8574 I/O extenders
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I've got what should be a relatively simple question. I'm kinda new
to electronics (or just naturally stupid).
I'm using a Philips 8574 w/ the BS2p. Basically, the 8574 outputs a
negative voltage from its outputs (unlike the stamp which is +5v). I
want to create a relay driver from this output. I tried using a PNP
(3906) transistor, but it isnt working.
_______ -(LED)-______ GROUND
|
|
C
- output ----B ( )
E
|
|______ + 9VDC
Am I doing something wrong here? Do I have to invert the output? I'm
lost! If it was a positive output I'd just use an NPN (2222)
transistor and all would be good!
-Michael Brown
to electronics (or just naturally stupid).
I'm using a Philips 8574 w/ the BS2p. Basically, the 8574 outputs a
negative voltage from its outputs (unlike the stamp which is +5v). I
want to create a relay driver from this output. I tried using a PNP
(3906) transistor, but it isnt working.
_______ -(LED)-______ GROUND
|
|
C
- output ----B ( )
E
|
|______ + 9VDC
Am I doing something wrong here? Do I have to invert the output? I'm
lost! If it was a positive output I'd just use an NPN (2222)
transistor and all would be good!
-Michael Brown
Comments
In your diagram I don't see the cathode and anode terminals labeled on your LED
so maybe you might have it reversed? ie (since the led is a light emitting
*diode* it will only conduct one way).
-=Randy Knutson
mikeathome666 <mikeathome@x...> wrote: I've got what should be a
relatively simple question. I'm kinda new
to electronics (or just naturally stupid).
I'm using a Philips 8574 w/ the BS2p. Basically, the 8574 outputs a
negative voltage from its outputs (unlike the stamp which is +5v). I
want to create a relay driver from this output. I tried using a PNP
(3906) transistor, but it isnt working.
_______ -(LED)-______ GROUND
|
|
C
- output ----B ( )
E
|
|______ + 9VDC
Am I doing something wrong here? Do I have to invert the output? I'm
lost! If it was a positive output I'd just use an NPN (2222)
transistor and all would be good!
-Michael Brown
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[noparse][[/noparse]Non-text portions of this message have been removed]
>I've got what should be a relatively simple question. I'm kinda new
>to electronics (or just naturally stupid).
>
>I'm using a Philips 8574 w/ the BS2p. Basically, the 8574 outputs a
>negative voltage from its outputs (unlike the stamp which is +5v). I
>want to create a relay driver from this output. I tried using a PNP
>(3906) transistor, but it isnt working.
>
> __1k___ +(LED)-______ GROUND
> |
> |
> C
>-- 6k2 --+--- B ( PNP
> | E
> 1k |
> |___ ______+______ + 9VDC
>
>Am I doing something wrong here? Do I have to invert the output? I'm
>lost! If it was a positive output I'd just use an NPN (2222)
>transistor and all would be good!
You have to make the base of the transistor see less than 0.5V with respect
to the emitter for the transistor to remain OFF. Assume that the logic HI
from the 8574 is 5V. You need to drop 3.5V across the input series resistor.
The above resistor values should work. Don't forget the series resistor
for the LED!
Another method:
__1k___ -(LED)+______ +9V
|
|
C
+5V -- 4k7 -- B ( NPN
E
|
input from ______|
8574
This version is a simple level shifter. When the output from the 8574 is
at 5V, both the base and emitter of the transistor are at the same level
and the transistor is OFF. When the output of the 8574 goes LO, the
transistor turns ON. Note that all the load current winds up being sunk by
the output pin of the 8574.
One more method - suitable for driving LEDs:
_______ -(LED)+______ +9V
|
|
C
+5V -- B ( NPN
E
|
input from __470R_|
8574
This functions as a switched constant current sink. Similar deal as above
except that the base voltage is held at 5V. Output current is set at (5V -
Vbe) / R emitter and is relatively constant even though the 9V supply varies.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 18 years of Engineering Innovation (1984 - 2002)
.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
`-' `-' `-' `-' `-' `-' `-' `-' `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
dwayner@p... writes:
> This version is a simple level shifter. When the output from the 8574 is
> at 5V, both the base and emitter of the transistor are at the same level
> and the transistor is OFF. When the output of the 8574 goes LO, the
> transistor turns ON. Note that all the load current winds up being sunk by
>
Therein lies the problem -- the 8574 is only capable of sinking 3 mA.
[noparse][[/noparse]Non-text portions of this message have been removed]
> At 05:57 AM 6/20/02 +0000, mikeathome666 wrote:
> >I've got what should be a relatively simple
> question. I'm kinda new
> >to electronics (or just naturally stupid).
> >
> >I'm using a Philips 8574 w/ the BS2p. Basically,
> the 8574 outputs a
> >negative voltage from its outputs (unlike the stamp
> which is +5v). I
> >want to create a relay driver from this output. I
> tried using a PNP
> >(3906) transistor, but it isnt working.
> >
> > __1k___ +(LED)-______ GROUND
> > |
> > |
> > C
> >-- 6k2 --+--- B ( PNP
> > | E
> > 1k |
> > |___ ______+______ + 9VDC
> >
> >Am I doing something wrong here? Do I have to
> invert the output? I'm
> >lost! If it was a positive output I'd just use an
> NPN (2222)
> >transistor and all would be good!
>
> You have to make the base of the transistor see less
> than 0.5V with respect
> to the emitter for the transistor to remain OFF.
> Assume that the logic HI
> from the 8574 is 5V. You need to drop 3.5V across
> the input series resistor.
>
> The above resistor values should work. Don't forget
> the series resistor
> for the LED!
>
> Another method:
>
> __1k___ -(LED)+______ +9V
> |
> |
> C
> +5V -- 4k7 -- B ( NPN
> E
> |
> input from ______|
> 8574
>
> This version is a simple level shifter. When the
> output from the 8574 is
> at 5V, both the base and emitter of the transistor
> are at the same level
> and the transistor is OFF. When the output of the
> 8574 goes LO, the
> transistor turns ON. Note that all the load current
> winds up being sunk by
> the output pin of the 8574.
>
> One more method - suitable for driving LEDs:
>
> _______ -(LED)+______ +9V
> |
> |
> C
> +5V -- B ( NPN
> E
> |
> input from __470R_|
> 8574
>
> This functions as a switched constant current sink.
> Similar deal as above
> except that the base voltage is held at 5V. Output
> current is set at (5V -
> Vbe) / R emitter and is relatively constant even
> though the 9V supply varies.
>
> dwayne
>
> --
> Dwayne Reid <dwayner@p...>
> Trinity Electronics Systems Ltd Edmonton, AB,
> CANADA
> (780) 489-3199 voice (780) 487-6397 fax
>
> Celebrating 18 years of Engineering Innovation (1984
> - 2002)
> .-. .-. .-. .-. .-. .-. .-. .-.
> .-. .-
> `-' `-' `-' `-' `-' `-' `-' `-'
> `-'
> Do NOT send unsolicited commercial email to this
> email address.
> This message neither grants consent to receive
> unsolicited
> commercial email nor is intended to solicit
> commercial email.
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> 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/
>
>
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