DC Boarduino in 3.3V power supply

[beaststwo]

I purchased a DC Boarduino (one with an Atmega128) a few months ago and am just getting around to assembling it. My application will use the Boardino and an XBee Module to communicate electric fence (livestock fencing) voltages back to a central location. I'd like to run the whole thing on 3.3v or less to reduce power turned into heat by the power supply (this will run off batteries charged by a small solar cell).

Comparing the DC and USB Boarduino schematics, I see no reason I couldn't just leave most of the 5V power supply hardware (regulator, etc) off the Boarduino, install the decoupling capacitors and supply the board 3.3v and ground directly. I see no components that won't run off 3.3v in the kit. Is there any reason this wouldn't work?

Thanks!

[adafruit]

should work just fine!

[beaststwo]

Thanks Limor! Better to ask that feel REALLY stupid later...

BTW, Great products, well though-out and designed!

[seakoz]

I don't suppose either of you would be willing to proivde a bit more detail on how to do this conversion? I'd love to convert my boarduino into a 3.3v unit. I'm trying to make this last on batteries and I'd appreciate any help you'd be willing to give me!

Thanks!

[beaststwo]

I left off the following components when I built the Boarduino: J1, C1, D1, C3, IC2. Could have left off some more but the main thing I wanted was to get rid of the 5V power supply and use the parts elsewhere.

The biggest surprise was that the 3.3v USB cable (TTL-232R 3.3V from Adafruit), actually puts 5V into pin 3 of JP7 (the header at the end where you program the Boarduino). I had assumed that 3.3v meant that all voltages were 3.3v. To get around this, I set the power select jumper to "ext" and used a 3.3v regulator (the same part used on the Adafruit XBee Adapter, Microchip P/N MCP1700-3302E/TO) to drop the voltage to 3.3v from a 4v sealed lead-acid battery (a PowerSonic PS-445 that I bought from BatteryWeb,com), which will be charged by a 4.5v, 100ma solar cell I bought from SparkFun (P/N PRT-07845).

It may sound like charging a 4 volt battery from a 4.5v solar cell will damage the battery, but with a fixed mounting it will only produce peak voltage for 1-2 hours per day and most of the time will be less. The battery is a bit of an overkill, but it will be running the Boarduino and an XBee-PRO ZNET 2.5 module that sucks 250mA when transmitting. I don't need that much distance, but my line of sight path isn't great, so I'm looking for more power to communicate. I also can't have the Boarduino sleep much of the time as I'm sampling electric fence voltages (animal containment, not frying humans) and I have to watch for a 1 millisecond, 10 kilovolt pulse that happens about once a second. I'm still writing software and hope to be able to make some time predictions for the next pulse so that I can use sleep more, but I figured that I should design the first unit to be able to run without sleeping the Boarduino.

Hope it helps!

Tim

[seakoz]

This was incredible helpful! Thanks so much!

[beaststwo]

I did want to explain about the 5V USB thing (It was late and I was sleepy when I wrote last night). After I built the Boarduino (sans extra parts) and ran a test sketch to verify it worked, I was just about to test an XBee module (in an Adafruit XBee adapter board with only connectors - no voltage regulator or buffering). Xbee-PRO ZNET 2.5 modules are only rated for supply voltages in the range to 3.0-3.4v. I had the XBee wired to the Boarduino and was just about to power up, when I had a moment of panic (or maybe better judgement).

I unwired the XBee, plugged in the USB cable and was surprised to find that the Boarduino was running on 5V power (saved me from killing a $40 XBee module). That's why I went the route of the external voltage regulator. I could have swapped some wires in the USB cable to put a 3.3v power lead on the Boarduino, but I didn't want to mess up a cable I use for other purposes.

So if you want to run a DC (vice USB) Boarduino at 3.3V, be really careful when you plug in the USB cable to reprogram it as you'll be running it at 5V. Make sure any other components that can't take 5V are disconnected.

Hope it helps!

Tim

[seakoz]

Is this true even if I have the power jumper set to EXT?

[beaststwo]

No, this is why I set the power to "ext", so that I could guarantee that if a USB cable were hooked up that I wouldn't fry the Xbee module with 5v power. If you left out the same parts I did and set the jumper to ext, then the 5v line from the USB cable is broken and you can connect 3.3v power from a regulator to the +5v and GND pins (near the Vin and Reset pins). I think this would be JP4 pins 1 & 2 on the schematic.

Note: I think the +5v pin near Tx & Rx are the same as the + and - pins from the USB cable on the end of the Boarduino and would therefore have 5V on it. Limor would have to verify this to be sure.

[karlgg]

Looking at the traces on my Boarduino, the +5 pin from the FTDI connection goes straight to the power jumper and nothing else. So if you have the jumper set to external, it should not have any connection anywhere else.

Since you're wiring this thing up special, maybe you want to skip the jumper block too? Just solder a wire from the middle to the side you want... Except that you've already done the soldering.

[seakoz]

Two more questions:

1. If I built a 3.3v boarduino, can I still use my 5v ftdi cable? Or will 5v serial input fry it. (Note, I'm not actually going to use the usb for power, just for the signals).

2. If i run it on 3.3v, how many volts come out of an arduino digital pin?

Thanks!

Jonathan

[adafruit]

1. its 'ok' but not suggested
2. 3.3 of course!

[beaststwo]

My opinions (reasonable educated guesses from my memory of engineering school 27 years ago). Your mileage may vary.

1. The atmega's UART pins (the ones you program through) shouldn't be damaged by the 5V logic output from the FTDI cable as it can take power supply voltages between 2.7-5.5v. I believe that the low-to-high threshold voltages for 3.3v and 5v logic are compatible, so ones and zeroes should be recognized correctly. To be safe, you might want to order a 3.3v FTDI cable from Adafruit. It's a tough tradeoff (a $20 cable) to keep from potentially damaging the Boarduino ($17.50 kit) if something went wrong or not having it talk correctly. I wouldn't expect any problems, but one never really knows for sure. All my FTDI cables are 3.3v and they work great with 5v logic, so I've never gotten a 5v cable.

2. The atmega will raise digital outputs to the power supply level, i.e., 3.3v. As digital outputs operate in either saturation or cutoff (i.e., zero or one), there's not much chance of outputs being anywhere except the power supply voltage or ground. If that's not enough to trigger peripherals, see the Adafruit Xbee adapter for the buffer Limor uses to adapt 3.3v and 5v logic. The part number is in the documentation.

[grenik]

Nice project!
I have two questions:

1) In order to avoid the problems with the ftdi cable and the 5v can I download the sketch to the microcontroller using a Duemillenove (replace the microcontroller,download program as in a normal 2009,take out the uc again) and then use the microcontroller as boarduino on the 3.3v power supply that beaststwo proposed?

2) Can I make boarduino to work at 8Mhz to conserve power consumption or that ruins boarduino concept?

Thank you

[mtbf0]

the answers to both of your questions are yes, just not both at the same time.

if you build for a duemi..., (sorry, i can't type italian), you'll be building for a 16MHz target, so none of the clock dependent stuff, (like serial comms or millis and delay functions), will work properly on an 8MHz board.

there are a couple of ways you can mod a boarduino to run at 8MHz. one way is to replace the 16MHz resonator with an 8MHz resonator. the other is to omit the resonator and modify the fuses on the chip so that it runs off of its internal 8MHz clock. in either case you will need to burn an 8MHz version of the bootloader. then just pick an 8MHz target in the arduino ide and build your sketches.

one advantage of running using the internal clock is that you get two extra i/o pins.