[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

Oh this is gold, serious. I noticed that the datasheets for optoisolators 
don't give you the resistor to limit the LED (which is necessary). This 
calculator in conjunction with the datasheet WILL give you the necessary 
limiting resistor value.

http://led.linear1.org/1led.wiz

On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

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[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

Talking about multistaging things, I have several discretes networks (my 
"bag of tricks") that I kind of patch together when I need a given function.

One of my favorites is SSR-to-Relay. The link is to a microminiture PCB 
relay that what you do is you take a BBB GPIO out pin and then activate the 
SSR (which also gives optoisolation) and drive this relay. Its beefy enough 
for most applications, but you can further stage it to a big 120VAC power 
relay if you need to throw a really big switch.

http://www.mouser.com/ProductDetail/Omron-Electronics/G5LE-1-ASI-DC6/?qs=sGAEpiMZZMvi6wO7nhr1L1%252bISpNYs8NG

On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

OK, I think you can do a bidirectional line this way with a $1 SSR (solid 
state relay)...maybe

http://www.mouser.com/ProductDetail/Sharp-Microelectronics/PR26MF11NSZF/?qs=sGAEpiMZZMvD44QvxK4%2fifSkPJVuODho

I use these primarily for a MCU (like a BBB GPIO out) to driver stage in 
like ESC circuits, or to drive a beefier electromechanical relay. Right 
price, really versatile.

On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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[beagleboard] Re: correct connection of an opto-coupler

[mzimmers]

Hi Woody -

Thank you for the detailed replies. I appreciate the effort you put in on 
these. I'm currently tied up with another project, but I will return to 
this before long.

Thanks again.

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[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

OK, here is another idea that performs a similar process, however it can 
only work from a higher voltage level down to a lower one (like a TTL 5V 
down to a BBB 3.3v level, NOT the other way around...you'll have to use the 
optoisolator approach for that). How a voltage divider works is by the 
ratio of the two resistor values you select:




OK, how I always conceive of the schematic of a voltage divider is the 
schematic on the far left. Your Vin would be the 5V digital signal (NOT 
VCC, the signal itself) you were trying to divide down to 3.3v (remember it 
can't do the up conversion, it'll only go "downhill") which appears on the 
Vout.


I even have the optimal resistor values for you that make sure the currents 
right, strong enough to drive the logic but weak enough that it doesn't 
consume any power really.


For 5V to 3.3V signal conversion:


R1 = 1700 Ohms (or a 1.7K resistor)

R2 = 3300 Ohms (or a 3.3K resistor)


Breadboard it out first. Take two resistors of the previously stated 
values, manually apply a continuous +5VDC to the Vin, tap it between the 
two resistors (don't forget the ground connection its how voltage division 
actually occurs) and verify with DMM that the output voltage is indeed 
3.3v. Now you have a nice little way of doing that voltage conversion. Add 
it to you bag of tricks.


For the newbies, whatever you do DON'T dream of passing ALL your signals 
over big arrays of optoisolators and voltage dividers, just for elegance 
sake. What you do is you run multiple voltage "domains" on your board and 
then when you absolutely, positively have to do the conversion, do so. For 
example, don't surround a TTL MCU with several optoisolators, run it off of 
a 5V bus (common ground though) and thread through the single signal (like 
a UART_TX from the BBB through the optoisolator). On the board level 
remember you don't necessarily have to thread the UART_RX back a lot of 
time (like for status). A good example is my PIC based motor speed 
control...it only receives a TX. You don't have to check at the board 
level, it always works...saving you a voltage divider back to the BBB.




On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

OK, electrically let's talk about exactly what is going on here. Basically 
a phototransistor is like mounting a small solar cell to the gate of a 
transistor and running the transistor in a switching (rather than 
amplifying) mode. With the LED butted right up against the photosensitive 
gate it provides more than enough light energy to provide the switching 
effect we are seeking. Switching can be looked at as merely amplification 
past the limits of the transistor...the transistor merely becomes 
conductive, like a switch.

