43 Light and Proximity Readout Physics

43.1 Start With the Measurement Story

Imagine a lamp that should turn on when a hand moves nearby. The detector sees reflected light, not the hand itself. An analog-to-digital converter is the circuit that turns the detector’s changing voltage into a number a program can use. It is often shortened to ADC.

Start by measuring empty space and then a known object. Change the distance, angle, surface, and room light one at a time. Choose a threshold only after those readings are visible. Then test a dark object, direct sunlight, and a value close to the threshold. A single clean desk test cannot prove reliable detection.

Use a small reading table. Begin with no object. Add one pale object. Add one dark object. Try three distances. Turn room lights on and off. Face the sensor toward and away from a window. Save the raw number each time. Look for overlap between present and absent cases. Set a decision band, not a perfect point, when readings move around.

Now repeat the test after the sensor warms up. Repeat it with another unit. Record any missed or false detection. The threshold should follow the evidence and the allowed cost of each error.

This simple experiment leaves out noise, drift, lens shape, and the full detector response. The Practitioner section builds a repeatable readout test. Under the Hood explains the light, current, conversion, and uncertainty behind each number.

Light and proximity readouts depend on physics before code. Begin with reflection, distance, angle, ambient light, and detector response, then decide which reading pattern proves an object is truly present.

Phoebe’s Field Notes: What 1 $\mu$A Of Photocurrent Says About The Light Itself

The mathematical gist. A photon’s energy is $hc/\lambda$, so a photodiode with quantum efficiency $\eta$ has responsivity $R=\eta q\lambda/(hc)$. At 850 nm and $\eta=0.8$, $R=0.548$ A/W; this chapter’s $1\ \mu\text{A}$ example therefore represents $1\ \mu\text{A}/0.548\ \text{A/W}=1.82\ \mu\text{W}$ of optical power.

Math Bridge · guided foundationsHow does one microamp reveal the light power?Let Phoebe count photons and recover this chapter's 1.82 microwatt signal. Learn the maths

43.2 Learning Objectives

After this page, you should be able to:

• Separate light intensity, proximity, presence, touch, and distance as different measurement questions.
• Explain how photodiodes, LDR voltage dividers, PIR modules, ultrasonic sensors, and capacitive sensors produce readings.
• Compute ultrasonic distance from echo time and distinguish it from optical time-of-flight hardware.
• Diagnose reflectance ambiguity, ambient-light interference, temperature drift, and photodiode noise limits.
• Record thresholds with units, geometry, readout chain, and calibration evidence.

43.3 After Light and Proximity Labs

Light and Proximity Sensor Labs walks through practical devices and wiring. Those labs only become reliable when the reading is interpreted through the right physics. A BH1750 lux value, an LDR ADC count, a PIR digital pulse, an HC-SR04 echo time, and a capacitive touch count do not answer the same question.

Use this page when a threshold behaves differently after mounting, target material, sunlight, temperature, or divider orientation changes. The goal is to decide whether the sensor measures light level, reflected intensity, motion, field disturbance, or true distance before turning a number into an automation rule.

43.4 Overview: Photons In, Current Out

Every light and optical-proximity sensor rests on one idea: light carries energy, and certain semiconductors turn that energy into a measurable electrical signal. A photodiode is the workhorse — incoming photons free charge carriers, producing a current proportional to how much light lands on it. That is optical transduction.

Proximity is a related but distinct problem, and there are several very different ways to solve it. You can shine infrared light and measure how much bounces back — simple, but it only reliably tells you something is there. You can measure how long light or sound takes to return — time-of-flight — which gives a true distance. You can also sense field disturbance with a capacitive plate, which is useful for touch and near-field presence but not for long-range distance.

“Proximity” hides three different observables, so inspect Figure 43.1 before interpreting a threshold or distance value. The comparison links each excitation method to the quantity the electronics really measure and the claim that quantity can support.

