PICAXE-18 PCB

A

Arthur C

New Member
Can someone please advise me?

Using the PCB layout for the PICAXE-18 PCB as provided on the PICAXE website (http://www.rev-ed.co.uk/docs/axe001_pcb.pdf).......

Will the output current from the Darlington Driver IC (ULN2803A) be enough to drive a motor? ...also..is it possible to use this circuit to reverse the motor direction?

Cheers
 
Y

ylp88

Senior Member
You can drive a motor using the ULN2803A chip on that board, although the motor will have to be "relatively" modest as the chip can handle a max current of 500mA per channel. The board configuration will put ~6V though the motors.

Direction control can be implemented using several of the chip's driving transistors by making and H-bridge.

EDIT: sp.

<b><i>ylp88 </b> </i>

Edited by - ylp88 on 09/01/2007 13:24:58
 
B

BeanieBots

Moderator
How long is a piece of string?
It depends on your motor!

Each output is rated at 500mA.
(you will need to get a copy of the ULN2803A datasheet to confirm if all outputs can run 500mA at the same time)

500mA is not very much when dealing with motors but it is possible to double up outputs to give 1A.
Reversing cannot be done directly, but it is easy to achieve using a DPDT relay.

It is NOT possible to configure the ULN2803A for use as an H-bridge. The ULN2803A can only sink current. An H-bridge requires source and sink capability. There are custom chips for this such as the 293D found on many of the PICAXE project boards.

It is possible to make your own with descrete transistors but not recommended for a beginner.

Just found a data sheet which states that the TOTAL current through all eight outputs must not exceed 2.5A


Edited by - beaniebots on 09/01/2007 13:39:53
 
Y

ylp88

Senior Member
My bad. I forgot that the ULN2803 can only sink current.

In any case, it should be a simple process to connect one channel of the chip to a DPDT relay (as suggested by BeanieBots above) in order to provide directional control for your motor. Another channel may then be required in order to turn the motor on or off as the relay would act only as directional control and without the extra channel, the motor would continually run in forward or reverse.

Ideally you could use an L293D but keep in mind that that would require you to mount that chip on a separate circuit board (or similar). You could always try and mount it as they used to mount components in those old vintage radios (components minus PCB) but let's assume you aren't!

EDIT: Sorry again for the mistake and thanks for the correction, BeanieBots.

<b><i>ylp88 </b> </i>

Edited by - ylp88 on 09/01/2007 15:35:08
 
B

BeanieBots

Moderator
Easy oversight ylp88.
Just to expand on the reversing issue.
At least two outputs will ALWAYS be required to control a motor in both directions no matter what method is used.
The 'simplest' solution is to use a single output for on/off (PWM speed control is also possible) and another output to control the relay that does the reversing.
 
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