4051 multiplexer current

D

Data888

Guest
Hi.
Can anyone hell me what the maximum current through the signal
input-outputs on a 4051 analog multiplexer would be? Because I want to
drive a small reed relay, with a coil resistance of 500 ohms. So if I
put 5v on the input, and the relay coil on the output the current
through the IC would be 10mA. Is this ok, or will it burn out the IC?

Nick
 
Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
news:3F48668E.706@bigfoot.com.au:

Hi.
Can anyone hell me what the maximum current through the signal
input-outputs on a 4051 analog multiplexer would be? Because I want to
drive a small reed relay, with a coil resistance of 500 ohms. So if I
put 5v on the input, and the relay coil on the output the current
through the IC would be 10mA. Is this ok, or will it burn out the IC?

Nick
Click to expand...
Hi Nick,

I don't think this is gonna work. The ON resistance of the 4051 is about
the same as the resistance of your relay (at 5V supply voltage). So this
leaves only 2.5V for your relay.

You'd better use a transistor to drive the relay. And don't forget the
protection diode across the relay coil.



--
Gert van den Heuvel
webmaster www.HobbyElectronics.info
 
Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
news:3F4892EA.3060009@bigfoot.com.au:

Gert van den Heuvel wrote:
Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
news:3F48668E.706@bigfoot.com.au:


Hi.
Can anyone hell me what the maximum current through the signal
input-outputs on a 4051 analog multiplexer would be? Because I want
to drive a small reed relay, with a coil resistance of 500 ohms. So
if I put 5v on the input, and the relay coil on the output the
current through the IC would be 10mA. Is this ok, or will it burn out
the IC?

Nick




Hi Nick,

I don't think this is gonna work. The ON resistance of the 4051 is
about the same as the resistance of your relay (at 5V supply
voltage). So this leaves only 2.5V for your relay.

You'd better use a transistor to drive the relay. And don't forget
the protection diode across the relay coil.




Thanks. Is a BC547 transistor OK? Could you provide a diagram as to
how to connect them?
Click to expand...
Yep, a BC547 is just fine.
Connect the lot like this:
+5V
|
+------+
| |
relay --- Protection
coil / \ diode
| --- 1N4148
| |
+------+
|C
+---+ B|/
4051 --|10k|--| BC547
+---+ |\
|E
|
---
GND

BTW, the 4051 is acually meant to switch analog signals (like audio or
video). Since you only want to drive a relay, you may want to use a 3-to-8
decoder, such as a 74HC138. It might be somewhat cheaper. For more info, go
to http://www.semiconductors.philips.com/pip/74HC138D.html.

I just saw your previous post on the AV switch. You want to switch one
video source between 2 devices. You may still try using the 4051 to switch
the video signal. It really doesn't such a bad job. Even TV/VCR
manufacturers use similar switches. Just give it a try and see if it's good
enough for you.
The video amp comes between the 4051's output and the input of each target
device. If you have 2 target devices, you'll have to build the video amp
twice. Actually, the circuit around T1 (in
http://jap.hu/electronic/avswitch.html), isn't an amplifier, it's a video
buffer. Just like the 4051 isn't poweful enough to drive a relay, it also
isn't powerful enough to drive the video input of a TV or VCR. The buffer
doesn't amplify the voltage of the signal, it just makes it able to drive
low-resistance loads. A 5V power supply voltage is therefore not too high.
If you're absolutely sure, there will never be more than 2 target devices,
you may consider using a 4053.It contains 3 switches with 2 inputs and one
output each. See http://www.philipslogic.com/products/switches/4053/

Hope this helps,


--
Gert van den Heuvel
webmaster www.HobbyElectronics.info
 
BTW, the 4051 is acually meant to switch analog signals (like audio or
video). Since you only want to drive a relay, you may want to use a 3-to-8
decoder, such as a 74HC138. It might be somewhat cheaper. For more info, go
to http://www.semiconductors.philips.com/pip/74HC138D.html.