The Infrared reflection column in Figure 43.1 follows pulsed IR light to Return intensity and then to photodiode current or ADC counts. Its warning, Intensity ≠ distance, limits that path to presence or relative proximity unless target reflectivity and geometry are controlled. Ultrasonic time-of-flight measures Echo round-trip time Δt and uses d = cΔt/2, but temperature, soft targets, dead zone, and multipath remain explicit confounders. Capacitive field disturbance instead observes a capacitance or controller-count change from baseline, supporting touch or near conductive presence—not identity or metric distance. Those three paths explain why the lab questions below ask different things of an LDR, a reflective IR sensor, an echo timer, and a capacitive electrode.

Light-sensing labs often mix all three ideas, so read the sensor value before designing the threshold. An LDR divider gives an ADC count whose meaning depends on the fixed resistor and room lighting. A BH1750 gives calibrated lux over I2C, so thresholds can be written in physical units such as "turn on below 40 lux". A PIR module does not report lux or distance at all; it reports a changing infrared pattern caused by warm motion. An HC-SR04 ultrasonic module reports echo time, so its number becomes distance only after the round-trip calculation.

Infrared sensing also needs a wavelength note. Near-infrared sits just past visible light on the spectrum, at a wavelength short enough to behave like an optical system: emit or observe light and interpret reflection, interruption, or time-of-flight, the way a TV remote or a reflective proximity sensor does. Far-infrared sits at the opposite end, closest to microwave, and is the wavelength ordinary warm objects radiate on their own -- sunlight, a room radiator, a sun-warmed sidewalk -- so far-infrared and thermal imagers observe heat radiation from bodies and surfaces rather than reflected light, and they support presence, hot-spot, or temperature-pattern claims rather than ordinary colour or brightness claims. That split explains an easy-to-miss design fact: a near-infrared proximity sensor must supply its own emitter, because it is reading a reflection, while a far-infrared PIR module needs no emitter at all, because a warm body already radiates the signal it detects -- which is exactly why "passive" is in Passive Infrared. A PIR occupancy sensor sits in that thermal-infrared family but reports change across its zones, not a still person's identity or an absolute temperature map.

That distinction changes the lab question. For ambient light, ask "how bright is the scene?". For IR reflection, ask "did enough light return to prove a nearby target?". For ultrasonic or optical time-of-flight, ask "how long did the emitted pulse take to return?". Once students attach each sensor to the physical quantity it really measures, false conclusions are easier to catch: a black object may look farther to an IR reflector, a sunlit window may saturate a photodiode, and a cold room shifts an ultrasonic distance.

43.5 Transimpedance and ToF Readout

A photodiode's current is often tiny, so a transimpedance amplifier converts it to a usable voltage. An LDR is different: it is a variable resistor, so the normal lab circuit is a voltage divider into an ADC. For ranging, distance comes from an echo time. Keeping those three readout patterns separate prevents the common mistake of treating every light-related sensor as if it returned lux.

Transimpedance amp:  Vout = Iphoto × Rf
LDR divider:         Vadc = Vcc × Rfixed / (Rldr + Rfixed)   (one common orientation)
Time-of-flight:      distance = (speed × time) / 2   (round trip)
  light:      speed = 3.0e8 m/s
ultrasound: speed ≈ 343 m/s at 20 °C

Photodiode -> transimpedance amplifier:
  Iphoto = 1 µA, feedback Rf = 1 MΩ
  Vout = 1e-6 × 1e6 = 1.0 V   (readable by an ADC)

LDR divider, Vcc = 3.3 V, Rfixed = 10 kΩ to ground:
  bright desk: Rldr = 3 kΩ   -> Vadc = 3.3 × 10 / (3 + 10) = 2.54 V
  dim corner:  Rldr = 60 kΩ  -> Vadc = 3.3 × 10 / (60 + 10) = 0.47 V

BH1750 lux threshold:
  reading = 28 lux, rule = turn task light on below 40 lux
  action = ON, because the reading is a calibrated physical unit

Ultrasonic ranging (HC-SR04 style):
  Measured echo time = 5.83 ms
  distance = 343 × 0.00583 / 2 = 1.00 m

Light time-of-flight is far faster:
  1 m round trip = 2 / 3e8 = 6.67 ns
(needs dedicated timing hardware, e.g. a VL53L0X)

In a lab notebook, record the readout chain with the threshold, not just the final threshold. A threshold of 1800 ADC counts means little unless the divider orientation and supply voltage are known; reversing the LDR and fixed resistor reverses whether darkness makes the ADC count rise or fall. A lux threshold is more portable, but still depends on sensor placement, diffuser, and whether the application cares about human-perceived illumination or narrow-band light. An ultrasonic threshold should include temperature if the range boundary is tight: at 0 °C, using 343 m/s overestimates a 2.0 m target by roughly 7 cm because sound is closer to 331 m/s.

