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Topic: Pager motors (Read 1 time) previous topic - next topic

2MuchNFO

Hi all,
has anyone found a good source for pager motors (the little vibration motors)? Pref in the UK to reduce shipping costs? I've found a few but they seem to be really expensive.

Also, any advice on the safest way to connect to the Arduino?  I know that I can simply connect to a digital pin and spin the motor but my lack of knowledge of the component side of things means I'm not sure if I'm damaging one or the other.

Many thanks

Groove

Even tiny pager vibras can draw more current than an AVR pin can provide.
Always use a transistor.
Per Arduino ad Astra

Grumpy_Mike

#2
Sep 29, 2010, 12:18 pm Last Edit: Sep 29, 2010, 12:30 pm by Grumpy_Mike Reason: 1

2MuchNFO

Thanks for the links and info.

one that looks suitable states...1G vibration at 12,000rpm from 3V, drawing less than 80mA.

Now you electronics folk are all nodding sagely but i'm looking at it through the mist of ignorance (wow, quite poetic)

From my limited understanding I believe that this is still drawing more mA than the Arduino is happy to supply?  ...and what about the 3V side of things?
Would someone be kind enough to explain how I would connect this so that I don't blow myself up. :-[

Many thanks.

Groove

You won't blow yourself up (unless you're in an explosive atmosphere) but your Arduino is another matter.
80mA is twice the maximum current an AVR pin can supply.
Per Arduino ad Astra

Grumpy_Mike

#5
Sep 29, 2010, 01:39 pm Last Edit: Sep 29, 2010, 01:42 pm by Grumpy_Mike Reason: 1
Just treat it like a motor or relay:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html
A single transistor will do.

Quote
and what about the 3V side of things

Use a resistor.
you want 2 volts drop with 80mA going through it = 2 / 0.8 = 25R (ohms)

and it will burn 0.08 * 2 = 0.16 watts so get one that is at least 0.25 W rating

2MuchNFO

There is a danger that I may be starting to understand this..Not sure but I do see a little light in the distance.

If I power the arduino from a 9v battery, can I use the same battery to power the motor? Seem to remember reading that I had to ensure they have a commom ground?

I guess I would need a 6V drop.
Is that 6 / 0.8 =  75R resistor.

(first time I've understood that calculation - what a breakthrough!)

Can I use a similar calculation for working out what resistor I need to connect to the base of the transistor?  I have transistors in my spares box but not sure what type unfortunately. Can I use a multimeter to assist?


Grumpy_Mike

#7
Sep 29, 2010, 05:50 pm Last Edit: Sep 29, 2010, 05:51 pm by Grumpy_Mike Reason: 1
Quote
Is that 6 / 0.8 =  75R resistor.

Close, the motor takes 80mA not 800mA so that will be:-
6 / 0.08 = 750R
Quote
Can I use a similar calculation for working out what resistor I need to connect to the base of the transistor?


Yes the current you want in the collector is the current in the base time the gain of the transistor. If you don't know the gain then guess at say 50, if it is an under estimate it doesn't matter.

Quote
Seem to remember reading that I had to ensure they have a commom ground?

Yes here is why http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

2MuchNFO

Just when I though I had it.
If 2/.08 is 25R
why isn't 6/.08  = 75R  (accepting that I wrote .8 in error)

...and if it IS 75R the Power Rating works out at .48W (again 'I think') so not sure if my current stash of resistors are up to it.

Quote
Yes the current you want in the collector is the current in the base time the gain of the transistor. If you don't know the gain then guess at say 50, if it is an under estimate it doesn't matter.

Yep, that's it. I've gone.  :o Guess I have lots more reading to do  :-[

As ever, many thanks for your assistance GM

Grumpy_Mike

Quote
why isn't 6/.08  = 75R  (accepting that I wrote .8 in error)

Because I saw you had written 0.8 and thought you had the sum right so adjusted your answer.  :-[

Quote

Yep, that's it. I've gone

No you haven't. Suppose you have 1mA of base current in a transistor with a gain of 50 then you can get 50mA flowing from collector to base. I say can because if you have a resistor in line that couldn't let that amount of current  flow with the given voltage then we say the transistor would be saturated. That is it would be on like a switch. That is all you want here you don't want the exact amount of current to flow the collector resistor will take care of that you just want to saturate the transistor so the exact base current doesn't matter much so long as you have enough to get it saturated.

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