I just saw your previous post on the AV switch. You want to switch one
video source between 2 devices. You may still try using the 4051 to switch
the video signal. It really doesn't such a bad job. Even TV/VCR
manufacturers use similar switches. Just give it a try and see if it's good
enough for you.
The video amp comes between the 4051's output and the input of each target
device. If you have 2 target devices, you'll have to build the video amp
twice. Actually, the circuit around T1 (in
http://jap.hu/electronic/avswitch.html), isn't an amplifier, it's a video
buffer. Just like the 4051 isn't poweful enough to drive a relay, it also
isn't powerful enough to drive the video input of a TV or VCR. The buffer
doesn't amplify the voltage of the signal, it just makes it able to drive
low-resistance loads. A 5V power supply voltage is therefore not too high.
If you're absolutely sure, there will never be more than 2 target devices,
you may consider using a 4053.It contains 3 switches with 2 inputs and one
output each. See http://www.philipslogic.com/products/switches/4053/

Hope this helps,
Click to expand...
Thank you very much, you're being very helpful. I know I could just have
used an ordinary multiplexer to drive the relays, I just happened to
have an extra 4051 instead of going and getting another IC.
Basically I'm designing it using a 4029 to feed the control inputs of
the 3 4051s, to switch between up to 8 AV inputs. I might later use some
additional logic to allow the 4029 to reset itself when it gets to a
certain output, to limit the switching between a certain number of inputs.
The second output I'm just planning to manually switch between the
inputs with a rotary switch, so it's possible a video signal will be
switched 2 ways at the same time, but unlikely. So I need to put one of
these buffers on both of the outputs?

Nick
 
Gert van den Heuvel wrote:
Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
news:3F49A27D.8020907@bigfoot.com.au:


Basically I'm designing it using a 4029 to feed the control inputs of
the 3 4051s, to switch between up to 8 AV inputs. I might later use
some additional logic to allow the 4029 to reset itself when it gets
to a certain output, to limit the switching between a certain number
of inputs. The second output I'm just planning to manually switch
between the inputs with a rotary switch, so it's possible a video
signal will be switched 2 ways at the same time, but unlikely. So I
need to put one of these buffers on both of the outputs?

Nick



OK, so you have 8 inputs and 2 outputs. 1 output is switched using a 4051
and the other ouput is switched between the same 8 inputs with a rotary
switch.

I assume all 8 inputs are grounded via 75 ohms resistors (to minimize
reflection)? In that case you need buffers on both outputs. Yep, even on
the output with the rotary switch. It you don't, the internal impedance of
the device on the output will become parallel-connected to the 75 ohms
resistor at the input.
Click to expand...
I haven't terminated the lines, although I've been reading and it seems
like a good idea. Does the 75 ohm resistor go from input to
ground/shield, or just in series with the input?
 
In article <Xns93E6C5E2FB69newshobbyelectronics@195.121.6.83>,
Gert van den Heuvel <newsRemovethiS@hobbyelectronics.info> wrote:
-Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
-news:3F4EE6B7.8040804@bigfoot.com.au:
-
-> Gert van den Heuvel wrote:
->> Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
->> news:3F49A27D.8020907@bigfoot.com.au:
->>
->>
->>>Basically I'm designing it using a 4029 to feed the control inputs of
->>>the 3 4051s, to switch between up to 8 AV inputs. I might later use
->>>some additional logic to allow the 4029 to reset itself when it gets
->>>to a certain output, to limit the switching between a certain number
->>>of inputs. The second output I'm just planning to manually switch
->>>between the inputs with a rotary switch, so it's possible a video
->>>signal will be switched 2 ways at the same time, but unlikely. So I
->>>need to put one of these buffers on both of the outputs?
->>>
->>>Nick
->>>
->>
->>
->> OK, so you have 8 inputs and 2 outputs. 1 output is switched using a
->> 4051 and the other ouput is switched between the same 8 inputs with a
->> rotary switch.
->>
->> I assume all 8 inputs are grounded via 75 ohms resistors (to minimize
->> reflection)? In that case you need buffers on both outputs. Yep, even
->> on the output with the rotary switch. It you don't, the internal
->> impedance of the device on the output will become parallel-connected
->> to the 75 ohms resistor at the input.
->>
->>
->
-> I haven't terminated the lines, although I've been reading and it
-> seems like a good idea. Does the 75 ohm resistor go from input to
-> ground/shield, or just in series with the input?
->
->
-
-The resistors go from input to ground.

This is the magic thread for me. This is the next hardware project on my list.
It must be the fact that the new television season is right around the corner.

