How to get longest battery life

[dbemowsk]

@tbowmo A diode typically has a 0.7v drop. The diode that I am talking about is the one on the PIR sensor. The specs for the PIR say that the working voltage is DC 4.5-20V, but I am sure that they are basing that on the fact that it has a 3.3v regulator on it and don't really give the specs for if the regulator (and diode) are removed. I guess I will just have to hook up my bench supply and do some testing to see how low the voltage can get before it stops functioning with the regulator and diode removed.

[tbowmo]

@dbemowsk

the voltage drop across a diode varies with the technology.. Schottky diodes only have 0.2V drop, while germanium has 0.3V, and silicon has 0.7V.

the 0.5V was just an example.. But the message was, that you have to take the diode drop into account, if you have it inline with the supply, as a reverse polarity protection.. In my example (with 0.5V diode drop) then if you supply your circuit with 2V, then the circuitry after the diode will get 1.5V, which is below the 1.8V minimum operating voltage.

And by removing the diode on the PIR sensor (if it's in the supply line), you can achieve a lower operating voltage.

[Heizelmann]

Here are some measured values (not very precise):

The unmodified HC-SR50: Supply 5.0V, Standby 0.05mA, Tripped: 0.19mA

Regulator and diode removed: Supply 3.0V, Standby 0.03mA, Tripped 0.15mA
Regulator and diode removed: Supply 2.4V, Standby 0.02mA, Tripped 0.11mA

Below 2.4 V the Sensor doesn't work reliable.
The voltage drop of the diode depends on the current and is minimum 0.5V.
So if you would like to operate with 3 AA-Batteries it is recommended to remove/shortcut the diode.

[scalz]

@dbemowsk for an ultra low drop voltage, a ridiculous drop voltage, and regarding reverse polarity protection, at the node level, i would use a P mosfet, a lot better than a diode even a schottky, for low power nodes..
i do this for my nodes, works well ;)

Simple: connect GND to your Gate, VBAT to your Source and your 3VCC to the Drain.
When batt is well connected, VBAT normally flows. But when reversed, you have no GND connected to your board.

You can think this acting like a resistor then, so with a low Rdson for your mosfet, and U=R.I, you can easily calc this micro volt drop voltage !
About the power consumption of this, it's also ridiculous, as this power consumption does matter in low power "sleep" mode, in uA, power loss is negligeable.

Enjoy :)

about the min voltage of this PIR module, if i remember the onboard controller ic is given for 3v min. lucky it can go to 2.4v.

[NeverDie]

Here's a really good youtube video which elucidates what scalz just said:
How to protect circuits from reversed voltage polarity! – 06:46
— Afrotechmods

@scalz Any suggestions as to which p-channel mosfet to pick? Have you any favorites or found any that stand out as just generally better than all the rest at these lower voltages and currents?

[dbemowsk]

So I thought since the PIRs that I have say for the chip that the voltage minimum is 3 volts. If I am going to make sensors that run on 2 AA batteries, once the voltage drops a bit the PIR will be useless, thus decreasing the usable battery life for that sensor even though the pro mini and the radio will run at a significantly lower voltage. What about using something like this to step up the voltage just for the PIR, but leaving the radio and arduino directly connected to the battery? This one will work down to 0.8 volts.
Step up booster 1
Or this one which works down to 1.8 volts which is about what the drop off point of the radio and pro mini is. Step up booster 2
I figure the current draw from the PIR would be low enough at 65ma tripped, and a quiscent current of 50ua for the second step up module (can't find quiscent current for the first one) that it shouldn't be too much of a drain on the battery, correct?

[NeverDie]

Heizelman says (above) that the sensor works on voltages down to 2.4v. So, use two 1.5v lithium batteries. They have a flatter discharge curve than alkaline batteries.

[Nca78]

@dbemowsk you don't want to use a step up/step down to run a pir, these generate a lot of noise and can be trouble for radio transmission. With a PIR which is much more sensible it is not a viable option. The only thing you can use to change voltage is a voltage regulator which is less efficient when you have a big voltage drop, but will reduce noise instead of increasing it.
But the best option is I think as NeverDie says, use 2AA with lithium technology, the voltage will stay high enough to run the PIR for most of the capacity of the batteries.

I just made a test with 2 nimh AAA and the SR501 runs reliably at 2.7V after removing diode and regulator, but my measurements are higher:

• 36µA in idle mode
• 190µA when triggered.

I did a test with another one, from another supplier (it had different diode an regulator).

• 51µA in idle mode
• 130µA when triggered.

Too high for battery power imho :(

[dbemowsk]

@NeverDie Being that the specs say 3v at the low end, makes me wonder how reliable it is down at 2.4v. That is 0.6v lower than it's rating.

[Heizelmann]

@Nca78
Take a battery lifetimecalculator, e.g. like this
and you will see that battery life time is sufficient. In my opinion more than 1 year is enough.

[scalz]

the more lifetime, the less maintenance :)

@NeverDie
That's the beauty of mosfets, and Pmosfets :)
i have not really a favorite for this, actually i use what i have at hand. That can be Vishay's (i've multiple ref like si2323 for instance), or cheap ali CJ2305 also work ok etc.

