Need help from the pro... for a tally interface

[mezzrow]

Good day to all, this is the second time i ask for help and i thank you all even for the time spend to reed this.
I'm an old (51) videomaker with a new videomixer. the new machine do not have the tally interface, the little red light that say wich camera is on air.
Quite impossible to work this way so i decide to use some old parts (old mixer) and a 18M2 to fill my needs.

The question probabily silly... the old interface needs to short to ground one of a four contacts ( four cameras involved ) to light up the red led,
How can i do that with the output pins of the picaxe?
Solved this problem i will start to manage all.
Hope to have been clear enough... and forget my poor english.

Mezzrow

 

[Dippy]

Would it be possible for you to post a simple drawing?
A drawing is often more helpful than a description.

It may be as simple as a transistor + resistor for each contact, where the transistor acts like a switch/relay.
Each transistor would connect to a PICAXE output via a resistor. You also need to join the grounds.
For minimal wiring you may consider an optocoupler (aka opto isolator). A little harder but potentially safer.
You would then simply set the PICAXE output High to make it switch on and light the LED.
See Manual 3 page 6.

In addition, you will need to find out if this 'switch' will need any simple current limiting.
As in; a standard LED will need a current-limiting resistor.

It would be good if you could do a little (clear & accurate) drawing and a little more information.
Maybe you could do some testing using a Multimeter.
We wouldn't want to blow up your mixer

 

[mezzrow]

so quick response

Dippy, glad you join my mission... ;-)

difficoult do draw something, but i can explain a bit more :
The interface use 12 v. each contact to led have the limiting resistor, each contact use a transistor PNP BC178C as a switch
exactly as the first project in this page:
http://www.rason.org/Projects/transwit/transwit.htm

I have tried to use a output pin from a 08M but as High or Low the tally circuit is always on, the ground of the two supplyes 12 and 5 volts is the same.
Thank you all for the patience.
Mezzrow

 

[srnet]

difficoult do draw something

Got a pencil, paper and a digital camera ?

The circuit shown will always be on as when the PIC pin is high the transistor is still going to get enough base current to turn on.

You should also be carefull with that setup, depending on the value of the base resistor you could damage the PIC, draw a circuit diagram of your setup and put the values chosen on it.

 

[Dippy]

In Post 1 you said "short to ground" , but you said you used "the first project on this page" (+link).
That is a PNP as a high-sided switch which is not the best way and could damage the PIC if the resistors were too low value.
In any event it is NOT ideal.

If you looked at (as I suggested in my first reply) Manual 3 Page 6 you would see that it is closer to the second example in your link which uses an NPN transistor which is far easier and more logical.
If you are looking at a simple low-current switch to ground then that is basically the way to do it, unless there are things you haven't told us.

Have a practice with LEDs , a battery supply and some transistors and resistors on Breadboard before committing a design as it sounds like your transistor skill/experience is at the early stages.

 

[mezzrow]

sorry i was away.

is it true i have a little experience in this, but i need to use the thing as it is. It is already mounted in a specific box that i prefere not to destroy!
In the link they say:
" To turn on our transistor switch all that is needed is to short resistor R1 to the negative ground."

This is the task. How to do it... i don't know.

Anyway if it is necessary i will break the thing apart and use the solution Dippy propose.
It was my first tough:
just lit a led when necessary with a picaxe... can be more easy then that?

 

[mezzrow]

I'm going to made the light plan for a new year teatre show
Is the day after tomorrow evening, as you can immagine, so i'm not sure that i can read yours replays soon.

Excuse me.
;-)

 

[MartinM57]

..." To turn on our transistor switch all that is needed is to short resistor R1 to the negative ground."...

So have you tried that by using just a piece of wire? What happens?

 

[mezzrow]

Hi Martin, the old interface i have ( a box with 4 input and 4 looong cables ending with leds) works in this way.
The old mixer short to ground (negative of the power supply) the input of the cable to the camera on air, so the led goes on and the cameraman knows it.
I need to use picaxe to simulate this, the new mixer ( don't know why ) do not have this option, but on preview monitor change the color (red insted of white) of the border of the camera on air.
So with 4 ldr, 4 adc, 4 output, a picaxe... some easy code...

That is! Hope to have been sufficent clear, is not my language ;-)

 

[srnet]

So with 4 ldr, 4 adc, 4 output, a picaxe... some easy code...

What are the ldr and adc bits for ?

 

[mezzrow]

What are the ldr and adc bits for ?

Easy... the 4 micro Light Dependant Resistor glued on the correct position on the screen read by the READADC command on 4 adc inputs can give the 18M2 the clue to light up the correct led.

This is the easy part...