I'm planning on using 74HC4051s to switch video and stereo line level audio.
The thread has been helpful, but I have 3 issues:

1) What is the setup for line level audio? Does it require the impeadance
matching resistor too?
If so then what value?
Is an audio buffer required?

2) I know it's silly, but I presume that all of the video/audio shields are to
be grouded
to the 74HC4051's ground right? The reason I ask is while researching this
task I came across Zoomcat's parallel port switcher here:

http://www.geocities.com/zoomkat/ppswitcher3.htm

On the page he has the following statement:

-----------
All the grounds (RCA plug outside) are connected together, keeping audio
and video seperated. Note that there are no connections made to the ground
side of the parallel port connector (that would ground out the video and
audio signals). The chip grounds are connected to the common video out
(not video ground) going to the capture card.
-----------

And in fact on the diagram it does show the 74HCT259 ground connected to
the common video out.

3) The primary reason for using the HC4051 instead of individual transistors
was for packaging reasons. The video (and possibly audio) buffers will cost
another transistor per output (which certainly isn't as bad as a transitor
per input!). However it would simplify my task if I could use an opamp
package with a circuit such as:

http://ourworld.compuserve.com/homepages/g_knott/elect254.htm

#2 the only confusing thing I've found about the whole process. Any insight
would be appreciated.

BAJ
 
Byron A Jeff wrote:
This is the magic thread for me. This is the next hardware project on my list.
It must be the fact that the new television season is right around the corner.

I'm planning on using 74HC4051s to switch video and stereo line level audio.
The thread has been helpful, but I have 3 issues:

1) What is the setup for line level audio? Does it require the impeadance
matching resistor too?
If so then what value?
Is an audio buffer required?

2) I know it's silly, but I presume that all of the video/audio shields are to
be grouded
to the 74HC4051's ground right? The reason I ask is while researching this
task I came across Zoomcat's parallel port switcher here:

http://www.geocities.com/zoomkat/ppswitcher3.htm

On the page he has the following statement:

-----------
All the grounds (RCA plug outside) are connected together, keeping audio
and video seperated. Note that there are no connections made to the ground
side of the parallel port connector (that would ground out the video and
audio signals). The chip grounds are connected to the common video out
(not video ground) going to the capture card.
-----------

And in fact on the diagram it does show the 74HCT259 ground connected to
the common video out.

3) The primary reason for using the HC4051 instead of individual transistors
was for packaging reasons. The video (and possibly audio) buffers will cost
another transistor per output (which certainly isn't as bad as a transitor
per input!). However it would simplify my task if I could use an opamp
package with a circuit such as:

http://ourworld.compuserve.com/homepages/g_knott/elect254.htm

#2 the only confusing thing I've found about the whole process. Any insight
would be appreciated.

BAJ
Click to expand...
I haven't buffered anything yet. Unfortunatly the video buffer in the
design I previously linked to is not possible for me, and none of the
electronics places near me carry that transistor. I possibly have a
design for another, and I'll be posting that soon, so someone can tell
me if it's suitable.
I'm not planning on buffering the audio, as it seems to be fine as it is.
 
In article <3F502DF7.3030106@bigfoot.com.au>,
Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote:
-Byron A Jeff edited:
-> I'm planning on using 74HC4051s to switch video and stereo line level audio.
-> The thread has been helpful, but I have 3 issues:
->
-> 1) What is the setup for line level audio?
-> 2) ...I presume that all of the video/audio shields are to be grouded
-> to the 74HC4051's ground right?
-> 3) ...opamp video buffer?i
-
-I haven't buffered anything yet. Unfortunatly the video buffer in the
-design I previously linked to is not possible for me, and none of the
-electronics places near me carry that transistor. I possibly have a
-design for another, and I'll be posting that soon, so someone can tell
-me if it's suitable.

The only thing you have to worry about with the transistor is the switching
speed. Even a lowly nickel 2N2222 can easily buffer video with it's 250 Mhz
switching frequency.

-I'm not planning on buffering the audio, as it seems to be fine as it is.

Good to know.

How did you handle the grounding issue I asked about above?

