Pir AS 312 with 2 rechargeable AAA battery. Boost needed?



  • Hi,

    according to this datasheet this PIR requires an input voltage of 2.7 - 3.3V, so if powered with 2xAAA (or AA) rechargeable NiMH batteries (2.4V) it will run out of spec.

    Someone has tested it with this voltage?

    If it does not working well, I have found this ME218833C boost converter that seems adequate to the job:

    • Low quiescent current (13μA)

    • Chip Enable Pin to enable and disable the circuit on demand



  • @franz-unix i don't know how you will handle the pir in your software but generally the pir needs to be powered all the time so no need for shutdown mode in your boost converter. The other thing is that pir doesn't linke noisy power supply, and dc converters are rather noisy especially in low current area. I would recommend to use higher voltage and ldo voltage regulator. You can find some ldos with very low quiescent current. You can use li ion rechargable battery and 3v ldo.



  • Yes, the booster will be always on to power the PIR when the board is powered by rechargeable batteries (2.4V), but the idea is that if you use two disposable batteries (3.0V) or external power (micro usb port, ICSP or FTDI), the MCU reads the VCC voltage and then shuts down the booster and the PIR is powered directly.

    In other words the booster will be activated by the firmware only if VCC is below 2.7V.

    About the noise question... yes this could be a problem, but the manufacturer declare the booster as "Low ripple and Low noise", and in my schematic (see below) I have inserted a dedicated capacitor for the PIR, so I hope this is enough.

    0_1562786363239_output-1.rotated.jpg

    I prefer to stay with AAA batteries because I want to keep the global dimension of the node as low as possible and with this type of batteries I can power the radio module and the MCU directly from battery, saving some mA.



  • @franz-unix if the ME2188 was really low noise and low ripple it would be good idea to use it. Unfortunetely there is nothing about it in the datasheet axcept for manufacturer declaration ... no data to support this declarations so it sounds rather not convincing. But if You have time to test it go for it 🙂


  • Hardware Contributor

    @franz-unix said in Pir AS 312 with 2 rechargeable AAA battery. Boost needed?:

    About the noise question... yes this could be a problem, but the manufacturer declare the booster as "Low ripple and Low noise", and in my schematic (see below) I have inserted a dedicated capacitor for the PIR, so I hope this is enough.

    It's a PFM step up to be efficient with low loads, meaning the frequency of the noise will change a lot and it is much harder to filter. You might want to include on your PCB an option to go through a ldo (that you can of course bypass), so if the step up is too noisy you can use one with a bit higher output voltage and use the ldo to filter the output a bit more.

    But I agree with @rozpruwacz a li-ion battery with a ldo like XC6206 sounds like a way more simple option and it's also easy to make it more compact than with 2 AA/AAA.



  • @rozpruwacz The ME2188 is quite inexpensive (€0.0574 ) so I will test it for sure. It is available also with different Vout voltages, so if it does its job could be an interesting option to power other type of sensors. This article is about the ME2108, which seems to be the predecessor of the ME2188 and the author says that the output voltage is clean and stable (tested with oscilloscope).

    @Nca78 This XC6206 seems a good ldo vreg, thank you for the info. About the Li-ion battery, it seems to be a general agreement (see this topic for example) about the fact that the best option to power a node is with AA or AAA batteries. In fact one of the advantages is that you don't need to step up or down the voltage to power the radio and the MCU.

    Yesterday I have had another idea for the circuit: the substitution of the manual jumper JP3 with 3 mosfet (Q3-Q4-Q5).
    In this way you can completely exclude the booster circuit when not necessary (Vcc >= 2.7V) without manual adjustment of the jumper. Just put you batteries (disposable or rechargeable) in and... boom, the MCU will automatically do the job.

    0_1562874186490_output1-1.rotated.jpg



  • @franz-unix said in Pir AS 312 with 2 rechargeable AAA battery. Boost needed?:

    About the Li-ion battery, it seems to be a general agreement (see this topic for example) about the fact that the best option to power a node is with AA or AAA batteries. In fact one of the advantages is that you don't need to step up or down the voltage to power the radio and the MCU.

    There is no one "best option". The power supply heavily depends on the project requirements. The PIR sensor requires noiseless voltage between 2.7 and 3.3v. This makes use of 2xAA problematic. In my opinion for such a project higher voltage supply with low quiescent current ldo is the best option. And it happens that li-ion battery with its voltage range and 3v ldo makes them perfect match. At least this is how I did it in my home and it works nice ( https://www.openhardware.io/view/610/MySXMotion). I went even further in my project and there is two ldos, one for pir and one for mcu + rf module as those two components creates large spikes in current flow as they work making supply voltage noisy.

    PS. The other benefit of rechargable batteries is that they are rechargable 🙂 so you don't need to remember to buy batteries when you go out of them.



  • If you have room for a AA, an LiFePo4 battery is right in the sweet spot.
    I have seen LiFePo4 AAA batteries, but they are really hard to find.



  • @nagelc The LiFePo4 batteries will be perfect (AA and AAA), but I arrived at your same conclusion: a little bit to exotic respect to standard alkaline or NiMH batteries.

    @rozpruwacz Nice board, is similar to what I want, but unfortunately not exactly the same. I prefer to have one single board to simplify the build and external case design (3d-printable).

    Thanks to your suggestion, I have modified a little bit the circuit:

    0_1563104742468_output-1.rotated.jpg

    Some consideration and board features:

    • Added footprint for CR2032 coin cell and dedicated 100 uF capacitor to prevent problem related to the low discharge rate of the battery.
    • Added a flyback diode across the inductor to discharge voltage spikes when the circuit is disconnected (mosfet Q3 - Pin D4). Not sure if it is strictly necessary... but just in case!
    • Multiple power options: 2xAAA batteries, 1xCR2032, micro USB (phone charger), J6 pin header. When powered from an external source through USB port or J6, the voltage can be between 4 and 12V. When you power the board through FTDI or ICSP make sure that the voltage is 3.3V, otherwise the radio module will be damaged!
    • Mosfet Q1 offers a reverse polarity protection to the board if the batteries are inserted in the wrong way. It cut off also the batteries compartment when an external power source is present, so you don't have to remove the batteries while powering the board via FTDI, ICSP, USB or J6
    • When VCC is below 2.7V (AAA rechargeable batteries) the boost circuit is activated by the MCU (Pin D4). When VCC is above 2.7V (disposable AAA batteries or CR2032) the circuit is disabled and the PIR is powered directly form VCC (see Q3-Q4-Q5)
    • To increase the battery life, the photoresistor is powered through PIN D5, only when the node is awake. During sleep D5 will be set to LOW, so no current waste.
    • Two momentary switch: 1 for MCU reset and 1 to force the node to wake up
    • Motion led (D3) and low battery / external powerl ed (D5) configurable in the firmware, plus radio activity LEDs
    • Header for Si7021 breakout
    • AS312 Pir with mounting holes for a standard fresnel lens like the one present in the HC-SR501
    • MYSX 2.6 connector to add other sensors.
    • ATSHA204A for security signing
    • Size: 57x57 mm (not so small but also not so big)

    This is how the board looks like

    0_1563106285959_front.jpg

    0_1563106300980_rear.jpg

    It is still under development, but if someone is interested, the Kicad project is available on github.

    ... Well the next step is to test the ME2188 boost converter... I hope that it can do its job without triggering false positive on the PIR sensors!


 

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