Sensor shield for Arduino Pro Mini 3.3V with boost up regulator

  • Hello,

    I've been reading here for quite a while and now I want to share my idea of a project with you. I love all the wonderful projects that are shown here, but I wanted to have a shield, where I can put an Arduino pro mini (china clone) and NRF24L01+ into and have connections for sensors outside together with a boost up regulator so I can use 1 or 2 cells for powering.
    Here is what I came up with. I really like to hear your ideas and suggestions, I tried to incorporate as many ideas and thoughts that I found elsewhere on this forum. It's actually the first time, that I'm trying to make my own pcb.

    Here's the scheme:

    sensor shield rev 1.5.png

    So these are the parts:

    • arduino pro mini 3.3V (china clone, have to remove voltage regulator and led)

    • two places to plug in the NRF24L01+ module (in both cases the antenna will not be covered by the shield pcb)

    • for voltage regulation, there is a MCP1640 (less power consumption than NCP1402 when everything is asleep)

    • a voltage divider on A0 to check battery voltage

    • a push button on D5

    • ISP port for programming

    • I2C port with pull-ups on SDA and SCL

    • two digital (D3 & D4) and two analog (A1 & A2) pins together with GND and VCC for sensors on 3-pin connectors

    So my first pcb would look like this:

    sensor shield rev 1.5 pcb.png
    Size is 2.4 x 7.9 cm, it could be smaller but I need space for SMD soldering.
    What do you think? Is the design worth sending to China for pcb manufacturing or do you have any ideas how to make it better?

    Thanks a lot,

  • I like the formfactor and if the price is right i'd buy a couple (5-10pcs).

  • Hero Member

    @phil83 Well I don't know much about the MCP1640. It was tested here:
    It is probably fine. Pretty cheap but it's annoying having those two extra resistors on the board. Other than that you could shrink your inductor and capacitors a little. They look a little big. And spaced quite far apart? Soldering shouldn't be difficult as long as you do them all in the right order so you wont obstruct yourself.

    That surface mount electrolytic ... 470uF? Higher capacitance would yield lower ESR sure, but wouldn't a simple ceramic 4.7/10uF or whatever work just as well? Conveniently the same type also used for the boost regulator.

  • @bjornhallberg Thanks for your input! I used that inductor since I can get it quite cheap and it is shielded. I tried to space the capacitors closer to the MCP, the resistors are quite far away - thats right, but they only deliver the voltage to put the MCP into 3.3V output mode, so this should be fine. Regarding the electrolytic capacitor - 470uF was a mistake of mine, I wanted to use 4.7uF as shown on the mysensors main page - they also use an electrolytic one. So now I put in a smaller one. With your input I could shrink the pcbs to 2.4x7.4cm, thank you once again!

    sensor shield rev 1.5b.png

    I checked prices yesterday and should have a final price of 5-6$ for pcb with parts but without arduino and nrf.

    Have a nice sunday,

  • Hero Member

    @phil83 I think the 4.7uF electrolytic is a bad recommendation for this purpose. It has been discussed a couple of times but the recommendation hasn't been changed. Probably because no one has actually tested and come up with a solution that is proven. It doesn't really matter though, you could retrofit any cap you want later. I've mostly put 1206 ceramic no-name caps on mine, soldered between to the nrf24 pins themselves and it seems to work. But I have no oscilloscope or esr meter or anything so it is impossible to know what is going on.

  • @bjornhallberg thanks for the advice, I searched the web and now I'm most likely to use a 100uF tantalum capacitor close to the two female pin headers for the nrf24l01 module. Thank you!

  • Hi, just looking at the step up voltage converter MCP1640 that you've specified. This has a quiescent current of 19uA. Perhaps you could consider switching to something with lower quiescent current such as the Texas Instruments TPS61221 which has previously been suggested for use on the official MySensors Battery Board (which still seems to be a work in progress) which only consumes 5.5uA. Don't know what the cost difference is though.


  • Hero Member

    Yeah, my concern with the MCP1640 is that the link I posted to eevblog is on to something and that it is misbehaving. They never seemed to hit their uA targets, though it may be user error.

    My main motivation for other regulators though, like the TPS61221, is that you need two less components, and thus less board space. I think the TPS61221 is about twice as expensive as the MCP1640. And the LTC3525 is about twice as expensive as the TPS61221. Those are the only regulators I have on hand, but there have been MANY more discussed in various posts. For me it is convenient that both the TPS61221, LTC3525 (and MCP1640) are available on AliExpress so I can avoid expensive shipping costs and probable customs fees.

  • @phil83
    Why not put the pro mini circuitry on the board in the first place?

  • @GaryStofer What do you mean by putting the mini circuitry on the board? You mean just use an Atmega328 instead of the Pro Mini board? The reason for this is that the Pro Mini nearly costs as much as the parts and my soldering skills are not good enough to solder the Atmega directly onto the board.
    I had to create a new user name - I can't log into the old any more, the password reset feature is also not working - sorry.

  • @hawk_2050 @bjornhallberg Thanks for the information about the TPS61221. I checked the data sheet and also checked availability at AliExpress. I guess I'm going to switch to that one.

    ![sensor shield rev 1.6 pcb.png](/uploads/files/1426622255314-sensor shield rev 1.6 pcb.png)

  • Hero Member

    Yeah, you're probably not saving a whole lot of money doing your own arduino-like board from scratch. Can't seem to find cheap ATMEGA328P-MU on AliExpress either, only ATMEGA328P-AU. But then to really save space you'd have to use the "mini" nrf24 module as well.

