FAQ
TL;DR: To push an IR reflectance sensor from 2 cm to ~5 cm, use synchronous detection at your 4 kHz modulation and tighten optics; "The signal you are looking for is the difference between the on and off readings." [Elektroda, Olin Lathrop, post #21658959]
Why it matters: This FAQ helps hobbyists and engineers cut ambient-light noise and extend short-range IR detection reliably.
Quick Facts
- Baseline: 4 kHz drive, 5 V LED, 0–2.1 V output over 0–2 cm; target ≥5 cm. [Elektroda, Vinit Apte, post #21658956]
- Best-practice readout: subtract LED-off from LED-on, then low‑pass filter the result. [Elektroda, Olin Lathrop, post #21658959]
- Optics matter: narrow the transmit beam and align the receiver’s field of view. [Elektroda, Olin Lathrop, post #21658957]
- Avoid interference bands: do not center around 40–100 kHz used by many remotes. [Elektroda, Duane Gibbs, post #21658964]
- Visible color ≠ IR reflectivity; some black materials appear bright in IR. [Elektroda, Olin Lathrop, post #21658965]
How do I extend my IR sensor range from 2 cm to about 5 cm?
Increase signal-to-noise ratio. Modulate the LED, read synchronously, and subtract LED-off from LED-on. Add simple low-pass filtering. Improve optics by narrowing the transmit beam and matching the receiver’s view to the same path. Align mechanically and minimize stray reflections. [Elektroda, Olin Lathrop, post #21658957]
What’s the fastest way to cut ambient light noise without complex op-amps?
Use synchronous detection: take a reading with the LED on, another with it off, subtract them, then low-pass the difference. "You only care about the reflected light level, which is what you get by subtracting the two." [Elektroda, Olin Lathrop, post #21658959]
Why isn’t a 50 Hz high‑pass filter enough if I modulate at 4 kHz?
Because your signal lives at the modulation frequency, not at 50 Hz. Filter and detect at the transmit frequency and phase for much tighter noise rejection than a simple low-pass or high-pass around mains noise. [Elektroda, Olin Lathrop, post #21658957]
How do I sample with a microcontroller for synchronous detection?
- Turn the IR LED on; wait a few microseconds for the receiver to settle.
- Read ADC (LED-on). Turn LED off; wait briefly; read ADC (LED-off).
- Subtract (on–off) and low-pass the result over several cycles.
This sequence boosts SNR and rejects ambient drift. [Elektroda, Olin Lathrop, post #21658957]
What is synchronous detection in plain terms?
It’s measuring in lockstep with your transmitter. You know when light is emitted, so you compare LED-on and LED-off readings. The difference isolates only reflected IR from your source and cancels steady ambient light. [Elektroda, Olin Lathrop, post #21658959]
Do optics or mechanical shielding really help range?
Yes. Collimate the transmitter into a narrower beam and restrict the receiver’s field of view to the same path. This increases the proportion of received signal that comes from your emitter, improving range and stability. [Elektroda, Olin Lathrop, post #21658957]
What delay should I use after turning the LED on?
Wait a few microseconds for the receiver and front-end to stabilize, then sample. Keep the same delay each cycle so phase and timing stay consistent for accurate subtraction and filtering. [Elektroda, Olin Lathrop, post #21658957]
Should I add an IR-pass filter or mechanical tube?
Yes. An IR filter over the receiver blocks visible light that causes DC shifts, and a physical tube reduces off-axis ambient light. Both raise effective SNR and measurement repeatability. [Elektroda, Duane Gibbs, post #21658964]
What modulation frequency should I target or avoid?
Pick a clean frequency and design a matching band-pass if needed. Avoid 40–100 kHz, where many consumer IR remotes operate, to reduce false triggers and interference. [Elektroda, Duane Gibbs, post #21658964]
Does a white target always reflect best in IR?
Not necessarily. Materials differ between visible and IR. Some blacks look bright in IR, and some whites look dark. This affects range and threshold settings, so test your real targets. [Elektroda, Duane Gibbs, post #21658964]
Edge case: Why didn’t black electrical tape block my IR interrupter?
Certain black electrical tapes are surprisingly IR‑transmissive. Even three layers may fail to trip an IR photo‑interrupter, causing misreads and false clears. Plan fixtures and masks with verified IR‑opaque materials. [Elektroda, Olin Lathrop, post #21658965]
Will using glass as a window impact IR sensing?
Yes. Glass that looks clear in visible can attenuate IR strongly, changing your signal. If you must cover the sensor, choose materials characterized for your IR wavelength and test the stack. [Elektroda, Cody Miller, post #21658966]
Is ultrasonic distance sensing a better alternative around 5 cm?
Ultrasonic modules often provide longer usable range and less optical interference. For 5 cm, they can be robust, though beamwidth and surface angle still matter. Consider them if ambient light is severe. [Elektroda, Khaba Bulu, post #21658967]
My 4 kHz system only reaches ~2 cm. What concrete tweaks help first?
Tighten alignment and add a simple shroud, implement on–off subtraction, and average several cycles. These changes often yield immediate range gains without new hardware. [Elektroda, Olin Lathrop, post #21658957]
Should I abandon the 50 Hz high‑pass I built?
Repurpose your filtering around the transmit frequency instead. The 50 Hz filter won’t track your 4 kHz signal and leaves wide noise paths. Synchronous methods are more selective. [Elektroda, Olin Lathrop, post #21658957]
