Absolute Beginner

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Fitfurnowt

New Member
Hi everyone,

These questions (3) relate specifically to the PICAXE-28X1 Project Board, using it to drive motors and solenoids and suitable power supplies.

I've only recently become aware of what a microcontroller is, what it can do, and PICAXE. My electrical experience goes no further than having rewired my house, soldered wires for various connectors ect. I have picked up a bit about volts, amps and the like and understand what resistors and capacitors do, but I don't yet go much beyond that. So my questions come from that knowledge.

I want to build a system that that is initiated by a switch and will control two motors (forward only, not reverse) and a solenoid valve rated at 12volts, 0.6 amp. Once I've learnt how to do that, I intend to add a second switch to control a second solenoid, and as my knowledge increases, an LCD display.

Having read up through the manuals, and PICAXE web site, I got all excited and got a PICAXE-28X1 starter pack. It includes the ULN2803A chip with the Darlington drivers on each of the eight digital outputs. The literature say's that the outputs will each drive up to 800ma and that I can use a second 12v power source to drive the ULN2803A and it's outputs.
Will the ULN2803A drive up to 800ma in total across the 8 outputs, with up to 800ma on any one, or does this mean that it will drive 800ma on each output independently, making 6.4 amps in total? I got the PICAXE Microcontroller book with my order but it doesn’t cover the newer X1 chips.

Also, am I correct in thinking that I simply connect the solenoid and motors to the ULN2803A with no other components?

I've not got the motors yet (I thought I'd get 12volt to keep things simple) so don't know what current they will need (they are used for small dosing pumps), but assuming they require the same as the solenoid, that would mean a current requirement of 1.8 amps for my initial system.

The last question is about the power supply. Is a computer power supply a suitable option? I understand they supply 12 volts at a high wattage which (if the above is correct) would supply enough power.

I'm sure these will seem like bone questions to you guy's so I apologise for my ignorance. I can't seem to find an answer in any of the documents on the site or forum.
 
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eclectic

Moderator
Fitfurnowt

Welcome to Picaxeland.

I can't answer all your questions
(there are plenty of experts here to do that.)
However, some more reading.

ULN2803 datasheet
http://www.rev-ed.co.uk/docs/ULN2803A.pdf

page 5, fig 12, shows the maximum values, when different outputs conduct.

Picaxe manual 3.

Page 11 .... Motor connections. Note diodes / capacitors.

Page 23 shows a Solenoid / diode circuit.

And, if the motors are of suitable rating, you might think about adding
the optional L293 chip as another method.

e

Added.
In manual 3, search for MOSFET

And, a possible alternative to the AXE020 is the CHI035 board.
 
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B

BeanieBots

Moderator
Well a good read of the datasheet pointed out by eclectic is a good place to start.
NEVER assume anything. You MUST find out what current your motors will require or there are likely to be tears.

Talking of which, a PC power supply is NOT suitable.
Even a wapping great big 500W power supply will only be good for about an amp max. on the 12v line. The POWER is for the 5v line. The +/- 12v lines are only for auxiliaries and cannot supply much. More often than not they are just 7812 type regulators comming off a small gauge winding on the main switcher transformer.

Without a proper spec for the motors I would not like to comment on if the ULN2803 is suitable (for you) but it does contain catch diodes so it is possible to simply connect a load (eg solenoid and/or motor) to them without the need to worry about back emf issues.
 
F

Fitfurnowt

New Member
Hmmm,

So looking at your first link page 5, figure 12, I can drive 8 outputs at up to 400ma for 30% of the time? Is that right? If so, have I understood the 800ma figure correctly here

http://www.rev-ed.co.uk/docs/AXE020.pdf

first parragraph, page 1,

"Each of the outputs is driven through a darlington driver, which will drive upto 800mA per output."

The graph indicates a reccomended maximum current of 400ma, per output, no matter how many outputs are used.

What would the likley out come be if I put my solenoid at 12volts and 0.6 amps on one output and two motors on two others. The solenoid would be on permantly, and each motor could be run alternativly for about 5 mins per hour. would the chip go bang or would it just get hot, which could be countered with a heat sink/fan?
 
B

BeanieBots

Moderator
The current limit is the current limit. A fan won't help, it will still pop if you exceed the current limit.
You can double up on outputs but you would need to modify the board. Switching one on then the other would be too risky. There is a way of switching on two outputs at the same but if you got it wrong, there would be tears.

Wait until you have numbers for the motors before going too far down any particular path. It may well be that a different board is better suited to your needs.
 
E

eclectic

Moderator
Another possibility, if you just want simple on/off.

Using the ULN2803 outputs to energise relays?

Or even better perhaps, latching relays.

e
 
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Fitfurnowt

New Member
I've found that the motors are rated at 12 volts and 60 ma. So how about I drive the solenoid through a relay and two motors direct from the outputs on the ULN2803A? (with the relevent capacitors and diodes)
 
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eclectic

Moderator
Cheers for the sheets.
Those motors look well within the 2803 current limits.

What will be connected in the final version.

2 x Solenoids. 2x Motors. 1 x LCD. Anything else?

e
 
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Fitfurnowt

New Member
Theres nothing more to connect really.
I got the 28X1 project board because I understod that the motors and solenoids could be driven straight from it with a minimum of fuss. If I had other ideas later, there are enough outputs to cover them.
 
E

eclectic

Moderator
Fair enough

For future reference, if you're going to be using a serial LCD,
then please have a look at the photo.

You'll see a line of solderpads, between the 28X and the ULN2803.
These can be used for direct connection to one of the outputs.

You'll need one to connect to say the AXE033 LCD.
(Which is very easy to use, once you've learned a couple of commands).

e
 

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Fitfurnowt

New Member
Thanks

Well thanks for the help, BeaneBots and eclectic, I think I know what I'm doing now. If not we'll be buying another board!!!
 
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