  • @bjornhallberg Yes, I totally agree... I could save probably 2cm in length of the board but have much more to solder.

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  • So this is my final board... what do you think?

    sensor shield rev 1.6 pcb.png

  • No no, this is not your final board. The traces are routed really close to each other.

  • @maha you mean the traces close to the GND polygon? I used the design rules of my pcb supplier.

  • An example is the radio module. The traces are super close to the pads, or are they even touching some of the pads? But I like the form factor of the card!

  • Hero Member

    Just curious - if Vin < VCC, why are you using a voltage divider to get the battery voltage? If you are using VCC as the ADC reference, you should be good without a divider, I'd think.

    It would be possible to use the 1.1v internal reference (ONLY when measuring battery, not most sensors), in which case your divider could make sense, and that would remove the stability of the VCC power supply from the measurement - tho I rarely see that approach taken.

  • @maha Yes you're right, they are quite close... but they should be fine with my manufacturer. I'll try tonight to see what I can do to get the traces a little more apart, let's see. When I use the Gerber file, I can't find any spots where the lines are touching. The picture taken directly from Eagle is also probably not that accurate.

  • @Zeph I wanted to use the internal reference as described here:
    What do you think?

  • Hero Member

    Another nitpick ... there are a lot of different Pro Mini pin layouts ( I don't know where you buy yours and if you can be sure of a certain pin layout, but I would probably not bet on anything other than the side rows that are present on all boards. A4 and A5 are fairly certain, but other than that ....

  • @bjornhallberg completely right - I have some here from Aliexpress and have a good and fast seller... but I asked for exactly that design before ordering...

  • sensor shield rev 1.6 pcba.png
    @maha I'm going to check again this weekend and then I'm going to send it to China for manufacturing, thanks for all your help!

  • Hero Member

    @phil83 said:

    @Zeph I wanted to use the internal reference as described here:

    The voltage divider in that circuit diagram is for measuring an input voltage (into the regulator) higher than Vcc (out of the regulator). Bad idea without a voltage divider. Since you are instead using an up-converter and Vin < Vcc, I think you can do without the voltage divider resistors.

    As an example, suppose you used one cell at 1.5v draining to 1.ov. With a Vcc of 3.3v a direct reading would give ADC readings of 465 falling to 310. Using that voltage divider would yield 149 falling to 99. (all out of 0 to 1023). You just lose resolution by inserting the divider. (WIth two cells double these numbers).

    The caveat is that I'm uncertain about is startup transients - with Vin applied to the pin before Vcc stabilizes. Not sure if that's a real problem or not, tho - maybe somebody else would have thoughts.

  • Hardware Contributor

    I think you're wrong. The divider works perfectly when measuring against internal reference like shown at the build-site. I think it's very well proven and I have never had any issues. And this is for battery voltage LOWER than Vcc.
    I think I had a link to one of my code example here as well.
    But sure, if you have a stable and known Vcc you could use that as reference instead. Without divider.

    Why don't you use the recommended 0.1uF capacitor in parallel with R2 ?

  • @m26872 Thanks for reminding me of the capacitor, I added it!

  • Hero Member

    @m26872 As I said in the first post, if you measure against the internal 1.1v reference (rather than the more common Vcc), it can work. And I see that's in fact what you are doing, so no problem, carry on.

  • Hey folks,

    I just received my first boards today. They really look nice, I never believed that it is so easy! Especially the manufacture in china was pretty cheap and looks really good. I had to wait four weeks, nearly perfect!

    pcb_16042015 Kopie.jpg

    Now I'm waiting for two more capacitors, all the other parts were delivered already. I hope to be able to solder next week and will keep you updated afterwards!
    I think I got infected by a virus by now since I'm already planning the next board ;-).

    Thanks again for all your help,

  • Very nice board.
    Do you intend to share gerber files/eagle file?

  • I'm going to check the board in the next few weeks, when everything is fine, I will share...

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  • Hello,

    I would be interessted in the wiring diagram, as I want to build something similar but with no SMD parts.
    Would you share your actual diagram please?


  • Hi everyone,

    I received the last parts on monday and populated 2 boards so far. I found out, that one header for nrf24 is not working properly... most likely since the wires are too far away from the capacitor. The other header is working perfect. Here are the first pictures of the populated board:

    PFC sensor shield 1.6 A.jpg

    PFC sensor shield 1.6 B.jpg

    PFC sensor shield 1.6 C.jpg

    Though the hand soldering of the sot23-6 was not easy at all, I managed to do it. I tested the boards with the BatteryPoweredSensor and the voltage divider is working properly. I will now start to program more and see which sensor I can build with that board.

    Thanks again for all your help,

  • @gloob : The scheme shown above is still the same, I only added a capacitor in parallel to the voltage divider!

  • turn out very nice.
    are you planing to share eagle files?

  • Hero Member

    Great work! Really impressive!

  • Long time no see... I had time to work on my sensors lately and here are some pictures of my humidity sensor which is running here for three months now:


    I left out the power regulator shown above and also desoldered the LED and voltage regulator of the Arduino to safe power. The sensor is directly powered by 3.7V of the lithium battery. The voltage is measured and the battery will be recharged when it goes down.
    Since I also built a 3D printer lately, I was able to put the sensor into a little box. Now I can easily measure humidity and temperature in my bath room and see it in fhem on a raspberry.

    Currently I'm working on my next pcb... I will show the next one when I'm finished in a new thread.