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8-Pin Development Board

chopnhack

Apr 28, 2014
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I just completed a schematic for a 8 pin PIC dev. board to get some hands on experience with a simple mcu. Six I/O pins shouldn't get me into much trouble, I hope!

Can anyone review this?
I am looking for errors as well as suggestions.
I think that what I have shown will allow some simple switching of LED's, A/D conversion of the pot and user input from the the tact. switch.


Top half of circuit is power supply as well as a protection circuit for the master clear of PIC.
PIC used is 12F675
S2 is reset for PIC
JP3-7 allows me to disconnect the soldered components if I want to jumper out the pins to a breadboard for other development not yet envisioned.

Thanks in advance!

upload_2015-5-18_13-53-42.png
 

Arouse1973

Adam
Dec 18, 2013
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Hey John

Your getting the hang of this now, look how far you have come from knowing nothing at the start. Good effort. The only thing I will say about the drawing is the bottom of the circuit looks a bit scruffy. See if you can neaten it up a bit also add a 0V symbol to let everyone know the common point of the circuit. Design wise I would add a 2.2 uF - 10 uF ceramic across the PIC power pins for a bit of PDS support, because you don't know what your going to be driving on the outputs. It's good practice anyway. You can probably do away with R12, the PIC output pins are all current limited. MCLR (GP3) is an input only so you can't drive an LED with it. If you are going to use GP3 as an input for a switch then could you just use the reset circuitry and switch for this.

Thanks
Adam
 

chopnhack

Apr 28, 2014
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Hey John

Your getting the hang of this now, look how far you have come from knowing nothing at the start. Good effort. The only thing I will say about the drawing is the bottom of the circuit looks a bit scruffy. See if you can neaten it up a bit also add a 0V symbol to let everyone know the common point of the circuit. Design wise I would add a 2.2 uF - 10 uF ceramic across the PIC power pins for a bit of PDS support, because you don't know what your going to be driving on the outputs. It's good practice anyway. You can probably do away with R12, the PIC output pins are all current limited. MCLR (GP3) is an input only so you can't drive an LED with it. If you are going to use GP3 as an input for a switch then could you just use the reset circuitry and switch for this.

Thanks
Adam

Thanks for catching that mistake Adam! I appreciate the words of encouragement - it is getting slightly easier, but there is so much to learn still!
I do have a cap across the pins of the PIC, best I have on hand is a 0.1uF ceramic. Here is the updated schematic:

Any other features I should add at this point or should I etch a board and call it 1.0? ;-)

upload_2015-5-18_16-10-40.png
 

Arouse1973

Adam
Dec 18, 2013
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Ok that's better. The cap I was talking about was in addition to the 100nF. Go and get some 2.2uF caps :) As you progress in electronics and your designs become more sophisticated you will find many times that just using what you have lying around won't always work.
Cheers
Adam
 

chopnhack

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Ok that's better. The cap I was talking about was in addition to the 100nF. Go and get some 2.2uF caps :) As you progress in electronics and your designs become more sophisticated you will find many times that just using what you have lying around won't always work.
Cheers
Adam
Easier said than done, the local equivalent of Maplins (Radio Shack) has gone bankcrupt and liquidated their components supply. Of course I was busy during this period and missed out on some scores. There weren't many in this area. I rely mostly on E-Bay for sourcing bulk components.

Can I use a 2.2uF electrolytic instead of a ceramic? If not why?
Thanks mate!

(I think I know why you are saying to use both, one filters noise at a different frequency)
 

Arouse1973

Adam
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Yes you can use an electrolytic. It's not really about filtering noise it's about supplying packets of energy for the circuit when it's needed to try and maintain a stable supply voltage.
Adam
 

chopnhack

Apr 28, 2014
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The small 100nF is a decoupling cap (this is for noise). The 2.2uF is to act as a battery for the PIC so that when it is in operation if there is a dip, such as when peripherals draw too much current, it can buffer the 5+ rail - got it!

Where do you suggest placing the 2.2 lytic? In parallel with the decoupling cap? or elsewhere between +v and gnd?

Thanks again mate, I might be able to etch this tonight!
 

Arouse1973

Adam
Dec 18, 2013
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The small 100nF is a decoupling cap (this is for noise). The 2.2uF is to act as a battery for the PIC so that when it is in operation if there is a dip, such as when peripherals draw too much current, it can buffer the 5+ rail - got it!

Where do you suggest placing the 2.2 lytic? In parallel with the decoupling cap? or elsewhere between +v and gnd?

Thanks again mate, I might be able to etch this tonight!

Place the capacitors as close to the power pins as you can. Although all capacitors will filter noise to a certain extent the main purpose of the capacitors is to supply additional current to the circuit which is usually in short bursts for the small ceramics and longer duration burst for the larger electrolytics. Most regulators are far too slow to do this, by the time they have adjusted their output it's too late. This is what the capacitors do. The smaller capacitors like the ceramics supply fast switching currents for logic gates and microprocessors. The larger capacitors like electrolytics will supply much more current but can't respond as fast because of their series inductance.

