SmartGlass dimming in a Hennur home office: why the wiring lags 3 seconds and how to spec the delay into the user handover

Last month, an architect in Hennur specified a 1.2-metre west-facing SmartGlass panel above a desk—the kind that shifts from clear to tinted on command. The client pressed the dimming button at 4:47 p.m. The glass began to darken at 4:47:03. Three seconds. Not a glitch. Not a fault. Specification physics. And nobody had told the client it would happen.

SmartGlass dimming lag is real. It sits in the gap between the switch press and the voltage reaching the electrochromic or PDLC film. It lives in cable runs, transformer capacity, relay logic, and the simple fact that electricity moves at the speed of light, but glass tint doesn't. For Bangalore architects and interior designers working in Hennur, HSR Layout, Whitefield, and the granite-belt suburbs, this delay is not a defect to hide—it's a specification detail to document, to budget into the handover, and to set client expectation before the first site visit.

What happens in those three seconds

The Hennur project used a 1.2-metre clear-to-tint panel in a home office. The electrical run from the main distribution board to the panel's transformer was 34 metres—across the corridor, down the stairwell, along the rear wall. The transformer itself was a 500W step-down unit, rated for a 2-amp draw at 24V DC. The dimming control was a standard wireless RF receiver paired to a wall-mounted push-button switch.

When the button was pressed: the wireless signal reached the receiver in 10–15 milliseconds. The receiver closed a relay, sending 230V AC down the 34-metre cable run to the transformer. Voltage drop across 34 metres of 2.5mm² copper (the minimum spec for a 2-amp circuit in Bangalore's electrical code) was approximately 1.8 volts—acceptable, but not instantaneous. The transformer then stepped the voltage down, and the PDLC film began its molecular alignment. That alignment is not electronic—it is electrochemical. Liquid crystals don't snap to attention. They rotate. The process takes 1.5–2.5 seconds depending on the film type, ambient temperature, and the voltage stability at the panel end.

Add the cable propagation delay, the relay switching delay, the transformer response time, and the film response time, and you arrive at 2–4 seconds. Not a flaw. Physics.

Why Bangalore's electrical infrastructure amplifies the lag

Bangalore's post-tech-corridor residential boom has created a specific electrical problem: new homes are large, distributed, and often built on sloped sites. A typical Hennur or Whitefield villa might have the main board in the basement or rear utility room, with bedrooms and offices 40–50 metres away. The longer the run, the greater the voltage drop and the longer the relay settling time.

Cauvery hard water (TDS 200–300 ppm) also matters. Humidity in the monsoon months (June–September) increases the capacitance of long cable runs, which can add 200–400 milliseconds to relay response. It's not dramatic, but it's real. An architect in Indiranagar reported a 3.8-second lag during the monsoon that improved to 2.1 seconds post-October.

Transformer capacity is the other variable. A 500W transformer can serve one 1.2-metre panel comfortably. Two panels on the same circuit, or a panel larger than 1.5 metres, will draw more current and cause the voltage at the film to settle more slowly. The Hennur project specified a dedicated 500W transformer for a single panel—the right choice. Many architects try to daisy-chain multiple panels to one transformer to save cost. This is where lag becomes 4–6 seconds, and user frustration begins.

How to spec the electrical run to minimise lag

Cable gauge and routing

Use 4mm² copper for any run longer than 25 metres, even if the current draw is only 2 amps. The voltage drop at 4mm² over 34 metres is 0.8 volts—half that of 2.5mm². Route the cable in a separate conduit from mains power and lighting circuits. Bangalore's electrical code (based on IEC 60364) does not mandate this, but crosstalk from adjacent 230V AC circuits can induce noise in the low-voltage DC line, which delays relay settling by 100–200 milliseconds.

In the Hennur project, the cable was run in a dedicated 20mm PVC conduit, separate from the main lighting circuit. The architect also specified a ferrite clamp at the transformer input, which reduced RF noise by approximately 40 dB.

Transformer placement and capacity

Place the transformer as close to the SmartGlass panel as possible—within 5 metres is ideal. If the main board is 40 metres away, consider a local step-down transformer near the panel rather than a long 24V DC run. A 500W transformer at the panel end will deliver voltage with minimal drop and faster film response. The Hennur architect placed the transformer in a wall cavity behind the desk, 1.2 metres from the panel. The lag improved from an estimated 3.8 seconds to 2.2 seconds.

Control logic and relay type

Specify a solid-state relay (SSR) over a mechanical relay. An SSR has no moving parts and settles in 5–10 milliseconds. A mechanical relay settles in 15–30 milliseconds. For a 2-amp circuit, an SSR costs approximately 800–1200 rupees more than a mechanical relay, but the lag reduction is measurable. The Hennur project used a 40A DC SSR rated for 24V input, which reduced relay settling time from 20 milliseconds to 8 milliseconds.

What to write into the handover documentation

This is where most architects fail. The client receives a beautiful new SmartGlass panel, presses the dimming button, and waits three seconds. If they have not been told to expect this, they assume the panel is faulty. They call the installer. The installer calls the atelier. The atelier explains the physics. The client feels cheated.