BAJ
 
In article <bioopq$so4@cleon.cc.gatech.edu>,
Byron A Jeff <byron@cc.gatech.edu> wrote:

====
I'm trying to get wider coverage to my questions. The original thread can be
found in sci.electronics.basics - BAJ
====

In article <Xns93E6C5E2FB69newshobbyelectronics@195.121.6.83>,
Gert van den Heuvel <newsRemovethiS@hobbyelectronics.info> wrote:
-Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
-news:3F4EE6B7.8040804@bigfoot.com.au:
-
-> Gert van den Heuvel wrote:
->> Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
->> news:3F49A27D.8020907@bigfoot.com.au:
->>
->>
->>>Basically I'm designing it using a 4029 to feed the control inputs of
->>>the 3 4051s, to switch between up to 8 AV inputs. I might later use
->>>some additional logic to allow the 4029 to reset itself when it gets
->>>to a certain output, to limit the switching between a certain number
->>>of inputs. The second output I'm just planning to manually switch
->>>between the inputs with a rotary switch, so it's possible a video
->>>signal will be switched 2 ways at the same time, but unlikely. So I
->>>need to put one of these buffers on both of the outputs?
->>>
->>>Nick
->>>
->>
->>
->> OK, so you have 8 inputs and 2 outputs. 1 output is switched using a
->> 4051 and the other ouput is switched between the same 8 inputs with a
->> rotary switch.
->>
->> I assume all 8 inputs are grounded via 75 ohms resistors (to minimize
->> reflection)? In that case you need buffers on both outputs. Yep, even
->> on the output with the rotary switch. It you don't, the internal
->> impedance of the device on the output will become parallel-connected
->> to the 75 ohms resistor at the input.
->>
->>
->
-> I haven't terminated the lines, although I've been reading and it
-> seems like a good idea. Does the 75 ohm resistor go from input to
-> ground/shield, or just in series with the input?
->
->
-
-The resistors go from input to ground.

This is the magic thread for me. This is the next hardware project on my list.
It must be the fact that the new television season is right around the corner.

I'm planning on using 74HC4051s to switch video and stereo line level audio.
The thread has been helpful, but I have 3 issues:

1) What is the setup for line level audio? Does it require the impeadance
matching resistor too?
If so then what value?
Is an audio buffer required?

2) I know it's silly, but I presume that all of the video/audio shields are to
be grouded
to the 74HC4051's ground right? The reason I ask is while researching this
task I came across Zoomcat's parallel port switcher here:

http://www.geocities.com/zoomkat/ppswitcher3.htm

On the page he has the following statement:

-----------
All the grounds (RCA plug outside) are connected together, keeping audio
and video seperated. Note that there are no connections made to the ground
side of the parallel port connector (that would ground out the video and
audio signals). The chip grounds are connected to the common video out
(not video ground) going to the capture card.
-----------

And in fact on the diagram it does show the 74HCT259 ground connected to
the common video out.

3) The primary reason for using the HC4051 instead of individual transistors
was for packaging reasons. The video (and possibly audio) buffers will cost
another transistor per output (which certainly isn't as bad as a transitor
per input!). However it would simplify my task if I could use an opamp
package with a circuit such as:

http://ourworld.compuserve.com/homepages/g_knott/elect254.htm

#2 the only confusing thing I've found about the whole process. Any insight
would be appreciated.

BAJ
 
Byron A Jeff wrote:
In article <3F502DF7.3030106@bigfoot.com.au>,
Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote:
-Byron A Jeff edited:
-> I'm planning on using 74HC4051s to switch video and stereo line level audio.
-> The thread has been helpful, but I have 3 issues:
-
-> 1) What is the setup for line level audio?
-> 2) ...I presume that all of the video/audio shields are to be grouded
-> to the 74HC4051's ground right?
-> 3) ...opamp video buffer?i
-
-I haven't buffered anything yet. Unfortunatly the video buffer in the
-design I previously linked to is not possible for me, and none of the
-electronics places near me carry that transistor. I possibly have a
-design for another, and I'll be posting that soon, so someone can tell
-me if it's suitable.

The only thing you have to worry about with the transistor is the switching
speed. Even a lowly nickel 2N2222 can easily buffer video with it's 250 Mhz
switching frequency.

-I'm not planning on buffering the audio, as it seems to be fine as it is.

Good to know.

How did you handle the grounding issue I asked about above?

