Status indicator that doesn't use power?

[brichards42]

I'm working on a little project which will be battery powered and it will have a low battery indicator.  Of course this will be a led of some sort but it made me wonder if there is or ever was some type of indicator made that can be triggered with a one-time pulse and it would mechanically change and retain that state without using the battery until reset by the user (spring-loaded or something like that)? 

Would be much better than draining the batteries to notify of low batteries.

[Fredderic]

There absolutely must be electromechanical "flippy-dot" style indicators…  I'm sure someone will suggest something soon.

In the meantime, a small e-ink display works rather well for displaying information that doesn't change often.

[Alex Eisenhut]

https://www.ckswitches.com/media/1402/fpush.pdf

[Someone]

https://en.wikipedia.org/wiki/Flip-disc_display
Never? miniaturised for handheld applications. As above, e-ink is the major player in that space these days (cheap, multi source, volume applications supporting availability).

[brichards42]

In the meantime, a small e-ink display works rather well for displaying information that doesn't change often.

The project I am working on has a very small mcu with no pins free, but a small e-ink display is something I will remember the next time. Surprising though the ones I saw on digikey seem to need a minimum of 5V.

[brichards42]

I may not be understanding but it looks like this is a actual switch with a mechanical indicator.  That is to say it not something that can be toggled from a mcu?

[Terry Bites]

https://flipdots.com/en/products-services/status-indicators/

or https://www.flamecorp.com/l3harris-technologies for your next MILSPEC project.

If you're mad, you could make a split flap or a drop flap like your pretend digital clockradio had in 1974, the one next to your water bed. Actually you can still buy flappy clock calendars. https://www.google.co.uk/url?sa=i&url=https%3A%2F%2Fwww.redcandy.co.uk%2Fclocks&psig=AOvVaw3rMPTkGYKjnJTkK9ezYdJE&ust=1649248504114000&source=images&cd=vfe&ved=0CAcQjRxqFwoTCMDXicz3_PYCFQAAAAAdAAAAABAH

[inse]

There are also some low power LED flasher circuits that consume very little average current.
Those last for years on a single battery.
Just do a search on the buzzwords or "ewiger Blinker“ if you‘re not afraid of German.

[Someone]

or https://www.flamecorp.com/l3harris-technologies for your next MILSPEC project.

Those rotating trefoil pattern ones scream industrial (bearing such a close resemblance to the ionising radiation trefoil).

[brichards42]

Flipdots looks very interesting but I couldn't find them on digikey or mouser so probably have to buy bulk quantities direct.

I found the web page for "ewiger Blinker" and I will build the circuit and see how it works.


Thanks to everyone for their ideas.

[AndyC_772]

A colleague and I came up with an idea for this which turned out to be patentable:

https://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=6690449&OS=6690449&RS=6690449

[Short version: a battery-shaped polariser applied to an LCD, which requires a small current to keep it NOT visible. Remove the batteries and it goes black!]

By a bizarre coincidence it comes up to its 20 year expiry in just a few weeks, so enjoy...!

[ataradov]

Both eInk and flip dot displays require quite a bit of power when switching. If you are indicating low battery condition, that battery may no longer be able to reliably flip them.

At the same time, modern LEDs light up at single uA currents. Any dead battery would be able to power an LED while not being able to power anything else.

[brichards42]

Both eInk and flip dot displays require quite a bit of power when switching. If you are indicating low battery condition, that battery may no longer be able to reliably flip them.

At the same time, modern LEDs light up at single uA currents. Any dead battery would be able to power an LED while not being able to power anything else.

I had not considered the power requirements for the eink and flipdots, probably not a good idea with a low battery.

Can you give me a example part number for a led that can run at uA?
I see Lumens/Watt as a spec on digikey, maybe that is meaningful for low power?

[ataradov]

I think most modern LEDs will work. Here is an example green LED I use for general purpose status indication on PCBs - https://lcsc.com/product-detail/Light-Emitting-Diodes-LED_Orient-ORH-G36G_C205443.html  I run them at ~5 uA (literal 100kOhm current limiting resistors @ 3.3V), otherwise they are too bright. Blindingly bright.

This is true for a normally lit room, of course. Obviously if you need them to be visible in a direct sunlight, the situation would be different.

You can't really get this info from the datasheet, as vendors specify maximum specs.

[Conrad Hoffman]

That's amazing. I didn't know anything would be visible at that level. My previous thought was that a low batt indicator is a dumb idea. I once had an answering machine that flipped on an LED to warn of low batteries. That quickly drained them, causing them to leak and destroy the device. Brilliant design or stupid design, I'm not sure. As for "any" modern LED, I'd suggest testing it first, and making sure most of the batch was useful at that level if this is a production device.

