I put an arduino on a PCB board and after some power saving techniques like going to sleep, and 3.3v, 8Mhz, etc, I was able to bring down my power draw to 2.5-3.0 mA from 4 AA cells in series.
I have an NRF24l01 connected to it and tried to use the NRF power saving techniques, but still drew 2.5mA. I removed the NRF24l01 radio and power dropped to .6mA (may have been lower).
Anyway, I am changing tactic now, instead of the arduino being in sleep mode, I am thinking of keeping it off entirely, no loop, it does all it needs to do in setup, then something external turns it off. This is what I was thinking
Note that I have the power as 5V should 6V
The transistor I used is a 2n2222 maybe I should be using a mosfet, also I could not calculate a proper R1 value, my Electronics Foo is weak.
Also read somewhere the base voltage and collector voltage need to be the same, they are not, the PIR needs to be powered with at least 5V and its signal is 3.3V.
Anyone have any better ideas or thoughts? the ProMini 5. with the NRF24l01 draws about 40mA
Circuit should work, try 1K for R1. 6V is the absolute max supply voltage for the microcontroller. The transistor will drop a couple tenths but fresh AA cells will deliver a bit more than 6V. So it's close.
The power LED on the Pro Mini might account for the 2.5-3mA current while the MCU is sleeping. Some folks remove the LED for low-power projects.
This is why I simply avoid mosfets, I don't think I understand them..
Eg, take a look at my circuit, why is the led not fully lit? Yup it's not saturated..is the simple answer buy a logic level mosfet? So how's it more of an advantage using a mosfet over a 2222?
According to attached datasheet for MIC5205 Operating Voltage starts at 2.5Vdc. How is that possible for a linear regulator that is not a Boost switching regulator ?
Just wanted to clarify that my intent was to have the arduino off, and when motion is sensed by the PIR (which is always on) then starts the arduino (boots it up) then when motion is gone the PIR signal drops back down to 0, and the transistor stops the flow of electricity to the arduino.
I have relays, but the PIR wont be able to turn them on. Part of this for me is to learn more about transistors, I thought a transistor would be more appropriate in this case.
This is different than most implementations I have seen, most of them have a loop, an interrupt, or a watch dog. In this case if there is no motion, there should be no power to the arduino.
Your intent is fine.
MOSFET will act most like a relay in terms of a switch, with lowest voltage drop across the component, vs a BFT with 0.5V drop. Just make sure you use logic level, low Rds component that will turn full on with 3.3V (look at Rds with Vgs = 3.3V).
apolonio:
I have relays, but the PIR wont be able to turn them on. Part of this for me is to learn more about transistors, I thought a transistor would be more appropriate in this case.
It's fine. Although I missed that in your drawing the supply was connected to the RAW pin. I wouldn't count on the Pro Mini's onboard regulator working very well on 6V, especially as the battery ages. So connect the supply to the Pro Mini's Vcc pin instead. A MOSFET will also work fine, but I think has very little advantage in this case (and will be the more expensive option). Collector-emitter saturation voltage for a 2N2222 shouldn't exceed 200mV until collector current gets above around 250mA, which it shouldn't unless the Nordic radio module draws a lot more than I think (I didn't look it up).
It works, so I think I just made a mistake somewhere earlier. I did buy a mosfet but now I have something higher current in mind.
But now the power draw when there is no motion is 48uA and when there is motion it jumps to 32mA.
Based on the 48uA and 2890mAH I think this should last 60,000 hours. Of course there is the whole self discharge thing I am not considering, so I will just replace the batteries once a year.
BTW I am not sure why, but when I put a multimeter on the red (6v rail) and the white wire (collector on the transistor) the voltage only goes to 5v and some change, not more than 5.5. Something tells me a should have a fried pro mini but I am lucky.
The ATmega328 chip is rated to 5.5 V, with an absolute maximum of 6.0 V. If you want to be safe, put a diode between the + terminal of the battery pack and everything else. The diode will drop around 0.6 - 0.7 V. In addition the transistor switching the pro mini will drop a few tenths of a volt, if properly saturated.
One drawback of your circuit is that "ground" is not well defined for the processor and anything attached to it, since there is a voltage drop across the grounded transistor. That can cause difficulties, especially for circuits that involve radio frequencies. For this reason people usually use PNP transistors to form a "high side switch".
Thanks, pretty sure I got couple 2N2907's around here somewhere, I will redo my project again and put the PNP on the other side of the arduino. Although my project "works" part of this is to learn.
You expressed a desire to learn about transistors. You should Google "Series Pass transistor configuration.".
If you had used this configuration, everything would have had a Common Ground. It is commonly found in a " Series Pass Regulator".
Or, High Side switch with PNP/P-channel MOSFET (vs low side switch with NPN/N-channel MOSFET).
Generally has the disadvantage that the Gate Off voltage has to equal the source voltage, and turning it on requires pulling to Gnd. If Arduino was 3.3V limited, then another transistor would be needed to buffer the 5/6V to the Gate. I don't know if the PIR can tolerate 5-6V, or if similar buffering would be needed (consisting of NPN transistor with open collector output wired to the gate - PIR/Arduino drives base high, turns on NPN and collector is connected to Emitter/Gnd to turn on the PNP/P-Channel MOSFET.
Yes, there are design issues with high side switches, especially with MOSFETS. I prefer bipolar transistors for this purpose and have used the following circuit with a 3 volt PIR sensor. It has the same logic as the OP's original design (a high from the PIR turns on the pro mini). I added a diode to drop the 6V battery to < 5.5 which also provides reverse polarity protection. And now, everything has a common ground.
This circuit presents a very light load to the PIR (the one I used runs off of 2xAAA batteries for a couple of years and can't provide much drive).
The resistor values chosen will allow the pro-mini to draw over 100 mA, but of course it hasn't been tested with one. R2 could probably be increased to 5K or 10K (and R1 to 1M) if the pro mini draws much less.