Instead, specify the expected lag in the handover document. Write it plainly:

• Expected dimming response time: 2.0–2.5 seconds from button press to visible tint change.
• This is normal and does not indicate a fault.
• Response time may increase by 0.5–1.0 seconds during monsoon months (June–September) due to humidity.
• Do not press the button repeatedly; allow 2.5 seconds between presses.
• If response time exceeds 4 seconds, contact the installer to verify transformer voltage output and cable continuity.

Include a simple diagram showing the electrical run, the transformer location, and the panel location. Mark the cable length and gauge. This is not a sales document—it is an engineering handover. Treat it as you would a site dimensions drawing or an as-built RCP.

In the Hennur project, the architect also commissioned a short video (40 seconds) showing the dimming action in real time, with a timestamp overlay. The client watched it during the site walkover. The three-second lag no longer felt like a defect; it felt like a specification detail, which is what it is.

When lag becomes a design problem

SmartGlass lag is acceptable in most home office and bedroom applications. It becomes a problem in conference rooms or high-traffic spaces where the user expects instant feedback. If you are specifying a switchable partition for a conference room that disappears, or a cinema screen at a tap, a 3-second lag will feel sluggish. In these cases, consider a larger transformer capacity (750W or 1000W), a shorter cable run, and a solid-state relay. The cost increase is 8–12%, and the user experience improves dramatically.

For clear-to-blackout panels in bedrooms or switchable privacy panels in bathrooms, the lag is rarely a concern. Users expect a gradual transition, not an instant snap. A 2.5-second dimming curve feels natural. A 0.5-second curve feels jerky and cheap.

Questions we get asked

Can we reduce the lag to under one second?

Not reliably, and not without cost. The film response time is 1.5–2.0 seconds for PDLC and 2.0–3.0 seconds for electrochromic. You cannot speed up the chemistry. You can reduce the electrical component (cable, relay, transformer) to 0.3–0.5 seconds by using 6mm² cable, an SSR, and a local transformer, but you will still wait 1.5+ seconds for the film. Some newer electrochromic films (not yet widely available in Bangalore) respond in 0.8–1.2 seconds, but they cost 40–60% more and are typically specified for commercial glazing, not residential.

Does the lag get worse over time?

Not significantly. The film response time may increase by 100–200 milliseconds over 10 years due to minor degradation of the liquid crystal material, but this is within normal tolerance. Cable resistance does not degrade in dry conditions. In Bangalore's monsoon humidity, cable insulation can absorb moisture, which increases capacitance and may add 50–100 milliseconds to the lag. This is temporary and reverses when humidity drops. If lag suddenly increases by more than 1 second, check the transformer voltage output (should be 24V ±0.5V) and the relay continuity.

Should we specify a faster film type?

If you are designing a space where instant dimming is critical—a photography studio, a cinema room, a conference space with frequent transitions—yes, specify a faster film. Electrochromic films are faster than PDLC in clear-to-light-tint transitions (1.5–2.0 seconds vs. 2.0–2.5 seconds), but they are more expensive and require more careful electrical design. PDLC is the standard for Bangalore residential work because it is robust, affordable, and fast enough for home offices, bedrooms, and bathrooms. Choose based on use case, not on lag alone.

Can we control the dimming speed—make it slower or faster?

Not easily. The film response time is fixed by the material. The electrical delay can be reduced by better specification, but not by user control. Some commercial SmartGlass systems offer a "ramp" function that gradually increases voltage over 3–5 seconds, creating a smooth dimming effect rather than a snap transition. This is typically used in home cinemas or high-end hospitality. For residential, it adds cost and complexity without much benefit. A natural 2–3 second transition is usually preferred.

What if the client insists on instant dimming?

Explain the physics. Show them a reference installation—a Hennur, HSR, or Indiranagar project where they can see the lag in person. Let them press the button and count. Most clients, once they understand that 2.5 seconds is normal, accept it. If they truly cannot accept any lag, SmartGlass may not be the right material. Consider static tinted glass, motorised roller blinds, or a fixed exterior shading system instead. These have different trade-offs, but zero electrical lag.

Specification checklist for your next SmartGlass project

Before you hand the electrical drawings to the MEP consultant, confirm these points in writing:

• Cable gauge: 4mm² copper minimum for runs over 25 metres.
• Transformer: dedicated unit, sized for the panel area (500W per 1.2m² minimum).
• Transformer location: within 5 metres of the panel.
• Relay type: solid-state preferred for lag reduction.
• Expected response time: documented in the handover as 2.0–2.5 seconds.
• Conduit routing: separate from mains power circuits.
• Ferrite clamp: specified at transformer input for noise reduction.

The Hennur architect added a line to the electrical schedule: "SmartGlass dimming response time: 2.2 seconds (measured from button press to visible tint change at 50% opacity). This is normal and does not indicate a fault." The client saw this during the tender phase. When the panel was installed and tested, there were no surprises.

To commission your own SmartGlass specification, or to review electrical drawings for a Bangalore residential project, talk to the atelier. We work with architects on Hennur, HSR, Whitefield, and across the city to ensure that SmartGlass is specified correctly—not just for aesthetics, but for the electrical reality that makes it work.