circuit question
Archiver
Posts: 46,084
does it matter which order you put leds and resistors?
Is p0>---\/\/\--->|---Vss = p0>--->|---\/\/\---Vss ???
also, is the resistor protecting the led or is it protecting the stamp - really
what is its purpose?
mkl
[noparse][[/noparse]Non-text portions of this message have been removed]
Is p0>---\/\/\--->|---Vss = p0>--->|---\/\/\---Vss ???
also, is the resistor protecting the led or is it protecting the stamp - really
what is its purpose?
mkl
[noparse][[/noparse]Non-text portions of this message have been removed]
Comments
positive supply then the LED. The reason is incase there is a short in the
circuit, worst case is you will ground the resistor and the current will be
limited to .....voltage value / resistor value.
The purpose of the resistor is to limit the current through the LED and to
protect the Stamp.
Resistor selection is done as follows.......A typical LED consumes 2 volts in
the forward bias direction.
1) You select what current you want through the LED (say 10mA for ex)
2) You subtract the LED forward voltage from the voltage used to power the
LED (for example 5 volts, minus 2 volts, leaves three volts)
3) Take this 3 volts and divide it by the current you want (10mA) and that is
your resistor value.
Therefore 3 volts / 10mA is 300 ohms. 270 ohms will work just fine and give
you a little more than 10 ma. Remember that the BS2 can only sink (ground)
25 mA at any one time . Therefore if you want several LED's on at once you
will need to have the Stamp turn on a transistor that will in turn control
your LED. This way the stamp needs to supply just a fraction (less than 1
ma) of the 10 mA the LED needs.
A simple transistor like a 2n3904 will do just fine. If you need additional
information on how to hook up the transistor, write back, it is very easy to
do....
[noparse][[/noparse]Non-text portions of this message have been removed]
outputs." and am likely to light up 32 + leds at once by the time I am done.
I have several transistors i have desoldered from various old answering
machines etc. I understand the basics of how the transistor works with its
3 prongs, the middle being the control. I understand that some are NPN and
others are PNP. I have no other idea about them and what their numbers mean
or how to tell how much volts/amps i'm allowed to put on their prongs.
Also, is this the main application of a transistor - to take the load off of
the controlling circuit?
Further, I am wondering about the 74hc595 chip used in expirements 23 & 23b
of stampworks:
Expirement 23b daisy chains 2 74hc595s together and controls them with the
SHIFTOUT command. This is great - 16 outputs for 3 stamp pins!
The last parameter of the SHIFTOUT command is "bits" and takes a value of
1 - 16.
Does this mean that the stamp can only control 2 74hc595s using only 3 pins
on the stamp or can you daisy chain say 4 74hc595s and it is just that you
have to use SHIFTOUT twice to set all 32 pins.
(I would just try this instead of asking but I only have the 2 74hc595s that
came with the kit.)
Original Message
From: <smartdim@a...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, November 24, 2002 2:32 PM
Subject: Re: [noparse][[/noparse]basicstamps] circuit question
> The order does not matter. However, I like to put the resitor to the
> positive supply then the LED. The reason is incase there is a short in the
> circuit, worst case is you will ground the resistor and the current will
be
> limited to .....voltage value / resistor value.
>
> The purpose of the resistor is to limit the current through the LED and to
> protect the Stamp.
>
> Resistor selection is done as follows.......A typical LED consumes 2 volts
in
> the forward bias direction.
> 1) You select what current you want through the LED (say 10mA for ex)
> 2) You subtract the LED forward voltage from the voltage used to power the
> LED (for example 5 volts, minus 2 volts, leaves three volts)
> 3) Take this 3 volts and divide it by the current you want (10mA) and that
is
> your resistor value.
> Therefore 3 volts / 10mA is 300 ohms. 270 ohms will work just fine and
give
> you a little more than 10 ma. Remember that the BS2 can only sink
(ground)
> 25 mA at any one time . Therefore if you want several LED's on at once you
> will need to have the Stamp turn on a transistor that will in turn control
> your LED. This way the stamp needs to supply just a fraction (less than 1
> ma) of the 10 mA the LED needs.
>
> A simple transistor like a 2n3904 will do just fine. If you need
additional
> information on how to hook up the transistor, write back, it is very easy
to
> do....
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
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Body of the message will be ignored.
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>
>
mklorenz@c... writes:
> I will take you up on the advise about transistors as I am "expanding [noparse][[/noparse]my]
> outputs." and am likely to light up 32 + leds at once by the time I am
> done.
>
> I have several transistors i have desoldered from various old answering
> machines etc. I understand the basics of how the transistor works with its
> 3 prongs, the middle being the control. I understand that some are NPN and
> others are PNP. I have no other idea about them and what their numbers
> mean
> or how to tell how much volts/amps i'm allowed to put on their prongs.
>
> Also, is this the main application of a transistor - to take the load off
> of
> the controlling circuit? YES
>
> Further, I am wondering about the 74hc595 chip used in expirements 23 &23b
> of stampworks:
>
> Expirement 23b daisy chains 2 74hc595s together and controls them with the
> SHIFTOUT command. This is great - 16 outputs for 3 stamp pins!
>
> The last parameter of the SHIFTOUT command is "bits" and takes a value of
> 1 - 16.
>
> Does this mean that the stamp can only control 2 74hc595s using only 3 pins
> on the stamp or can you daisy chain say 4 74hc595s and it is just that you
> have to use SHIFTOUT twice to set all 32 pins. YES you can daisy chain, I
> don't immediately remember but i think there is a maximum mumber of bits
> you can send out using SHIFTOUT ( I will check data book on Monday)
>
> (I would just try this instead of asking but I only have the 2 74hc595s
> that
> came with the kit.)
>
>
>
Regarding the transistors.....USE NPN transistors. The emitter goes to
ground, the base will go to the output of your 74 hc 595. Make the
particular bit high to turn on the transistor which will ground the LED
resistor circuit.
Typical values as follows....
Connect a 10k resistor between the output pin of the '595 and the base of the
NPN transistor.
Connect the emitter of the transistor to ground.
Connect the collector of the transistor to the "ground side" of your LED
RESISTOR and the other end of the LED RESISTOR to +5 Volts.
When the bit out of the '595 if high, the transistor will turn on providing
ground for the LED Reistor pair. Conversely, when the bit is low, the
transistor is off and the led off.
Don't hesitate to ask more questions if you need help
[noparse][[/noparse]Non-text portions of this message have been removed]
> Does this mean that the stamp can only control 2 74hc595s using only 3 pins
> on the stamp or can you daisy chain say 4 74hc595s and it is just that you
> have to use SHIFTOUT twice to set all 32 pins.
>
> (I would just try this instead of asking but I only have the 2 74hc595s that
> came with the kit.)
You can hook up as many as you want up to a limit dictated by signal
strengths, line lengths, etc.
8 cascaded 595's for 64 outputs are no problem using a PIC16F84, so should
be fine with the stamp too.
HTH.
Mos.
--
84 AE86, 90 ST185GrpA, 91 MX83Gr, Sydney, Oz.