The practical pattern is to calibrate with the same geometry the device will use. For a bin-fill sensor, mount the ultrasonic transducer at the final lid height and measure empty, half-full, and full echoes. For an IR proximity sensor, test the real target materials rather than only a white card. For a daylight sensor, collect readings at dawn, midday, and night in the enclosure, then choose a threshold with hysteresis so the output does not chatter at the boundary.

43.6 Light, Reflectance, and Sound

Each optical and proximity method has a characteristic failure mode that separates a reliable reading from a misleading one. The important engineering move is not memorising one "best" sensor; it is matching the failure mode to the environment. A reflective IR sensor can be excellent inside a printer paper path and poor outdoors. Ultrasonic ranging can be stable in a tank headspace and unreliable near soft fabric. A lux sensor can make sensible lighting decisions but says nothing about whether a person is nearby.

The pattern to remember: intensity methods are cheap and answer "is something there?"; timing methods cost more hardware and answer "how far?". Capacitive methods answer a third question, "has a conductive or high-dielectric object disturbed the field near this electrode?". That is why touch buttons work through thin plastic overlays but do not replace an ultrasonic sensor for distance.

The simple capacitor model is C = epsilon * A / d. A capacitive water-level probe changes the effective dielectric epsilon as water replaces air around the electrodes. A MEMS capacitive accelerometer changes the plate spacing d as the proof mass moves. A capacitive slider changes overlapping area A as a finger or wiper moves along the electrode pattern. The formula is not there for decoration; it tells the review which physical variable the installation is actually changing and which variables must stay controlled.

Good firmware treats these effects as part of the measurement, not as afterthoughts. Subtract a dark reading or ambient baseline when the emitter is off, then compare it with the emitter-on reading. Average only after removing obvious outliers, because a single missed echo should not drag a tank level estimate. Add hysteresis around thresholds: for example, turn a light on below 35 lux and off above 45 lux instead of toggling at exactly 40 lux. Store calibration constants with units so a later maintainer knows whether a number is counts, volts, lux, centimetres, or a dimensionless proximity score.

43.7 Compare Two Returns at the Bench

A black carton and a white carton stop at the same point in front of an infrared proximity head. The white surface returns more light, so raw intensity makes it appear nearer even though a ruler says both are 40 cm away. In Figure 43.1, follow the infrared reflection path, ultrasonic echo path, and capacitive field path separately; each labelled route responds to a different physical property.

An ultrasonic echo taking 5.83 ms has travelled to the target and back. Using 343 m/s, the one-way distance is (343\times0.00583/2=0.9998\ \text{m}), or about 1.00 m. Dividing by two is essential because the timer includes both legs. The infrared channel cannot borrow that result: return intensity also changes with colour, angle, emitter current, and dust.

43.8 Release Checklist

Before releasing a light or proximity lab result, confirm the evidence record includes:

Start by the physical quantity measured: lux, ADC count, presence, echo time, capacitance, or distance. Then the readout chain: I2C part, voltage-divider orientation, PIR module output, transimpedance amplifier, or trigger/echo timing. Next the threshold units and mounting geometry used during calibration. After that ambient-light, reflectance, temperature, and crosstalk checks relevant to the sensor type. Finally hysteresis or debounce rules for thresholds that can chatter near the boundary.

43.9 See Also

Start by Light and Proximity Sensor Labs for the hands-on wiring and lab sequence. Then Sensor Circuits and Signal Conditioning for divider, filter, and front-end design. Next Sensor Data Processing for filtering, calibration, validation, and persistence. Finally Sampling, Aliasing, and Anti-Alias Boundaries for acquisition checks before digital filtering.

43.10 Next

Return to Light and Proximity Sensor Labs with the readout model documented, then move to Best Practices & Labs for wiring standards and deployment checks.