So you just hook up the input like you would any other LED (because it IS 
an LED) that part is easy, however what you do is you set up the output 
like in the schematic, by connecting the appropriate leg to VCC (in the 
case of TTL +5V) determined by which way the diode effect goes (you don't 
want the voltage to go against it, or it just won't switch...just logically 
figure it out, or breadboard it first). And then the other output leg 
becomes your isolated, voltage-converted output.

Sorry I'm overexplaining this, but for the newbies I think it good to 
explain it operation thoroughly and logically.

OK, now in this schematic, you'll notice its got like a zener or something 
and its multistage. I don't think you need this sophisticated a set up 
necessarily (assuming you are selecting one from mouser or somewhere). You 
can use just a regular phototransistor (1 stage) optoisolator. Why? Because 
the driving LED has more than enough juice to saturate the gate on the 
phototransistor single stage or not.

On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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For more options, visit https://groups.google.com/d/optout.

[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

OK, I haven't breadboarded this myself, but here's A circuit that looks 
like it will do the 3 to 5 V conversion (and back interestingly 
enough...just check if the driving volt level has enough juice to drive the 
LED).

Its unidirectional, remember this so two can implement a UART TX and RX, 
however there are some hardware protocols that require line 
bidirectionality that I can't easily figure out how to do.



On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

http://www.mouser.com/ProductDetail/Lite-On/LTV-817/?qs=sGAEpiMZZMteimceiIVCB4BEdbJlF99qoohwWIG77es%3d

An idea. Kind of reconsidering here. I'll look for a good way of connecting 
something like this up. Nice for passing thru a single UART signal thru. I 
wouldn't glue entire subsystems together this way, but if you had to pass a 
UART_TX to a slave TTL MCU this might be the way to do it. Inexpensive too.

On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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[beagleboard] Re: correct connection of an opto-coupler

['woody stanford' via BeagleBoard]

I agree with my fellow posters that using an optoisolator here is not the 
solution.

The solution is to use an isolated (preferably optoisolated) logic gate. In 
this case an OR gate. The critical thing to mention here is that the BBB 
uses 3.3V stuff which isn't TTL (which is 5V+ for on and 0V for off). Which 
makes it interesting because most of the stuff you'll connect is still TTL.

Here is a link to A chip (not the right one but has a lot of the 
characteristics we are looking forand the price is 
right). 
http://www.mouser.com/ProductDetail/Texas-Instruments/CD4075BE/?qs=sGAEpiMZZMtYFXwiBRPs05FGKgXIMzep

What you do is you connect up the power (GND and VCC) and tie one input of 
an OR gate to ground and then the other input to your BBB. Then you take 
the output of the gate as your isolated output. Most of these logic chips 
have such amazing isolation that you can do this, I think. And if you blow 
it you buy a cheapo plastic DIP socket and replace the logic chip like a 
fuse.

This is such a great question at its core, I think I'm going to shop for an 
inexpensive DIP chip that performs isolation AND the converstion from 3,3v 
to 5v necessary for any civilized conversation about BBB connectivity.

Thoughts?

On Tuesday, November 8, 2016 at 7:39:33 AM UTC-7, mzimmers wrote:
>
> Hi, all - I thought I posted something about this last week, but a search 
> doesn't turn it up, so here goes again.
>
> I'm working through Molloy's book, and trying to build the opto-coupler 
> circuit in chapter 6. I'm not a hardware guy, so I'm feeling my way along 
> here. The diagram doesn't show specifically how to wire up the four 
> connectors. I looked at the data sheet for the device, which was helpful, 
> but still doesn't get me home.
>
> I could trial and error, but I've already fried one component, and they're 
> not easy to come by in my area. Can anyone help clarify this configuration?
>
> This is the specific device: opto-coupler 
> 
>
> Thanks...
>

-- 
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