ON / ON transistor circuits

Hello All,

I have project I'm working on and could use some advice on the electronic interfacing. There's a thread about programming this project in the LED/Multipexing section of the forum but I thought the General Electronics might be a better section for this question.

The project: approximately 50-75 units, 100-150 PC fans in all, with two fans per unit. Each unit will have two fans which inflate and deflate the object. ( I cannot find many reversible PC fans ) The fans are stacked and face opposite directions. Fan A on, Fan B off = inflates. Fan A off, Fan B on = deflates.

To reduce the number of pins used to control these motors I was considering using both a NPN circuit and PNP circuit. When the pin is HIGH, one is on and the other off. When LOW, the opposite is true.

I can easily source 16 channel NPN boards but am having a hard time sourcing PNP boards ( where LOW turns output ON ). Any suggestions on where to find this or an alternative solution to controlling these two motors off one pin?

DPDT relays are an option but the clicking is an issue. ( particular with 75 units )

thanks

You have described an H-Bridge Circuit. h-bridge

Wouldn't an H-Bridge require numerous pins to control it?

Lubby:
To reduce the number of pins used to control these motors I was considering using both a NPN circuit and PNP circuit. When the pin is HIGH, one is on and the other off. When LOW, the opposite is true.

You may find you won't be able to inflate anything much with a PC fan, they don't do much back-pressure
typically (but it should be on the datasheet).
I suspect a blower is needed, something like:http://uk.farnell.com/sanyo-denki-sanace-fans/9bmb12p2k01/blower-97x33mm-12vdc/dp/2292897 (that example can do over 1kPa back pressure)

And if you have two of them you'll just be pushing air through the other fan anyway, so you'll need flap valves of
some sort...

Perhaps if you were more forthcoming about what you are trying to do, we'd be less in xyproblem territory
and more able to suggest realistic approaches. Inflate what? How fast?

Lubby:
( I cannot find many reversible PC fans )

That's because aerofoils only work in one direction.

Lubby:
Wouldn't an H-Bridge require numerous pins to control it?

Theoretically, a minimum of one - direction. But an enable allows for freewheeling (an off state), and independent control signals for each branch allows for dynamic braking. So a maximum of three for all the features you could use. Not too many compared with many other devices...

Hi,
I think you need something like this;

TwoFanControl.jpg2066×1012 122 KB

Tom..

TwoFanControl.jpg2066×1012 122 KB

Nice schematic TomGeorge!
That would do it.

After some testing the fans are inflating plastic bags well. There's not much pressure required. A higher current 120 x 120mm fan does it. When off things deflate quick enough without any fan deflating. Just an ON /OFF of one fan should do it.

Thanks for the feedback.