Do I need a relay for my normally open circuit?

[Kit333]

Hello,
I think the easiest way to describe my question is to give you the application for this first.

I work for a small TV station and we are replacing our large studio cameras with new cheaper DV cameras. On the new DV cameras they do not have tally lights (the red light on the top of the camera to tell you it is on). So my solution to this problem is to convert one of those garage door indicators (like the ones at Home Depot that tell you when your garage door is open or closed) and use the status LED light as the tally light.

Normally, tally lights have two wires that I connect to the switcher. I know it uses a normally closed circuit to turn on the light and that the lights are being powered from an external source, not from the switcher.

So for my application I would like to take the transmitter (garage door sensor) and take out the tilt sensor and instead connect it to the switcher. So that when I select the camera the LED on the reciever will light, and when I select another source it will turn off.

My concern is that the transmitter is powered by a 3V CR2032 battery and when I place a volt meter across where the tilt sensor used to be I do read 3Vs. So, now if I connect that to the switcher will I have problems? I'm thinking that maybe I need to connect a relay to the switcher and then to the transmitter - so the 3Vs does not go through the switcher. Or maybe this is so little voltage it would not really matter.

The smallest one the had at RadioShack is a 5VDC relay with a 3.5V pickup and a coil resistance of 56 ohms. I don't think that would work anyway.

Hopefully that makes sense, sorry for the long post.
Kit

 

[Kit333]

Update....
Ok I got my relay to work by testing it with two AA batteries and when I apply voltage across the coil I can get the light to turn on. The problem I think I'm going to have is that on the switcher that will be connected to the transmitter I believe that it is a dry contact closure - so I think that means there is no voltage and therefore it would not work with the relay? Is that correct? If so, any ideas?
Thanks

 

[richs]

Hiya-

Well, let's think about this for a bit. The power from the old talley lights had to come from somewhere.

So drag out the schematics from the OLD cameras and see what source and current that drove the old talley lights. Reed relays *might* be able to be driven by the same supply.

Or, better yet, MEASURE the current and voltage across the switcher tally light circuit. Check for AC and or DC. That will give you a good start at a "not to exceed" for current and voltage. See if the power is sourced by the CCU (camera control unit) or the switcher. It could be AC or DC (the lights don't care). Most likely, it will be a lower voltage (not like 110VAC) and will be on the order of hundreds of ma or so.

Watch out for the inductive "kick" from the relay coil and if you possibly can, put a healthy diode back biased on the coil.

If you are worried about this, you can always use a simple
bipolar transistor and a (couple of) resistors for bias as
a switch, and you can play all you want without damaging your switcher.......

IF you feel gutsy, and indeed the switcher IS dry contact, then you can just eliminate the relay alltogether. Just supply a suitable power source (you ARE planning on elminating batteries. I sudder to think that you would ever have batteries in ANYTHING except wireless mikes in a studio).

The downside of this is that with the directional properties of the LED, the tally lights will be hard for the floor director to see since he/she is off axis. I can just see my floor director when that happened. She would be throwing a FIT! Maybe you should think about having FOUR LEDs for each camera. I know that when I run camera and I'm not on, its nice to look at the other cameras to see WHO (if anybody, I might have missed the tape drop from the TD) is on. So 4 of them, front back and both sides. Yeah, I could and do look at the studio feed in the viewfinder, but I also like to keep an eye on the floor director too!

best of luck.

Cheers,

Rich S.

 

[Kit333]

Hi Rich,
You bring up some good points, first on the old camera equipment I don't have the schematics for them - they were installed long before I got there. They must be powered from the cameras, if I take the leads from the tally light and connect them together the light turns on (on the cameras that have tally lights). So, the switcher might just be closing the circuit - I'll have to put a voltmeter on this Monday.

Oh, also on the running on batteries comment - I plan to wire in an AC/DC adapter to the unit, so it will be powered.

Right now cameras 1 and 2 have working tally lights, since they are the older type - but we are trying to use a new HVX200 on the jib, since it is lighter in weight and much cheaper than purchasing a full blown studio camera. So the floor director (who is usually running a camera - we are a small station) can assume that if camera 1 and 2 tallys are not on camera 3 is on. I also use a lot of voice notification on the headsets. But the jib operator would really like a tally light, instead of relying on me telling him his camera is hot.


I'm interest in learning more about how to use a simple bipolar transistor and a couple of resistors for a switch. Could you describe how to hook this up, my electronics background is not quite good enough for me to figure this out on my own.

Thanks for the help! Nice to see some other video people in the forum too.
Kit

 

[Kit333]

Ok, after doing some research I think I understand the correct way to connect the transistor. The Base would be connected to the switcher tally and to the C side of the transistor. The C side would then also be connected to the transmitter and the power and then to the E side. All that matters is that the proper current directions are maintained for the sake of correct junction biasing.

I'm guessing the resistors are to get the transistor in a saturation state. I'm not quite sure as to how to go about finding out which ones to use.

Thanks!
Kit

 

[Skogsgurra]

Switcher? Can't really follow you.

Is it the switch you are referring to? Or a switching PSU?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Kit333]

Sorry getting video terms mixed in there... when I said switcher above, I was refering to a device that switches video signals - basically I take the video signal from camera 1,2,3 and then I can select which one I want to record and output that to tape. Basically the switcher tally is a device that turns on the red lights on the top of the camera that is being output to the tape at that time and that is done by a dry closed contact.
Hopefully that helps.

 

[felixc]

Are you able to tell what exactly is the control for the tally light? You may have an answer if you can get the spec sheets of the switcher unit. To repeat richs' question, does the switcher supply the power to the tally light, of does the switcher just have some relay contact for that function.

If it is some relay contact from the switcher, then you can tap power from the camera power cable (probably some 6Vdc) to feed the tally light circuit, why not just a bulb, and switch it from the control of the switcher.

 

[waross]

I am wondering if a relay may not take more power than an LED or indicator light.
If the switcher is controlling the tally lights can you measure the voltage and current output to one of the older camera tally lights? This will give you a good indication of what is a safe load on the switcher output.
respectfully

 

[bogeyman]

The tally lights on our analogue Sony stuff are powered from the kits internal supplies and switched by a two wire interface to a remote switch, Closed = lamp on.

You could use the existing wiring and fit a lamp (or LEDS + resistors)to the cameras and add a single common supply (to suit the chosen lamps) at the desk/control end, modifying the wiring to include the PSU in each circuit.

 

[LionelHutz]

If the switcher really is a dry closed contact then just use it to switch the power directly. You already somewhat verified this by saying that you could short the wires from the other cameras together to get the light to come on.

bogeyman has a good idea in putting a common power supply at the switcher so you don't need one at each camera.