Adding to a circuit the ability to "remember" the last chosen settings when turned off or disconnected

Question

If you could please bear with me: I'm just starting out in electronics and I've had an idea for a nice experimental mod that will also be useful.

I've recently bought a pretty cool little 8 x 8 LED spectrum analyzer off eBay:

And I'm currently building a container for it. I'm going to add a couple of mods. Among them, an ON/OFF switch, so I can get rid of the USB connection and have it permanently connected, but only ON whenever I really need it.

The thing is, this little device has a couple of configurable settings, none of which are, by default, the ones I prefer. So after installing the ON/OFF toggle, every time I toggle the device off, I'll lose my settings, and have to reconfigure them when toggling it back on... which is of course a hassle, because this is meant to be a little decorative plus when I turn on my music and relax, not a task to complete every single time.

So I've been researching what would be the best (slash-maybe-not-the-most-complex) way of adding the possibility for the device to remember its settings even when disconnected to power (or powered off).

I've read a couple of forums and articles and have come across the terms latching relay and slow-discharge capacitor... but I'm still mostly lost. I'm even wondering if just adding a coin cell at some point will do the trick but I'm almost sure it can't be that easy!

So if anyone could put me on the right track, in other words tell me, based on this specific project, what's my best bet, that'd be GREAT! Because right now, after a couple of days of research, I've turned out nothing, at least nothing I can make any sense of.

Thank you in advance!

P.S. Or... is there a way to reconfigure the WD1088S chip?

P.S.S. I've just had another idea... it might be a bad one, but I'm not sure if it won't actually work OK. This device works with 3-5V and has a "static working current [of] less than 3mA". So what if I attach it to a 3.6V 1240 mAh camera battery and have the charger connected to the battery with the ON/OFF toggle in between. That way, at least, I won't be using the electrical current while the device is not in use. Yes, the device won't really be OFF, but as long as it's kept idle, it should have enough power to not turn OFF during about 12 days. I can even use it with the battery (it should work about a day, with an working current of 30mA). I know it's not an elegant solution, more like a workaround... But it's the only thing I can think of. What I'm afraid is I'll be contributing to shortening the lifespan of the whole thing... Anyway, I'm out to learn, so what do you think of this idea?

Edit: I found the PCB schematic and the IC datasheet. Perhaps that can help...

Answer 1

Assuming it doesn't display anything when then is no audio input, then leave the circuitry on all the time and have a SPDT switch (or DPDT if stereo) that either passes through the audio, or shorts out the input so nothing is displayed. That becomes your on/off switch.

Answer 2

The basic idea of adding a battery to keep memory from being lost is an OK idea. But you have to look into various potential problems.

Is the full range of battery voltage acceptable (at first glance, based on your description, it sounds like it is).

Is the switch connected and of such a design that it disconnects the battery before it connects the external power source? I think the wiring is OK (based on your description) but is the switch a break before make type? You don't want to short the battery to the power supply momentarily while switching. Some switches DO temporarily short all three terminals together during switching. That would not be ideal. There would be some high currents flowing every time you switch. It could lead to long-term reliability problems with the switch or the power supply. It might even damage the battery.

Finally, if you do use a break-before-make type of switch, is there enough capacitance on the board to hold up the input voltage during the transition from one power source to the other? Since it only uses 3mA, you don't need a lot of capacitance, but it would be nice to have at least 100 uF on there. There is no way for me to know how much might already be on the board. You could just add 100uF to be safe. If that black cylindrical component is an electrolytic capacitor, then you probably are OK on capacitance. Otherwise I would advise adding a 100uF capacitor. Probably aluminum electrolytic will be most practical.

Have fun and keep experimenting!