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Xbee circuit help
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Xbee circuit help

by nemo000111 on Thu May 02, 2013 2:07 pm

Hi all,

I'm working on a remote sensor network using xbee's. I could use some help on the shchematic and the board layout. I've attached what I've come up with, however I'm sure there are better ways to do it. The sensor is battery powered (2x AA batteries) and runs at 3.3v via a bost conerter, so low waste power is important. Currently it's got a mag switch (normally open) and a tmp36 on it (more will come later). There is also an ISP header. The way have the p-channel mosfet in the power supply should provide reverse voltage protection with minimal waste power. I'm not sure how to all over voltate protections to this without a signifigant cost (money and power). Everything in the circuit can survive 5 volts execpt the xbee. For this reason, there's a manual switch to enable and disable the isp header. When the header is enabled the xbee's power and serial input are disconnected. As I'm typing this, I'm thinking I should probably add a simple diode on the vcc line of the header to protect from reverse voltage. As far as the board layout goes. There is a ground plane on the top and bottom layers with vias near components. Any via labeled "tp#" is a test point. All buyt 2 traces are on the top of the board. The signal traces are 12mil and the power traces are 21mil. The micro enables/disables the xbee and tmp36 via their sleep pins and checks if the mag switch goes from ground to high. Any help/suggestions would be much apprechiated as I've never designed something like this before . Thanks.
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board.jpg (162.95 KiB) Viewed 931 times
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Re: Xbee circuit help

by franklin97355 on Fri May 03, 2013 11:38 pm

You have quite a few components that don't seem to have connections on both ends, am I missing something here?
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Re: Xbee circuit help

by nemo000111 on Sat May 04, 2013 10:32 pm

The ends that don't look like they're connected are connected to the ground plane. All of the unlabeled vias connect the ground planes on the top and bottom of the board. It was a cleaner picture to leave off the ground plane.
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Re: Xbee circuit help

by extrabee on Wed May 15, 2013 11:42 pm

A couple thoughts:
1. Make sure the micro does not drive the DTR/DI8 pin of the XBee when programming or you could easily exceed 3.3V on the module.
2. The disconnect switch is a good plan to protect the module as well
3. Are you going to do the reed switch debouncing in software? If you want to reduce that burden, consider a 0.1 or 0.01 uF capacitor in parallel with R3.
4. Consider adding a pull-up resistor to RESET even though it has an internal one. I've had electrical noise trigger resets sometimes.
5. If you want to be able to use an XBee-PRO, make your power traces wider to reduce voltage drop and maybe include a big capacitor to help provide the current pulses (like a big 47 or 100 uF electrolytic or Tantalum)

Good luck!
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Re: Xbee circuit help

by nemo000111 on Thu May 16, 2013 4:41 pm

Thanks for the reply. At the moment, I physically disconnect vcc and serial in on the xbee. I haven't done anything to protect teh dtr pin. Will it matter if dtr is getting 5v and tvcc on the module is disconnected? I was planning on doing the debouncing in software to keep the costs down. If that proves too problematic, I'll at the cap. Good catch on the pull up resistor, I'll work that in in the next rev. I hadn't really considered using a pro. This is made for indoor use (so 150ft or so max) and the switch I'm using isn't rated for the extra current. Ideally, I'd like to make the micro able to be flashed via the xbee, however I'm not sure if the extra componets are worth the gain. Thanks again.
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