But that just keeps power flowing from the battery to the hub and prevents backlow into the batteries. It won't shut the flow from the batteries off.
Also, how are you dealing with the fact that you'll have two voltages? Assuming you have 4 AA's in series, that will be 6v. The wall will deliver 5.
This circuit is going to use the batteries while using the power from the wall-wart. The batteries will be dead long before you lose power and need the backup. It will be a matter of hours. Because there is nothing shutting the flow off from the batteries while power is being supplied from the wall-wart. Why will it choose to take it from the wall-wart? Because you want it to?
The red line and the blue line will both be supplying power at the same time. Why would it choose to only use the red line instead of using both? In fact, since the blue line has the higher voltage, at first, it's going to use MORE power from the batteries than the wall.
You need a PNP transistor in there to shut the flow off from the batteries when the wall is powered on. It should go in between the diodes and the base should go to the line supplied by the wall.
That is why there are two diodes in series with the batteries and I mentioned the actual circuit may need a third. Remember this is the "simple(st)" circuit I can come up with.
Besides with an additional switch (PNP or whatever) there could be a delay causing a dip in power that might effect the HUB
Upon further inspection, it looks like those are Schottky diodes in the diagram. Those should minimize the voltage drop. Perhaps use regular diodes for the battery voltage drop, and the Schottky for the power supply? Just thinking out loud....
Perhaps use regular diodes for the battery voltage drop, and the Schottky for the power supply?
Good thought
I had considered a 1N400x diode and was on the fence about using 2 different diodes or multiple schottky's. You could go either way. If you had parts laying around the 1N400x would do the trick but if you have to purchase these parts (this schottky is a ultra low drop device) maybe more of the same is better.
I'm working on one that will power off the hub after is shuts down. Which means it will power up automatically. I'm trying to stay with simple parts even though a small µP could easily do the trick. I think if anyone wants to try and build one a µP could easily put them off.
Still I'm really happy with the AA Lithium's for simplicity.
I also like simple and cheap things.
I did for a specific simple project a very simple "UPS" (Ryan, I know, it's not technically an UPS )
I took a 12V car battery, used but in good shape (plenty of amps), connected to a basic trickle charger plugged on the wall. An electronic device was connected to the battery, so it was in fact not connected to the main power, just connected on a big buffer.
A small 12v to usb converter and, voilà !
In case of power failure (Quebec here...), the battery is always fully charged (because of the trickle charger) and can power my devices for weeks (because it's a car battery).
No electronic (except the regular trickle charger and the converter) and nothing to do. If you plug a HE hub on this unit (and other small electronic appliances), it will last "forever" without shutting down.
In fact, I use this kind of things for almost important in case of (power failures):
small 12V led lamps in my garage (frustrated to have no lights when power outages), my sump pump and controller, a small Li-ion battery bank with 12V/5V outputs, etc.
All based on [bad] experiences
I remember a flood years ago in my son's house and his big sump pump (120V AC) was not running because the power was down, because of the flood... And the water stayed too long just because of this stupid setup...
I have in my home a 12V Bilge 4700 GPH pump and it can remove a lot of water in no time... whatever the main power status.
Some years ago I purchased a portable generator. Its good for long outages but too much effort to setup for short ones.
Funny thing is, I bought it during a winter storm when the power company said the power would be returned in 3 days. I went to Home Depot, they had already sold out of the smaller ones so I ended up purchasing a 5kW unit. By the time I got home the power was back on!
I was thinking of doing something like this recently, not to power the hub but to keep repeaters alive in the event of an outage or if main power has to be turned off for whatever reason. Especially xiaomi sensors which may fall off the mesh if the connection is interrupted.
I was thinking of doing this for 1 or 2 of mine as well. My concern was with regards to sensors and HSM. If the power fails and a repeater is not available, HSM is pretty much useless. It won't be able to receive the motion or contact sensors signals. I think this would best be accomplished for a zigbee mesh using Xbee's since they can be powered off 3.3vDC and there are inexpensive OTS systems you can buy for UPS systems for those.
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