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Scenario:

I have a functioning weather station (temp/humid, MQ-2 smoke sensor, anemometer), and want to add a barometer. The weather station is outside, linked to my inside display and RTC by the I2c&Power modules and ethernet cable. The cable supplies 12v and I use a small converter to provide the 5v to run the I2c&Power board and the rest.

So I have 12v and 5v available out at the weather box.

Inside along with the RTC and LCD display I have a 2-port I2C&Power which will be used in the future (with the new pgpio library) to feed the info to an Rpi3 to create a web page to show changes over time.

The barometer module I bought is the BMP-180. It is nice and tiny and looks perfect for the job, but the instructions all over the web tell me that it will burn up if powered by 5V. The Arduino Nano does not have a 3.3v output pin like the Uno does.

Q: What is the easiest way to accomplish adding this 3.3v board to the weather station?

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Consider attaching your +5V supply to a USB plug, and using that to power the Nano via its USB socket.

When the Nano is powered via its USB socket, the CH340 or FTDI FT232 supply up to 50mA of 3.3V power to the 3V3 pin located between Aref and d13. See for example 3v3 output from FTDI chip on Arduino Nano 3 at arduino.cc.

Actually, that thread apparently says the 3V3 is available no matter how the board is powered; you may want to run some tests and measurements. The only test I ran was to measure about 3.2 V at the 3V3 pin when a clone Nano 3 with CH340 was connected to a USB-power adapter.

  • That is interesting. I sure have noticed that power via USB is not the same as 5v on the Vin pin. Thanks for the info. - However, if you read the OP you will see that I am feeding 12v out to the box where there is a 12-to-5 converter. It measures at exactly 5v to the Vin and to the other devices (except the anemometer, which uses 12v). This is a clever solution but might upset the applecart. – SDsolar Nov 23 '16 at 19:04
  • I meant to add that the analog input from the anemometer uses a conversion constant in order to convert the readings to MPH. It took some trial and error to figure it out... Since it is working I would prefer to leave alone the part that is working. Still, I will remember what you wrote here for the future. – SDsolar Nov 23 '16 at 19:13
  • Just did an experiment where I fed a lab power supply into an analog input, with the Arduino powered by the USB. I used a voltage divider of 10K/2K so I can measure up to 24V. I take 10 readings at 500ms intervals then average them. When running on USB, the plotter readings are all over the place. With Vin at +5 it is much more stable - when displayed on the LCD and the USB disconnected. – SDsolar Nov 23 '16 at 20:05
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    @Archway, by "running on USB" do you mean USB-from-computer, or USB-power-adapter? If both USB ways give bad ADC readings [presumably due to noise?], sounds like +5-to-Vin is what you have to use. But you could still see if a usable 3.3 V is appearing on the 3V3 pin (next to Aref on the Nano) when using +5-to-Vin. – James Waldby - jwpat7 Nov 23 '16 at 21:39
  • I did mean running on USB from a computer. But to be sure, I ran the Nano Vin on the lab power supply at a strict 5v and the 3V3 pin measures 2.81 volts. Interesting. – SDsolar Nov 24 '16 at 0:39
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Here is the solution I settled on: a small part that will step 5 volts down to 3.3 volts. The price is right, but it will be shipped by the slow boat so will take a month or so to receive.

5Pcs 5V to 3.3V DC-DC Step-Down Power Supply Buck Module AMS1117 LDO 800MA

http://www.ebay.com/itm/5Pcs-5V-to-3-3V-DC-DC-Step-Down-Power-Supply-Buck-Module-AMS1117-LDO-800MA-/261983164332

It has 3 pins - Vin, Vout, GND. Easy to hook up.

Upvote to everybody for the discussion.

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