then the headers, test points, potentiometer and the LDR.
  2. Check that RV1 and LDR1 sit flat and that nothing blocks the light path to the LDR.
  3. With a multimeter in continuity mode, confirm there is no short between +5V and GND.

10.2 Powered test

Set the multimeter to DC volts. Keep the black probe on TP2 (GND) throughout.

  1. Leave JP1 open. Apply 5 V to J1 (pin 1 = +5 V, pin 2 = GND). Measure TP1: this is your actual supply voltage.
  2. Measure TP3 (VA). It should be half the supply.
  3. Fit the shunt on JP1. TP3 should fall to about one third of the supply, 1.67 V at 5.00 V.
  4. Turn RV1 from one end stop to the other while measuring TP4 (VB). It should sweep from about 0 V to about the supply voltage.
  5. Measure TP5 (VC) in normal room light. Cover LDR1 with a finger: it should fall. Shine a phone torch on it: it should rise.
  6. Calculate the LDR resistance from your readings: $R_{LDR} = 10\ \text{k}\Omega \times (V_{TP1} - V_{TP5}) / V_{TP5}$.
Measurement Expected Measured
Supply at TP1 5.0 V (4.75 V to 5.25 V)  
VA, JP1 open half the supply, 2.475 V to 2.525 V at 5.00 V  
VA, JP1 fitted about one third of the supply, 1.645 V to 1.689 V at 5.00 V  
VB at one end stop about 0 V  
VB at the other end stop about the supply voltage  
VB at mid-travel about half the supply  
VC, finger over LDR (dim) low, typically below 1 V  
VC, normal room light roughly 3.5 V to 4.5 V (varies a lot between parts and rooms)  
VC, phone torch close to LDR close to the supply voltage  
Calculated R_LDR in room light about 1 kΩ to 4 kΩ  
Supply current about 1 mA (0.95 mA typical, 1.29 mA maximum)  

10.3 Troubleshooting

Symptom Likely cause
VA reads 0 V or the full supply R1 or R2 missing or open, or a solder bridge
VA with JP1 fitted does not change Shunt not making contact, or R3 not soldered
VB does not change when turning RV1 Wiper pin (pin 2) not soldered, or the pot is in the wrong footprint orientation
VB sweeps the wrong way Pot pins 1 and 3 swapped relative to the shaft direction. This is not a fault, and you can swap the end connections if you prefer
VC stuck near 0 V LDR1 open, or R4 open and the LDR in the dark
VC stuck near the supply R4 shorted to the supply, or the LDR is shorted
Readings drift or are noisy Flux residue on the board, or a poor probe contact. Clean with isopropyl alcohol
VC changes slowly when the light changes Normal: the LDR lags by tens of milliseconds and remembers recent light for seconds

11. Lessons learned

12. References

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