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1  Using Arduino / General Electronics / DC-DC Power Isolation on: October 07, 2012, 09:19:14 am
I would like to add some sensors to my controller project which i completed recently (,109316.msg855545.html) . 

The sensor circuitry requires its power  to be isolated from the controller.   The data output from the sensor will be opto-isolated from the input pins on the arduino chip.

The question is will the DCP021212 do the trick:   

The sensor circuitry is low power draw and requires +12V input.      Would using the DCP021212 achieve the same result as using a separate transformer power supply to galvanically isolate the sensor from the controller board?

Thx in adavance as usual... sorry for the n00b questions in advance.
2  Using Arduino / Project Guidance / Re: PCB Layout review on: July 10, 2012, 07:31:39 pm
Hey Folks - thought i would update - PCB arrived from Seeed and i finished populating and wrote some test programs over the weekend.  Here are some pics.

I ended up with 3 stuipid mistakes but they were all fixable - 1) a missing capacitor i had on my hand drawn schematic but not on eagle, 2) a pin was connected to GND that s/b connected to VCC (same cause as item 1) and 3) one of the power trace widths was incorrect.

Thanks everyone for your help it made my first PCB order a successful one.

3  Using Arduino / Project Guidance / Re: DS1307 misbehavior on: June 15, 2012, 02:54:52 pm
Mali -

*** disclaimer *** - i am by NO means an expert but i ran into similar issues with the same chips ....

You only need one set of pullup resistors on SDA and SCL.  if you are not using the square wave output of the DS1307 for anything i would leave that pin floating - i have never connected anything to that pin.   

Can you provide a diagram of how you are interfacing your SSR's to the MCP23018?   Also, is the length of your i2c bus unusually long?  I do not believe it should be more than a few feet, but optimally the bus should all be on the circuit board.

What kind of SSR's are you using? i had trouble switching ballasts with SSR's and use regular relays for this.    I believe this is because my MH ballasts are actually an inductive load (seems counter-intuitive being that its ultimately a light).       I had this issue with an IceCap MH ballast, and my Phillips T5 ballasts.     

For the garbage response you are getting back from the DS1307 - is it gibberish, or just an incorrect time? 
If you are getting a time back, but not the correct time, i would replace the crystal and make sure its placed as close to the pins as possible.    I have run into this issue, i have found that when using a DS1307 on a breadboard, an easy solution is to put the DS1307 in a dip socket with the leads from the crystal carefully inserted into the same 2 holes the pins from the chip go in.   Then cutoff the 2 pins at the bottom of the socket. 


4  Using Arduino / Project Guidance / Re: microcontrollers for RS485 on: June 14, 2012, 07:25:16 pm
It looks like the collector/emitter on the sensor can take up to 30V according to the datasheet.  I would try something like this, hopefully someone with more expertise could chime in.

Basically use telephone or intercom cable (4 conductors), 2 for the LED and 2 for the sensor collector/emitter.  Run the sensor circuit at 12V rather than TTL levels.

RS485 network of uC's seems like overkill for transmitting the state of a sensor that is only sensing a low or high state and the switch time is relatively long.

Interested to hear how it turns out!

5  Using Arduino / Project Guidance / Re: PCB Layout review on: June 14, 2012, 06:57:35 pm
Something I noticed was the traces coming off the row of transistors up the top. If the through hole pads are on both sides, the trace from the middle pad gets VERY close to the right pad on some of them. Some of the other clearances look a bit tight too.

You can set up EAGLE to run DRC to check for minimum clearances etc, might be worth checking the specs of the fab house you're going to use and check your layout with those values.

Good luck!

Hi Squarebear - i see exactly what you are talking about.   Firstly, it passed DRC with 8mil clearance and the fabhouse DRC file has 6mil clearance.   It passes DRC until i change the clearance to > 10mil.     It does look VERY close though...  If I hide the bottom layer, you can more clearly see the space between the top trace on the middle pad and the pad to the right of it. As it turns out it looks like i fixed 2 of the traces on the file I submitted to the fabhouse, but one is still there like in the last picture I posted....   We will see what happens... this is my first fabricated PCB so i budgeted for one round of mistakes  smiley-eek-blue


6  Using Arduino / Project Guidance / Re: LED: Can PWM be used to substitute for a potentiometer on a buck puck? on: June 12, 2012, 08:14:29 pm
For a 0-10 V signal you would need to use an op-amp or similar since the logic signal is 5v...  I use a variation of this circuit for a similar light dimming application on my aquarium.

7  Using Arduino / Project Guidance / Re: PCB Layout review on: June 12, 2012, 07:29:53 am
I ordered the pcb from seeeed on Sunday! Will update with pics in due time.

How about one of these little assemblies for 12V to 5v conversion?

This will be going into my next mouser order as well smiley

Thanks everyone for the feedback.
8  Using Arduino / Project Guidance / Re: PCB Layout review on: June 10, 2012, 06:04:38 pm
The pulldown would ensure it doesn't start on while the outputs are high impedance, once they are set as outputs and either high or low it doesn't matter, you could probably get away without them but thereon almost always there
how much draw on the 5v line are you using? Dropping ffrom 12v to 5v is basically wasting more than half the power as heat with a linear regulator

the 5v line draw is minimal - if i remove the status LED's (or put them on the emitter side of the transistors) - the draw would be the Arduino, DS1307 & MCP23008 + the current draw on the transistor gates =  i am guessing it is not using very much power but its not sipping power either... also there are a ton of pulldown/pullup resistors on the pins.   The draw on the opamp is < 10ma. I was going to limit the LED draw to 140ma.

