PCF8574 to control relays
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I was thinking of using the I/O expander to drive a set of relays (7
of them). Now since there is no way it can source or sink that much
current, I want to switch some transistors to turn the relays on and
off.
Since the I/O expander defaults to high on all pins at startup I
can't use the normal pnp transistor setup (can't have everything
turned on at once during power up). I'm thinking I can go NPN and
have the relays turned on when the pin is low.
Is this the correct thinking? If so, what would be a good transistor
to use? I'm using 5v reed relays to control 12v ac 8watt solenoids.
If not, whats the correct way?
of them). Now since there is no way it can source or sink that much
current, I want to switch some transistors to turn the relays on and
off.
Since the I/O expander defaults to high on all pins at startup I
can't use the normal pnp transistor setup (can't have everything
turned on at once during power up). I'm thinking I can go NPN and
have the relays turned on when the pin is low.
Is this the correct thinking? If so, what would be a good transistor
to use? I'm using 5v reed relays to control 12v ac 8watt solenoids.
If not, whats the correct way?
Comments
There's a commonly used darlington, TIP120, that may be what you
need...
--- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> I was thinking of using the I/O expander to drive a set of relays
(7
> of them). Now since there is no way it can source or sink that much
> current, I want to switch some transistors to turn the relays on
and
> off.
>
> Since the I/O expander defaults to high on all pins at startup I
> can't use the normal pnp transistor setup (can't have everything
> turned on at once during power up). I'm thinking I can go NPN and
> have the relays turned on when the pin is low.
>
> Is this the correct thinking? If so, what would be a good
transistor
> to use? I'm using 5v reed relays to control 12v ac 8watt solenoids.
> If not, whats the correct way?
understand what I'm looking at. How can you tell what current it
needs to stay off? The PCF8574 can only source 300uA...
--- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> Sounds good to me.
> There's a commonly used darlington, TIP120, that may be what you
> need...
>
> --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> > I was thinking of using the I/O expander to drive a set of relays
> (7
> > of them). Now since there is no way it can source or sink that
much
> > current, I want to switch some transistors to turn the relays on
> and
> > off.
> >
> > Since the I/O expander defaults to high on all pins at startup I
> > can't use the normal pnp transistor setup (can't have everything
> > turned on at once during power up). I'm thinking I can go NPN and
> > have the relays turned on when the pin is low.
> >
> > Is this the correct thinking? If so, what would be a good
> transistor
> > to use? I'm using 5v reed relays to control 12v ac 8watt
solenoids.
> > If not, whats the correct way?
sourcing. That gives me 20mA per pin but 100ma max total for the chip.
I was also looking for low power npn darlingtons, the BC517 seems
closer to the specs I need (assuming its actually available anywhere).
--- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> I was just checking out the datasheet for that and I just don't
> understand what I'm looking at. How can you tell what current it
> needs to stay off? The PCF8574 can only source 300uA...
>
> --- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> > Sounds good to me.
> > There's a commonly used darlington, TIP120, that may be what you
> > need...
> >
> > --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> > > I was thinking of using the I/O expander to drive a set of
relays
> > (7
> > > of them). Now since there is no way it can source or sink that
> much
> > > current, I want to switch some transistors to turn the relays
on
> > and
> > > off.
> > >
> > > Since the I/O expander defaults to high on all pins at startup
I
> > > can't use the normal pnp transistor setup (can't have
everything
> > > turned on at once during power up). I'm thinking I can go NPN
and
> > > have the relays turned on when the pin is low.
> > >
> > > Is this the correct thinking? If so, what would be a good
> > transistor
> > > to use? I'm using 5v reed relays to control 12v ac 8watt
> solenoids.
> > > If not, whats the correct way?
The 74HC245 is a bidirectional buffer. I use it in a LED monitor circuit so
I can monitor buses without loading the circuit I'm monitoring...
Cheers,
Ben.
> Actually what am I smokin'... I'll be sinking with the 8574, not
> sourcing. That gives me 20mA per pin but 100ma max total for the chip.
>
> I was also looking for low power npn darlingtons, the BC517 seems
> closer to the specs I need (assuming its actually available anywhere).
>
> --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
>> I was just checking out the datasheet for that and I just don't
>> understand what I'm looking at. How can you tell what current it
>> needs to stay off? The PCF8574 can only source 300uA...
>>
>> --- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
>>> Sounds good to me.
>>> There's a commonly used darlington, TIP120, that may be what you
>>> need...
>>>
>>> --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
>>>> I was thinking of using the I/O expander to drive a set of
> relays
>>> (7
>>>> of them). Now since there is no way it can source or sink that
>> much
>>>> current, I want to switch some transistors to turn the relays
> on
>>> and
>>>> off.