[Nca78]

@Heizelmann said:

@Nca78
Take a battery lifetimecalculator, e.g. like this
and you will see that battery life time is sufficient. In my opinion more than 1 year is enough.

I like to use CR2032 batteries :P
And when you use the calculator, you have to adapt the really available capacity to stay over a minimum voltage that will keep the sensor running reliably. If sensors can go down to 2.5 - 2.4V and still run reliably with enough range and sensibility, then with some agressive power management yes you can probably survive about one year with 2AAA.

@NeverDie said:

If it could be done cheaply enough, maybe this would be a good use-case for energy harvesting. If the energy harvesting were expensive, though, then it would be cheaper to just use a more energy efficient PIR. So far I've been quite surprised at just how expensive the best energy harvesting chips seem to be.

I will give it a try "the cheap way" :)
5.5V solar panel, small lipo taken from a 2$ mp3 player and CN3063 to manage the charge.
I will need good lighting to be able to charge but I can put the panel in the corner of a window and wires to the sensor at the corner of the window/ceiling which is a good position for a PIR.

[korttoma]

@Nca78 said:

I will give it a try "the cheap way" :)
5.5V solar panel, small lipo taken from a 2$ mp3 player and CN3063 to manage the charge.

just make sure you share your results ;)

[Nca78]

@korttoma said:

just make sure you share your results ;)

I will of course, but at the moment it's a bit early I just prepared 2 PIRs, 2 panels and soldered CN3063 and MCP73831 on adapter boards. I will compare the two, CN3063 is supposed to be better at this task but you never know...

[dbemowsk]

What about using 2 of these in parallel? That would give 5200 mAh at 3.7v
Lithium ion rechargeable

[Nca78]

@dbemowsk I would put only one. Capacity is already very good and you won't gain battery life by using 2 cells as they self-discharge in around one year...
And if you buy those, just throw away the charger. It's made with very low quality charging ICs that might destroy the batteries...

[NeverDie]

@dbemowsk said:

What about using 2 of these in parallel? That would give 5200 mAh at 3.7v
Lithium ion rechargeable

It seems to defeat the purpose of buying a $1 PIR if it takes $16 to make it work in your use case. For instance, the last time I checked, you could buy a really nice, small, highly integrated Panasonic PIR with about 1ua standby current for around $19.

Of course, it hardly matters either way if you're making just one. But if you want to make a lot of them (and I don't know that you do), maybe it becomes a consideration.

[NeverDie]

@dbemowsk said:

So I thought since the PIRs that I have say for the chip that the voltage minimum is 3 volts. If I am going to make sensors that run on 2 AA batteries, once the voltage drops a bit the PIR will be useless, thus decreasing the usable battery life for that sensor even though the pro mini and the radio will run at a significantly lower voltage. What about using something like this to step up the voltage just for the PIR, but leaving the radio and arduino directly connected to the battery? This one will work down to 0.8 volts.
Step up booster 1
Or this one which works down to 1.8 volts which is about what the drop off point of the radio and pro mini is. Step up booster 2
I figure the current draw from the PIR would be low enough at 65ma tripped, and a quiscent current of 50ua for the second step up module (can't find quiscent current for the first one) that it shouldn't be too much of a drain on the battery, correct?

Anyhow, to answer your question, I have a hunch those voltage converters would drain your battery even faster for a number of reasons:

1. Inefficiency introduced by the up-converter itself. Purely guessing, but it might be only 30-60% efficient at converting lower voltages and currents.
2. It takes a lot more low voltage current to produce a higher voltage current. Ultimately, your battery is limited by the amount of mah in it, and so those higher currents will drain it faster.
3. The quiescent current you already mentioned.

#3 could be managed by using a storage capacitor and running the up-converter only intermittently, as needed, to recharge the capacitor. For instance, you could use a voltage detector to decide when to turn it on (for instance, maybe http://www.digikey.com/product-detail/en/rohm-semiconductor/BD49K25G-TL/BD49K25G-TLCT-ND/3693402 , which itself draws less than 1ua of current while monitoring the voltage). However, I don't see an easy way around #1 and #2, except maybe this: run on battery power only (i.e. no boosting) until your batteries are down to around 2.4-2.5v, and only then turn on your booster circuit. That would, in fact, give you at least some extra run-time versus just giving up and replacing the batteries after they became drained down to 2.4-2-5v. You might also want to boost to a lower voltage than 3.3v, as that would mitigate #2.

Maybe others here also have suggestions on how you might approach it.

[dbemowsk]

@NeverDie I was just using that link as an example. Here is a 10 pack for $9.99 US. That's $1 per battery. And, these boast that they can supply 5800 mAh. Not sure if I believe that, but even if it could source half of that, it should last a while. Also, being rechargeable I wouldn't have to keep buying batteries.

[NeverDie]

I'd wadger there's 0.0000% chance of them being 5400mah. The best that I'm aware of are the one's made by panasonic, and those are reputedly 3400mah.