Help with Prop video switcher
kochevnik
Posts: 30
I am working on a system to switch CCTV signals (not power) on/off using the Prop & a transistor - just trying to get this to work in it's most simple form).
I found a website where someone did what I am trying to do - only with a parallel port signal instead of a signal from a Prop.
http://www.vwlowen.co.uk/radio/videoswitch2/index.htm
and this one seems to be wired the same way :
http://web.comporium.net/~shb/ppswitcher.htm
My problem is I cannot get this simple circuit to work, and worse, I don't see how it is supposed to work. My understanding of using an NPN transistor as a switch is that the signal goes from the Prop to the base on the transistor and then to a ground thru the emitter. This schematic just puts a signal on the base, but I dont see where the current flows thru to ground other than back to the ground on the parallel port - I dont see any kind of ground on the transistor side (if that all makes sense)
But I wired it all up anyways and tried it, using 2 different NPN tranistors and for me, it does not work. I get the LED to light on the base side which means I have current to the base, but the tranistor is not turning ON to allow the CCTV signal to pass thru. I tried an LED + resistor as a load thru the emitter/collector and that did not work either, unless I tied the ground for the signal and the ground from the emitter together - then the transistor worked exactly like the switch is supposed to be.
I know this is a pretty simple circuit - but maybe someone who knows a lot more than I do can give me some pointers as to what I am doing wrong ?
Many Thx
===========================
Solution :
Using a transistor to switch the video works (at least in my application) - the wires were just hooked up incorrectly. As Phil P helpfully pointed out, the camera ground, the video in ground and the ground from the propeller (or whatever ground your 3.3v signal comes from) need to be tied together. In addition, the transistor needs to be used to switch the Ground / - wire on the cctv signal, NOT the + wire.
I hooked it up thusly and it works fine - caveat being that my video input goes to a GVI CCTV mux so it might be doing some synching and adjusting before it sends the signal on to the monitor.
Cheap, works good. Wiring Diagram attached.
I found a website where someone did what I am trying to do - only with a parallel port signal instead of a signal from a Prop.
http://www.vwlowen.co.uk/radio/videoswitch2/index.htm
and this one seems to be wired the same way :
http://web.comporium.net/~shb/ppswitcher.htm
My problem is I cannot get this simple circuit to work, and worse, I don't see how it is supposed to work. My understanding of using an NPN transistor as a switch is that the signal goes from the Prop to the base on the transistor and then to a ground thru the emitter. This schematic just puts a signal on the base, but I dont see where the current flows thru to ground other than back to the ground on the parallel port - I dont see any kind of ground on the transistor side (if that all makes sense)
But I wired it all up anyways and tried it, using 2 different NPN tranistors and for me, it does not work. I get the LED to light on the base side which means I have current to the base, but the tranistor is not turning ON to allow the CCTV signal to pass thru. I tried an LED + resistor as a load thru the emitter/collector and that did not work either, unless I tied the ground for the signal and the ground from the emitter together - then the transistor worked exactly like the switch is supposed to be.
I know this is a pretty simple circuit - but maybe someone who knows a lot more than I do can give me some pointers as to what I am doing wrong ?
Many Thx
===========================
Solution :
Using a transistor to switch the video works (at least in my application) - the wires were just hooked up incorrectly. As Phil P helpfully pointed out, the camera ground, the video in ground and the ground from the propeller (or whatever ground your 3.3v signal comes from) need to be tied together. In addition, the transistor needs to be used to switch the Ground / - wire on the cctv signal, NOT the + wire.
I hooked it up thusly and it works fine - caveat being that my video input goes to a GVI CCTV mux so it might be doing some synching and adjusting before it sends the signal on to the monitor.
Cheap, works good. Wiring Diagram attached.
640 x 512 - 13K
Comments
For the base/emitter junction to be biased the emmiter would have to be referenced to ground somehow. I wonder if they are assumming that this has the 75 Ohm, to deck, without any capacitors ??
The audio bit would be the same but probably about 1K Ohm ??
-Phil
So if you want a cheap video switch solution without individual transistors then you may want to try the 4016. I only used it on low end BW cameras, so I can't comment on color, if you are using you'd have to experiment. You can also buy video mux chips that will do this, probably more expensive than a 4016. But they will probably have higher bandwidth, better stability and designed to handle different electical conditions (power surges etc) or at least have design notes that would give you design considerations.
If there is a measurable DC level in the video, run the circuit through spice and see what it says. use a 1vP-P wave form to simulate RS-170 video.
Anyways, works, and thx to all who responded.
Zoopy - I ordered some mux IC's and am going to see if those work as well.
I took a video of it and had some free time this evening to upload it to YouTube.
http://www.youtube.com/watch?v=oPF8Dfoue2A
Enjoy.
-Phil
Documentation in Spanish.
(and if the little toads get stuck then all your years of training, and self improvement, allow you to select a suitable sized screwdriver handle to give them a good thwack)
Frank