BAJ
Click to expand...
I just grounded the shields and the ICs to the same ground as the power
supply.
 
byron@cc.gatech.edu (Byron A Jeff) wrote in
news:bir0li$e37@cleon.cc.gatech.edu:

In article <bioopq$so4@cleon.cc.gatech.edu>,
Byron A Jeff <byron@cc.gatech.edu> wrote:

====
I'm trying to get wider coverage to my questions. The original thread
can be found in sci.electronics.basics - BAJ
====

In article <Xns93E6C5E2FB69newshobbyelectronics@195.121.6.83>,
Gert van den Heuvel <newsRemovethiS@hobbyelectronics.info> wrote:
-Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
-news:3F4EE6B7.8040804@bigfoot.com.au:
-
-> Gert van den Heuvel wrote:
->> Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
->> news:3F49A27D.8020907@bigfoot.com.au:
-
-
->>>Basically I'm designing it using a 4029 to feed the control inputs
of ->>>the 3 4051s, to switch between up to 8 AV inputs. I might later
use ->>>some additional logic to allow the 4029 to reset itself when
it gets ->>>to a certain output, to limit the switching between a
certain number ->>>of inputs. The second output I'm just planning to
manually switch ->>>between the inputs with a rotary switch, so it's
possible a video ->>>signal will be switched 2 ways at the same time,
but unlikely. So I ->>>need to put one of these buffers on both of the
outputs? -
->>>Nick
-
-
-
->> OK, so you have 8 inputs and 2 outputs. 1 output is switched using
a ->> 4051 and the other ouput is switched between the same 8 inputs
with a ->> rotary switch.
-
->> I assume all 8 inputs are grounded via 75 ohms resistors (to
minimize ->> reflection)? In that case you need buffers on both
outputs. Yep, even ->> on the output with the rotary switch. It you
don't, the internal ->> impedance of the device on the output will
become parallel-connected ->> to the 75 ohms resistor at the input.
-
-
-
-> I haven't terminated the lines, although I've been reading and it
-> seems like a good idea. Does the 75 ohm resistor go from input to
-> ground/shield, or just in series with the input?
-
-
-
-The resistors go from input to ground.

This is the magic thread for me. This is the next hardware project on
my list. It must be the fact that the new television season is right
around the corner.

I'm planning on using 74HC4051s to switch video and stereo line level
audio. The thread has been helpful, but I have 3 issues:

1) What is the setup for line level audio? Does it require the
impeadance
matching resistor too?
If so then what value?
Is an audio buffer required?

2) I know it's silly, but I presume that all of the video/audio
shields are to
be grouded
to the 74HC4051's ground right? The reason I ask is while
researching this task I came across Zoomcat's parallel port
switcher here:

http://www.geocities.com/zoomkat/ppswitcher3.htm

On the page he has the following statement:

-----------
All the grounds (RCA plug outside) are connected together, keeping
audio and video seperated. Note that there are no connections made
to the ground side of the parallel port connector (that would
ground out the video and audio signals). The chip grounds are
connected to the common video out (not video ground) going to the
capture card.
-----------

And in fact on the diagram it does show the 74HCT259 ground
connected to the common video out.

3) The primary reason for using the HC4051 instead of individual
transistors
was for packaging reasons. The video (and possibly audio) buffers
will cost another transistor per output (which certainly isn't as
bad as a transitor per input!). However it would simplify my task
if I could use an opamp package with a circuit such as:

http://ourworld.compuserve.com/homepages/g_knott/elect254.htm

#2 the only confusing thing I've found about the whole process. Any
insight would be appreciated.

BAJ


1) There's no audio buffer required. The input resistance of of audio
Click to expand...
devices is usually 10k or more. The 4051 just adds 500 ohms or so. This
also means that there's no need to terminate the audio lines.

2) Yes, all audio and video grounds must be connected to pin 7 of the 4051.
The digital ground is connected to pin 8.
Zoomcat uses an NPN transistor to switch audio and video. The ground of the
74HCT259 is NOT connected to ground, but to the emitter of the switching
transistor, which is connected to the center (signal) pin of the RCA
connector.

3) You only need to buffer the video signal. Opamps for video frequencies
are quite expensive, but if that's not an issue, feel free to use them.