I have pretty much -0- experience with power supplies - i have always used a 9 or 12 v wallwart with LM7805 .... having said that, my prototype circuit is using the L4931 regulator in a TO-92 (!) package - it is running a bit hot.      I was planning on switching out the L4931 in the RBBB i am using for the prototype with a LM7805 but have not gotten around to doing it (desoldering is my least favorite activity)

If you have any power supply recommendations to improve the circuit, i would appreciate!

9  Using Arduino / Project Guidance / Re: PCB Layout review on: June 10, 2012, 08:55:01 am
Is this gonna be in an enclosure?  using a mosfet like the stp40nf12 would allow you to run cool

I have a handful FDP8860's handy let me review the datasheet and see if this is the same as the mosfet you quoted i beleive it is      Is it correct to say the circuit would be the same as TIP120 but i need to pulldown the Gate to ground?
10  Using Arduino / Project Guidance / Re: PCB Layout review on: June 10, 2012, 08:45:53 am
PS: I didn't want to sully my nice computer with the likes of Eagle but may install it to see this stuff smiley
What CAD software do you use normally?  The schematics on your website are very clean and easy to understand.

Is this gonna be in an enclosure?  using a mosfet like the stp40nf12 would allow you to run cool
a linear regulator is burning alot of heat, a switching one would save on that, but why do need regulated 12v? Fans aren't that picky if you just use something close from the external source

the 12V input is regulated (not on the board) so the lights controlled by the op-amp don't flicker/change.
The regulator on the board brings the 12V to VCC for everything else.    Agreed about the fans smiley
(There is a VCC pin on the power header for testing purposes only)
11  Using Arduino / Project Guidance / Re: PCB Layout review on: June 09, 2012, 10:19:26 pm
* Added Ground Plane, removed a bunch of ground traces on the top layer that became unneccessary
* Rerouted a few items on the bottom layer
* Added bypass cap on MCP23008 and ATMEGA
* Fixed mounting holes

Here is updated pic... THe eagle board/schematic link above in the first post has been refreshed

12  Using Arduino / Project Guidance / Re: PCB Layout review on: June 09, 2012, 09:10:17 pm
If all 7 transistors are on

  • 3 relays @ 20mA each
  • 2 fan circuits have 2 fans wired in parallel @ 0.580 A, for 1.16 A on each transistor   <-- these two TIP120's i have heatsinks on, they are hot to the touch w/o heatsink.
  • 2 fan circuits have 2 .140 A fans wired in parallel, .280 A each

Total current = 3A
Current in "normal" daytime condition = 60mA (relays) + 560mA (2 fan circuits)

The two high power fan circuits only turn on sporadically if the temperature rises above a threshold.   

Updated pics coming shortly.
13  Using Arduino / Project Guidance / Re: PCB Layout review on: June 09, 2012, 08:35:46 pm
Should also add ground plane.

DONE - rerouted various traces.   Question - there is a ground plane "island" that is connected to the rest of the plane by about a 50mil trace.  The island has quite a few pins attached to it.     To fix would require me to re-route about a quarter of the board - can you explain if this is an issue/what would the impact be?

First thought, if possible (and it seems that it is) I would align the mounting holes so they are square, currently they appear to be in random locations.


Fixed..  I dropped the mounting holes on so i wouldn't forget to put them on later, and then i never put them in the right spots - thanks for pointing that out!  Posting new files soon.
14  Using Arduino / Project Guidance / Re: PCB Layout review on: June 09, 2012, 04:24:54 pm
Also, i wanted to ask, should i add decoupling caps to all the IC's or are the caps near the regulator circuit sufficient?
15  Using Arduino / Project Guidance / PCB Layout review on: June 09, 2012, 04:11:11 pm
Hello -

I was hoping to get some feedback on my PCB design - this is my first attempt at having a PCB manufactured.    I re-routed this board maybe 15 times and I hope I finally got it right!  Also want to make sure i am not doing anything blatantly wrong.   I did have to redo and shrink the layout a few times to get it to 10cm x 10cm for the PCB fab.

What it is -  This will be my controller for my aquarium hood.
* Atmega168 with arduino bootloader
* MCP23008 GPIO controls 7 TIP120 transistors:  3 to turn on and off relays, the remaining 4 control 12VDC fans.   The fan headers are on the right, each of the 4 circuits drives 900 mA.
* DS1307 real time clock, also on the i2c bus
* TLV2374 rail to rail op-amp attached to PWM pins from Arduino.   These are for 3 0-10 VDC outputs which control the dimming of my T5 Ballasts.   
* 2 voltage divider circuits on the analog inputs with thermistors, to measure the temperature of the hood and the water.
* the remaining pins of the arduino route to a header for buttons, encoders, LCD, etc.
* input is regulated 12VDC, on the lower left of the board is the voltage regulator circuit.

i only had  small breadboards, so my prototype is a bit of a mess, but it works.

Here is the prototype:

Here is a picture of the layout and links to the Eagle Schematic & Board layout

I still need to complete putting labels on all the parts.   Any feedback, comments, etc is appreciated.
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