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Author Topic: will my opto-isolator t activate the Radio PTT??  (Read 1856 times)
WB0AXN
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Posts: 90




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« on: April 15, 2008, 03:31:29 PM »

Hello,

I built a sound card interface for my Icom 746Pro and my computer and it works fine.
Now, I want to add a PTT circuit using a 4N35 Opto-isolator.
Using my PSK31 program, I want to key the rig to transmit.  I am using the computer's RS-232 socket, which has the RTS output on Pin #7 to the input of the opto-isolator and the output of the isolator to short the PTT circuit on the rig to gound.
I question if my simple circuit will work....without
damaging my rig.  Why am I in doubt, you ask?
Well, there is a +7 volts on the PTT line on the rig.
There is a constant -11 volts coming out of pin #7 on the RS-232 plug, Pin #7.
When I select "send" or "tranmit" on my PSK31 program, the output on pin #7 goes to a +11.75 volts....which goes to the input of the 4N35.
That's quite a range in voltages and I don't understand why this is.
I don't know how an opto-isolator works, but it appears to me something is not right.

Question:: does the output of the opto-isolator short to ground when pin #7 goes to 11.75 volts positive?
What is the output voltage when the input is a minus 11 volts??   I wouldn't want a plus voltage there.

I thought this was going to be an easy project, but obviously, it's not to me...
For reference of what I'm building, here is the web site with the interface and PTT circuit:

http://www.geocities.com/n2uhc/interface.html

Roland


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G3LIV
Member

Posts: 17




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« Reply #1 on: April 15, 2008, 04:24:22 PM »

Hi there.

de Johnny G3LIV,

Ok on your Opto situation.
I did at first use optos in my interfaces see www.g3liv.co.uk

However I found I had a problem mostly triggering the PTT on the Yaesu 1000Mp range.

You should be ok with your ICOM with an opto.

Yes the Com port uses Pins 5 and 7 for PTT switching.
In receive say with DIGIPAN loaded Pin 7 will be NEG volts circa 10 to 12 when measured against PIN 5 ground.
When you switch to TX the port changes over.
And PIN 7 will now show 10 to 12 volts POSITIVE.
That will turn on the input LED in the opto which then drives the internal transitor.
I think that opto you mentioned is a darlington model but it works the same.
Giving a near short across its output pair of pins which shorts down your PTT so putting the rig into TX
But it does not go to Zero ohms, that seemed to be the problem on the 1k MP rigs.

Thats why in all my interfaces now I use a Mini Dil relay.

Not sure if that helps
73.

If you want a curcuit just email me off forum
at     johnny@melvin.com
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WB0AXN
Member

Posts: 90




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« Reply #2 on: April 15, 2008, 09:14:07 PM »

Hi Gang,

Well, I put everything together nice and neat on a small PC board....and the darn thing won't work.
I have the worst luck at these simple projects!!
In fact, my luck is SO bad, if a magician sawed a woman in half....I'd get the end that talked.

Here is what happens:
When I connect the output from the opto-isolater to the PTT line on my Icom 746pro....the transmit relay inside the rig flutters on and off at a stead pace.
Sounds like a large clock ticking.  

However, if I select "Send" or "TX" in the program, the rig starts transmitting like it's suppose to.
When it's done sending a message and returns to recieve, the relay fluttering starts up again.
And Yes,  I dis-connect the PTT cable as soon as the fluttering starts.
I tried adding a 100 and 500 resistor in the PTT line and that didn't stop the relay fluttering.  Added a larger resistance in line and the fluttering wouldn't start....however, the rig wouldn't start transmitting whem I selected "Send" or "TX".  
Not sure where the problem is amd I don't know what to do to fix it.
Maybe it's a good idea to forget the digital modes  and return to watching TV or taking long naps.

I give up....

Come to think of it.....I did see a schematic somewhere similar to mine, but it had a diode conected between the input of the opto IC and ground.
Maybe I will try that tomorrow....after a good night sleep.  
Maybe I should start drinking booze??  After a three or four shots....if something doesn't work....you don't give a ....!!

Roland
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WB0AXN
Member

Posts: 90




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« Reply #3 on: April 16, 2008, 09:09:30 AM »

Hi again,

Guess what!!   I found out why my PTT circuit wasn't working properly.  
The schematic calls for a 100 ohm resistor going to pin #1 of the opto IC.  
I could not find a 100 ohm resistor in my junk box, so I installed a 1800 ohm instead.  
Dug a little deeper in my junk box and found a 100 ohm resistor...and now the PTT interface works great.

Roland
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KM5Z
Member

Posts: 24


WWW

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« Reply #4 on: April 21, 2008, 08:22:13 AM »

> The schematic calls for a 100 ohm resistor going to pin #1 of the opto IC.
>I could not find a 100 ohm resistor in my junk box, so I installed a 1800 ohm instead.
>Dug a little deeper in my junk box and found a 100 ohm resistor...and now the PTT interface works great.

Yes, you can also use a LED in series with the optoisolator's Pin 1-2 - it would give you some visual feedback that the PTT is happening.

Mike Yancey
KM5Z
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