HTH,

--
Gert van den Heuvel
webmaster www.HobbyElectronics.info
 
byron@cc.gatech.edu (Byron A Jeff) wrote in
news:bithbn$2v4@cleon.cc.gatech.edu:

In article <Xns93E89B57B943Dnewshobbyelectronics@195.121.6.67>,
Gert van den Heuvel <newsRemovethiS@hobbyelectronics.info> wrote:
-byron@cc.gatech.edu (Byron A Jeff) wrote in
-news:bir0li$e37@cleon.cc.gatech.edu:
-
-> In article <bioopq$so4@cleon.cc.gatech.edu>,
-> Byron A Jeff <byron@cc.gatech.edu> wrote:
-
-> ====
-> I'm trying to get wider coverage to my questions. The original
thread -> can be found in sci.electronics.basics - BAJ
-> ====
-
-> In article <Xns93E6C5E2FB69newshobbyelectronics@195.121.6.83>,
-> Gert van den Heuvel <newsRemovethiS@hobbyelectronics.info> wrote:
-> -Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
-> -news:3F4EE6B7.8040804@bigfoot.com.au:
-> -
-> -> Gert van den Heuvel wrote:
-> ->> Data888 <kcinSPAMBLOCK@bigfoot.com.au> wrote in
-> ->> news:3F49A27D.8020907@bigfoot.com.au:
-> -
-> -
-> ->>>Basically I'm designing it using a 4029 to feed the control
inputs -> of ->>>the 3 4051s, to switch between up to 8 AV inputs. I
might later -> use ->>>some additional logic to allow the 4029 to
reset itself when -> it gets ->>>to a certain output, to limit the
switching between a -> certain number ->>>of inputs. The second output
I'm just planning to -> manually switch ->>>between the inputs with a
rotary switch, so it's -> possible a video ->>>signal will be switched
2 ways at the same time, -> but unlikely. So I ->>>need to put one of
these buffers on both of the -> outputs? -
-> ->>>Nick
-> -
-> -
-> -
-> ->> OK, so you have 8 inputs and 2 outputs. 1 output is switched
using -> a ->> 4051 and the other ouput is switched between the same 8
inputs -> with a ->> rotary switch.
-> -
-> ->> I assume all 8 inputs are grounded via 75 ohms resistors (to
-> minimize ->> reflection)? In that case you need buffers on both
-> outputs. Yep, even ->> on the output with the rotary switch. It you
-> don't, the internal ->> impedance of the device on the output will
-> become parallel-connected ->> to the 75 ohms resistor at the input.
-> -
-> -
-> -
-> -> I haven't terminated the lines, although I've been reading and
it -> -> seems like a good idea. Does the 75 ohm resistor go from
input to -> -> ground/shield, or just in series with the input?
-> -
-> -
-> -
-> -The resistors go from input to ground.
-
-> This is the magic thread for me. This is the next hardware project
on -> my list. It must be the fact that the new television season is
right -> around the corner.
-
-> I'm planning on using 74HC4051s to switch video and stereo line
level -> audio. The thread has been helpful, but I have 3 issues:
-
-> 1) What is the setup for line level audio? Does it require the
-> impeadance
-> matching resistor too?
-> If so then what value?
-> Is an audio buffer required?
-
-> 2) I know it's silly, but I presume that all of the video/audio
-> shields are to
-> be grouded
-> to the 74HC4051's ground right? The reason I ask is while
-> researching this task I came across Zoomcat's parallel port
-> switcher here:
-
-> http://www.geocities.com/zoomkat/ppswitcher3.htm
-
-> On the page he has the following statement:
-
-> -----------
-> All the grounds (RCA plug outside) are connected together,
keeping -> audio and video seperated. Note that there are no
connections made -> to the ground side of the parallel port
connector (that would -> ground out the video and audio signals).
The chip grounds are -> connected to the common video out (not
video ground) going to the -> capture card.
-> -----------
-
-> And in fact on the diagram it does show the 74HCT259 ground
-> connected to the common video out.
-
-> 3) The primary reason for using the HC4051 instead of individual
-> transistors
-> was for packaging reasons. The video (and possibly audio)
buffers -> will cost another transistor per output (which certainly
isn't as -> bad as a transitor per input!). However it would
simplify my task -> if I could use an opamp package with a circuit
such as: -
-> http://ourworld.compuserve.com/homepages/g_knott/elect254.htm
-
-> #2 the only confusing thing I've found about the whole process. Any
-> insight would be appreciated.
-
-> BAJ
-
-
-1) There's no audio buffer required. The input resistance of of audio
-devices is usually 10k or more. The 4051 just adds 500 ohms or so.
This -also means that there's no need to terminate the audio lines.