Adam
 

chopnhack

Apr 28, 2014
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Waiting on some tactile switches to complete the circuit, but then on to programming! (I have the PIC, didn't want it in until the switches come in)
I did a power on, green LED lit, nothing smoked, ;-) I checked all the traces before populating and found one shorted to another and three incomplete traces. The former was fixed with an exacto knife and the latter via some wire solder to the trace. The traces are 16 thou, so not the smallest, but nothing to sneeze at for home grown.

upload_2015-5-21_22-43-17.png

upload_2015-5-21_22-44-15.png
 

chopnhack

Apr 28, 2014
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Very good job.
Not as good as yours! But its only my second toner transfer, hopefully I can improve ;-)
What exactly does this circuit do ?
It's a simple circuit to help me learn how to program the PIC microcontrollers. The pic is set up to operate the 3 leds and take input from a tactile switch and pot. When I outgrow this limited hardwired functionality I have jumpers on those to remove them from the PIC and an 8 pin header to take the pinouts to another board/breadboard as my programming improves.
 

HellasTechn

Apr 14, 2013
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I noticed that the ground plane came out perfect on your board. "Hi res Printer i assume"
 

chopnhack

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Its an HP laserjet - 1200 dpi. I actually didnt create a ground plane and I'm glad i didnt! I would have had more shorts to trace down!! I didnt want to have undercutting of the thin traces - so I left those large pads unetched. Less copper to remove = shorter etch time.
 

HellasTechn

Apr 14, 2013
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Understood. Looks like ground plane though !

I noticed the trimmer you used. We dont have this type here in Greece. I have only seen them on boards made in the USA
 

chopnhack

Apr 28, 2014
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Understood. Looks like ground plane though !
The ground plane would have been made the same way, its just not connected.

I noticed the trimmer you used. We dont have this type here in Greece. I have only seen them on boards made in the USA

Really? Do you order any parts from China? E-Bay has many vendors that deal directly with the factories there so you can get super deals.
For instance I paid $1.85 shipped for #200 3mm red LED's.
There are lots of great deals on there - check it out when you have time.
 

HellasTechn

Apr 14, 2013
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Yes i order online from e-bay very frequently. But because it takes a month or so for the packages to arrive i only order what i cant find here in Greece or if i need larger quantities.
 

Colin Mitchell

Aug 31, 2014
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As you become more adept at laying out circuits you will find that your layout is just a jumble.
Place the LEDs near the pins that activate them and always start with the IC socket around the correct way. Place everything as close as possible to the way the circuit is laid out.
Never use a 10 turn pot because you have absolutely no idea where it is positioned and if the value is increasing or decreasing.
The board is absolutely useless without a legend and that is why I have stopped producing my own boards over 40 years ago. This type of board only costs a few dollars to be make with overlay, tinned lands and solder mask by Chinese PCB makers.
When you have 400 projects lying around, like I have, you will appreciate the fact they must be instantly recognisable and usable without any reference to information. Without an overlay you will be frustrated when you come back to it in 6 months, and having produced 20 new projects in the meantime.
The regulator does not need a heatsink because it not supplying any current.
2u2 is valueless on the front end. Never use anything less than 100u and 10u on the output.
The board must be laid out in a similar way to the circuit so that when you look at the board, you see the circuit.
When you see the circuit you can visualise what you are doing when you create experiments.
Its such a jumble that I don't see any inputs.
Draw the supply rail at the top and 0v at the bottom. Put all the circuit between the rails.
You cannot possible expect to understand electronics until you draw a circuit so that it presents its self instantly.
 

chopnhack

Apr 28, 2014
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Thanks for the input Colin.

As you become more adept at laying out circuits you will find that your layout is just a jumble.
Place the LEDs near the pins that activate them and always start with the IC socket around the correct way. Place everything as close as possible to the way the circuit is laid out.
The circuit does indeed look jumbled, what you see is my second draft of it, from the brain to the schematic capture software. The reason the IC is facing the wrong way is because the original had everything in a linear fashion flowing from left to right, but it would not fit on the webpage and was a bit unwieldy. I simply folded the circuit and that is how the IC came to be underneath and facing the wrong direction.
Never use a 10 turn pot because you have absolutely no idea where it is positioned and if the value is increasing or decreasing.
The pot comment is well taken. You did not mention what you would have used instead?
The board is absolutely useless without a legend and that is why I have stopped producing my own boards over 40 years ago. This type of board only costs a few dollars to be make with overlay, tinned lands and solder mask by Chinese PCB makers.
When you have 400 projects lying around, like I have, you will appreciate the fact they must be instantly recognisable and usable without any reference to information. Without an overlay you will be frustrated when you come back to it in 6 months, and having produced 20 new projects in the meantime.
I wouldn't say the board is worthless, it has no proven value as of yet, but its certainly not worthless, LOL. But seriously, the board had a legend, I had to wipe it off because the image was mis-transposed. I forgot the mirror image!
I disagree with using the PCB manufacturers for one of's - for hobby and experimentation there is certainly no need to send off a board to be made and wait 2-4 weeks. I would rather get my hands-on locally. I think it would be very disappointing to have waited so long to find out that there was an error in your board and have a dozen worthless boards on hand. I would rather prototype in my backyard, turnaround time is same day!

The regulator does not need a heatsink because it not supplying any current.
Yeah, not getting that at all... Is this based on your experience or some other source of factual data? The voltage regulator will be dropping 4v from the supply times whatever current the circuit will be using. I think the heatsink is a good idea, but I would love to hear your explanation.

The values of the caps where taken from the datasheet. I guess those are considered the minimum values. I have seen what you are referring to when I simulated some of these circuits with Spice. The larger caps create a smoother in/output.
 
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