>>>>
>>>> Since the I/O expander defaults to high on all pins at startup
> I
>>>> can't use the normal pnp transistor setup (can't have
> everything
>>>> turned on at once during power up). I'm thinking I can go NPN
> and
>>>> have the relays turned on when the pin is low.
>>>>
>>>> Is this the correct thinking? If so, what would be a good
>>> transistor
>>>> to use? I'm using 5v reed relays to control 12v ac 8watt
>> solenoids.
>>>> If not, whats the correct way?
>
>
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--
http://www.lennard.net.nz/
Ben Lennard, NCEE, Dip EE
Electronics R&D
Hm: +64 4 972 7567
Mb: +64 21 536 627
87 Spencer Street
Crofton Downs
Wellington
New Zealand
"To the optimist, the glass is half full. To the pessimist, the glass is
half empty. To the engineer, the glass is twice as big as it needs to be."
No animals were harmed in the transmission of this email, although the
Dog next door is living on borrowed time, let me tell you! Those of you
with an overwhelming fear of the unknown will be gratified to learn that
there is no hidden message revealed by reading this warning backwards.
sorry
--- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> I was just checking out the datasheet for that and I just don't
> understand what I'm looking at. How can you tell what current it
> needs to stay off? The PCF8574 can only source 300uA...
>
> --- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> > Sounds good to me.
> > There's a commonly used darlington, TIP120, that may be what you
> > need...
> >
> > --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> > > I was thinking of using the I/O expander to drive a set of
relays
> > (7
> > > of them). Now since there is no way it can source or sink that
> much
> > > current, I want to switch some transistors to turn the relays
on
> > and
> > > off.
> > >
> > > Since the I/O expander defaults to high on all pins at startup
I
> > > can't use the normal pnp transistor setup (can't have
everything
> > > turned on at once during power up). I'm thinking I can go NPN
and
> > > have the relays turned on when the pin is low.
> > >
> > > Is this the correct thinking? If so, what would be a good
> > transistor
> > > to use? I'm using 5v reed relays to control 12v ac 8watt
> solenoids.
> > > If not, whats the correct way?
to use that as a isolator from my parallel cable to a board for a
similar project.
Dave
--- In basicstamps@y..., Ben <ben@l...> wrote:
> I'd use a 74HC245 between the PCF8574 and the other components.
>
> The 74HC245 is a bidirectional buffer. I use it in a LED monitor
circuit so
> I can monitor buses without loading the circuit I'm monitoring...
>
> Cheers,
>
> Ben.
>
>
> > Actually what am I smokin'... I'll be sinking with the 8574, not
> > sourcing. That gives me 20mA per pin but 100ma max total for the
chip.
> >
> > I was also looking for low power npn darlingtons, the BC517 seems
> > closer to the specs I need (assuming its actually available
anywhere).
> >
> > --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> >> I was just checking out the datasheet for that and I just don't
> >> understand what I'm looking at. How can you tell what current it
> >> needs to stay off? The PCF8574 can only source 300uA...
> >>
> >> --- In basicstamps@y..., "kvasilak" <my427v8@h...> wrote:
> >>> Sounds good to me.
> >>> There's a commonly used darlington, TIP120, that may be what you
> >>> need...
> >>>
> >>> --- In basicstamps@y..., "ghidera2000" <ghidera2000@y...> wrote:
> >>>> I was thinking of using the I/O expander to drive a set of
> > relays
> >>> (7
> >>>> of them). Now since there is no way it can source or sink that
> >> much
> >>>> current, I want to switch some transistors to turn the relays
> > on
> >>> and
> >>>> off.
> >>>>
> >>>> Since the I/O expander defaults to high on all pins at startup
> > I
> >>>> can't use the normal pnp transistor setup (can't have
> > everything
> >>>> turned on at once during power up). I'm thinking I can go NPN
> > and
> >>>> have the relays turned on when the pin is low.
> >>>>
> >>>> Is this the correct thinking? If so, what would be a good
> >>> transistor
> >>>> to use? I'm using 5v reed relays to control 12v ac 8watt
> >> solenoids.
> >>>> If not, whats the correct way?
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@y...
> > from the same email address that you subscribed. Text in the
Subject and Body
> > of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >
> >
>
> --
> http://www.lennard.net.nz/
> Ben Lennard, NCEE, Dip EE
>
> Electronics R&D
>
> Hm: +64 4 972 7567
> Mb: +64 21 536 627
> 87 Spencer Street
> Crofton Downs
> Wellington
> New Zealand
>
> "To the optimist, the glass is half full. To the pessimist, the
glass is
> half empty. To the engineer, the glass is twice as big as it needs
to be."
>
> No animals were harmed in the transmission of this email, although
the
> Dog next door is living on borrowed time, let me tell you! Those
of you
> with an overwhelming fear of the unknown will be gratified to learn
that
> there is no hidden message revealed by reading this warning
backwards.