Cool. That simplifies the task.

-
-2) Yes, all audio and video grounds must be connected to pin 7 of the
4051. -The digital ground is connected to pin 8.

Now this raises another question. What voltage should Vee (pin 7) be
relative to the digital ground. Should it be -5V?

-Zoomcat uses an NPN transistor to switch audio and video. The ground
of the -74HCT259 is NOT connected to ground, but to the emitter of the
switching -transistor, which is connected to the center (signal) pin
of the RCA -connector.

I can see what he did. What I don't understand is why he did it. It's
important because from your statement below a trasistor based buffer
seems to be the ticket.

-
-3) You only need to buffer the video signal. Opamps for video
frequencies -are quite expensive, but if that's not an issue, feel
free to use them.

Since it's only the outputs for the video (which will end up being
only two or possibly 3) transistors will be fine. Also the audio
doesn't need it. So I think I'll go the transistor route.

So here's what I know:

1) Video needs to be buffered.
2) Video inputs needs to be terminated with 75 ohm resistors.
2) Shields should be connected to Vee.

Here's what I'm still confused about:

1) Should Vee also be tied to the power supply?
2) If so should it be -5V or 0V?
3) Will Zoomcat's transistor buffer be sufficient?
4) If so then what is the relationship of the digital and analog
grounds for
the transistor?

For the most part I'm a digital circuits dude. The concept of grounds
in pretty simple in the digital world. But I'm tres confused about
this mixed signal stuff. If I were using relays as an isolated
curcuit, then of course there is no issue. But the 4051 and the
transistors create a voltage relationship between the input, output,
and digital parts of the circuit, and frankly I get lost without the
concept of connected grounds.

Let's use Zoomcat's second switcher schematic here:

http://www.geocities.com/zoomkat/pix/para9.jpg

as a sample of what I mean. It's clear that he's using the transistor
as a switch. In a typical digital senario the base emitter current
relationship is established by grounding the emitter or tying the
collector to Vcc. In either case raising the base voltage turns on the
switch. But in Zoomcat's circuit there is doesn't seem to be a voltage
relationship between the three transistor components other than the
fact that the collector and emitter share a common ground. So how does
changing voltage on the base cause movement in the emitter?

Doing what Zoomcat says not to do "grounding the video" would make
sense to me. By tying the shared ground to the digital ground of the
parallel port, then a clear voltage relationship between the
transistor components can be established. When the voltage in the base
is raised relative to the now common ground, the emitter will follow
the collector relative to the now common ground.

But the way that he has it on the full schematic:

http://www.geocities.com/zoomkat/pix/swmulti3.jpg

I find completely baffling. Grounding the emitter establishes the
relationship between it and the base. I'm fine with that. But that
means that the center conductor of the output is fixed to ground
right? So then how in the heck does it work?

This is what happens when you become a computer
scientist/microcontroller engineer instead of listening to your
parents and becoming an electrical engineer!

BTW the OP of this thread stated that he tied all of the audio, video,
and Vee all to ground and it worked great. Is that in fact the ticket?

Any insight would be greatly appreciated.


BAJ
Click to expand...
OK, if you use a symmetrical power supply (I should have noticed that
before), things are a bit different. I suppose the digital part just uses
the positive 5V supply. In that case:
* All grounds are are connected together and to pin 8 of the 4051
* -5V is connected to pin 7 (Vee).
* The video buffer must use the full power supply.
Hopefully, this answers your four questions.

I don't like much what Zoomcat is doing. The transistors aren't used as a
buffer, but as a switch. If a pin on the parallel port becomes '1', a base
current (DC!) will flow from the port, through the resistor, the base, the
emitter, and the connected device. This will saturate the transistor,
allowing the video or audio current to flow from the input to the output.

I think the OP uses a asymmetrical supply. In that case, you can connect
pin 7 to pin 8 (and all other grounds), but make sure that the DC component
at each switch must be about half the power supply voltage. Take a peek at
my Macrovision killer that uses a 4053 switch:
http://home.planet.nl/~heuve345/electronics/projects/MacrovisionKiller.html
Here, R3 and R4 add the DC component. C1 ensures this DC component doesn't
reach the input.


--
Gert van den Heuvel
webmaster www.HobbyElectronics.info
 
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