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Comment by dragontamer | Hacker News Reader

> LED where controlling current is not feasible

Voltage-controlled current source is a pretty basic OpAmp circuit actually. There's also some tricks to turn some common voltage regulators into current controls. If your "information" is already encoded as current, you can somewhat current-mirror with a very basic BJT circuit (its not the best circuit, but if you don't care about accuracy its fine).

I'd say that current control is _more expensive_ than PWM though. PWM is just software or a 555 timer if you're oldschool, both are cheaper than OpAmps / Voltage Regulators.

Or maybe you mean, "not feasible for the costs people expect", which is probably true. PWM is just so much cheaper in practice.

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It's "feasible" to use dynamic current sources per pixel, and many LED pixel drivers do offer this for dot correction / colour balance / global brightness, but PWM is almost always used for pixel values; it's much easier to achieve the necessary resolution staying in the digital domain, and there's not really any downside. The other big issue with current control is that the simple ways to do dynamic current are linear, so effectively use constant power regardless of pixel state, and also burn a lot of silicon area and might start creating thermal issues in the driver. At high power levels, current regulated switch mode DC-DC drivers start to make sense, but doing that per pixel is definitely not feasible.

I really meant for large 4K (4096x2160) arrays of LED pixels like in this [1].

[1]: https://displaysolutions.samsung.com/led-signage/onyx

When I needed a beeper with a time delay for a fridge door, I looked at retail prices for a dual-555 IC (a 556) and for a dual-opamp IC (ended up going with a 6002) and was surprised to find the opamp was something like three times cheaper, even taking into account the hefty capacitor for the time delay. (Active buzzers wouldn’t run off a 3V cell so I did need a dual IC for the delay and the square-wave generator.) Is this just a retail-specific distortion?

All of the components you mentioned would be sub-cent in large volumes and surface mount parts.

I'd imagine the retail price is a combination of 'what the customer is willing to pay', and 'random'.

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