Interfacing to Relays

T

tomlamonby

New Member
Hi guys

Sure you heard it many times, but I'm no expert so I'm not blaming anyone if the theory of this post is comical..

I'm wanting to buy parts for a project I'm planning. The project will involve using a Picaxe to control six RS 4V submersible mini water pumps: http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0480140
I would like to use the 28x1 starter pack project board as I already have it, and for ease of use. The system should allow the pumps to be switched on and off seperately, and also all together.
So my question is, can I use the relay interfacing circuit on page 13, section 3 of the picaxe manual? Could the board support six of these circuits? The boards collective output is 100 mA correct? Which transistor is best for the job? I saw on another thread the 2N2222 NPN was suggested for speed. Also would this relay be suitable?: http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0102490 if not, could someone suggest a better one? Bearing in mind the pumps themselves would be powered by a recheargeable sealed lead acid battery.
Basically I like the circuits simplicity, but could my board give enough juice to support the six relays using this interfacing method?

Hope the post wasn't too long winded, look forwards to hearing your advice.
Thanks
Tom
 
E

eclectic

Moderator
Hello Tom and welcome to the Forum.

A couple of suggestions.

1. Download the latest version of the manuals, from Datasheets above.

2. The relay circuit is shown in Man. 3, page 7. You need 6.

Look back one page to page 6: The Darlington ULN2803.

You already own one. :)

It might suit your project very well.

e
 
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T

tomlamonby

New Member
works like this?

Thanks for the quick reply! Yeah this looks like a good solution, especially as its already on the board, and I don't have to worry about back EMF. So in that diagram, the motors would be replaced by the relays... is that right? which in turn I would wire up to the higher power source and my pumps? I really appreciate this suggestion of yours, just need to understand it good!

Thanks in advance :)

Tom
 
T

tomlamonby

New Member
also...

Sorry I forgot to add.. Would those relays from my first question be trippable even together as six from different outputs on the 28x board? or is it too much?

Thanks
 
hippy

hippy

Ex-Staff (retired)
Yes, that's the principle. Rather than the Darlington turning on, conducting current through the motor making it rotate, the Darlington turns on and conducts current through the relay to switch its contacts.

Each Darlington channel (ULN2803A) is rated at 500mA so as long as each relay is less than that it's fine. The maximum total current will be number of relays x relay current. That is drawn through the relay power supply and back through the 0V. As long as you have good enough cable and a battery to deliver the current, no problem.

The PICAXE will only use a small amount of current to switch the Darlingtons so it won't be affected by whatever current the relays use.
 
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T

tomlamonby

New Member
working

Ok thanks for helping everyone, got it all working. As Ibenson said the relays are somewhat industrial in nature, but I needed some fairly heavy duty components, as the system im building needs to be able to interface with some higher current dc stuff.
Thanks again
:)
Tom
 
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