Ask HN: How do LCD devices run without any tangible power source? | Hacker News Reader

Ask HN: How do LCD devices run without any tangible power source?

7 pointsPopAlongKid8 comments

I have a scale to weigh myself, I've had it for at least 30 years. The display is liquid crystal display (LCD). This scale has no user-accessible battery, yet it reliably displays my weight every time I step on it. I also have a MFA authentication card, recently acquired from a software vendor which requires MFA for login (TOTP), it is the exact size of every credit card I've ever had, so there is no apparent battery. Yet the LCD display on the card displays every time I press the "button". How is it that these LCD displays can work with no apparent source of electric power?

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Piezo-electric affect generates voltage enough for some lcd applications.

LCD displays without backlight consume almost no current. Both devices likely have a fairly sizeable 3 volt lithium in them, and are highly optimized to reduce "off" leakage current to less than the self-discharge rate of the battery.

The MFA card requires knowledge of the current time, if I understand them correctly... thus there must be a very low power clock chip inside it, along with the CPU, etc.

All of those TOTP credit cards do have batteries in them. Some of these cards use LCDs and some use e-paper.

Battery life is specced for 3-5 years mostly.

The scale is also using some sort of internal battery. It could be a gel type battery or a CR2032 or similar inside. My CR2032 powered scale is going on 12 years with the original battery.

TL;DR: there’s batteries inside.

I guess that must be it. The scale certainly has a plastic box component that could house a battery, but it does not appear to be user-accessible.

I guess I have to abandon my whacky theory that somehow the kinetic energy transferred every time I step on and off the scale was being converted into a tiny amount of electricity by some kind of internal generator.

This. They’ll run out for sure, after a certain number of presses.

there is a gel battery inside the card.

a scale is certainly large enough for a battery to be inside.

Which scale is that exactly?

The brand name is "Thinner", which as of now appears to be a product from Con-Air.

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