[RoGeorge]

Any e-Paper (e-Ink) display doesn't need power for retention.
https://en.wikipedia.org/wiki/Electronic_paper

[ataradov]

Any e-Paper (e-Ink) display doesn't need power for retention.

But it needs a ton of power to do the flipping. And dead battery may not be able to deliver that. It is more power efficient to run an LED for days than flip an eInk display.

As far as efficient LEDs go, I would also look at LEDs designed for outdoor use. Those are very efficient and require very little current for normal brightness. I recently added a few of Cree LEDs to the order (https://www.mouser.com/ProductDetail/941-CV94DFCCCYB7D3D3) for no particular reason, they just looked interesting. They become tolerable to look at for an extended time at 0.5 mA and remain very bright down to 20-30 uA.

[Someone]

Any e-Paper (e-Ink) display doesn't need power for retention.

But it needs a ton of power to do the flipping. And dead battery may not be able to deliver that. It is more power efficient to run an LED for days than flip an eInk display.

You are correct in saying there is a crossover point where one is going to last longer, but the OP hasn't fully explained their requirements, their desired duration is undefined. E-ink displays vary, the larger ones needing 10mA or more and seconds to flip the entire screen, but there are low power/smaller displays and/or partial updates which bring those down to more reasonable figures that you can support with a 100uF capacitor.

[Someone]

Both eInk and flip dot displays require quite a bit of power when switching. If you are indicating low battery condition, that battery may no longer be able to reliably flip them.

At the same time, modern LEDs light up at single uA currents. Any dead battery would be able to power an LED while not being able to power anything else.

I had not considered the power requirements for the eink and flipdots, probably not a good idea with a low battery.

Can you give me a example part number for a led that can run at uA?
I see Lumens/Watt as a spec on digikey, maybe that is meaningful for low power?

Think of the mechanical options as needing an energy reserve, rather than a sustained power (as an LED would need).

You can get LEDs specified for low currents, those that arent are usually ok but you might get a batch (or the occasional outlier part) with higher than average leakage currents and then it might not be visible anymore in a simple voltage source + resistive driver.

That's amazing. I didn't know anything would be visible at that level. My previous thought was that a low batt indicator is a dumb idea. I once had an answering machine that flipped on an LED to warn of low batteries. That quickly drained them, causing them to leak and destroy the device. Brilliant design or stupid design, I'm not sure. As for "any" modern LED, I'd suggest testing it first, and making sure most of the batch was useful at that level if this is a production device.

100% needs device testing. Bright enough to notice is also dominated by contrast, an LED sitting in a white panel needs a lot more power to be visible than one in a very dark recess. Lower power again, short blink every n seconds.

[jbeng]

We used large electromechanical flip displays in 7-segment format (6" & 12" tall) at one of my previous jobs (late 80's-early 90's); smaller ones were also available.  Back then, they were made by the Signalex division of Staver Company in Bay Shore, New York, USA.  Another company called Luminator (Texas, IIRC) also made flip-dot displays that were often used in the front windows of busses to indicate their route/destination.

Scoretronics, Inc. in Willowbrook, Illinois, USA makes flip-dots and displays... the thing is, they're quite large.
https://www.scoretronics.com/components/
Alfa-Zeta Ltd. in Poland is a another manufacturer and they make small dot indicators
https://flipdots.com/en/products-services/status-indicators/

[Doctorandus_P]

There are some tubes that have a bar in them somewhat like an old fashioned thermomenter and that bar changes with current put though the device. They used to be used for an indication of the amount of hours an gadget was in use.

Also:
Often it's also not preferable to use batteries until they die, but to just swap them on timed intervals, maybe once each year or so.

[inse]

@jbeng:
Minimum requirements for those miniature indicators: 4,5V 250mA - OK, only for 1.5ms but you will have to be able to supply this.

[razvan784]

From a teardown of an Automatic Electronic Defibrilator I remember seeing such a fail-safe indicator.
It was a classic LCD (same technology as the 7-segment ones) but with only one "segment" covering the whole screen, which would normally be ON, requiring very little power. When a fault is detected or when the battery runs out, the segment is turned off, revealing a big red X painted on the reflective back foil.

[Alex Eisenhut]

Oh guess I misunderstood the requirement.

There are some tubes that have a bar in them somewhat like an old fashioned thermomenter and that bar changes with current put though the device. They used to be used for an indication of the amount of hours an gadget was in use.

Also:
Often it's also not preferable to use batteries until they die, but to just swap them on timed intervals, maybe once each year or so.

https://electronics.stackexchange.com/questions/308735/what-is-this-cartridge-fuse-style-component

[mariush]

Why isn't anyone suggesting a very small lcd display with a few segments? Make it transflexive and you don't even need backlight... and you'd probably only need to pulse it once every few seconds to keep the liquid cristal working.
There's also some LCD displays that have very long retention and only need to be refreshed once a minute or something like that.