I am trying to multiplex two cameras (small cctv cameras attached its picture)but i am facing a problem when I open the gate of the transistor so that one of the comeras work but the problem when I but ground from the arduino to the transistor gate the camera is still on and the chamneldoesn't close.
Does anyone have an explanation for this?
Thanks
I suppose you switch the GND connection. It could be that the camera gets enough GND from the video connection to work.
Either switch the + connection or maybe add a capacitor in series with the video signal.
A better solution may be to use a MUX like 4051 to only switch the video and let the power stay on.
A schematic would be more use that a picture of a camera.
HKJ-lygte:
I suppose you switch the GND connection. It could be that the camera gets enough GND from the video connection to work.
Either switch the + connection or maybe add a capacitor in series with the video signal.A better solution may be to use a MUX like 4051 to only switch the video and let the power stay on.
I already tried the + connection but it always keeps on
When i tried to switch on the video signal it worked but it adds noise to the image of the camera
omaro200:
I already tried the + connection but it always keeps on
When i tried to switch on the video signal it worked but it adds noise to the image of the camera
Then you are not doing it right. For the plus connection you use a P-channel MOSFET and pull the gate to ground to turn it on. For off the gate must be at the + supply level.
Use the above when camera use same or lower voltage than the controlling.
If the camera voltage is higher than the processor voltage you can use the above circuit.
Could you recommend a pchannel mosfet iwas using irf640
Go to a site like Digikey
And use the selection filters.
You need p-Channel and a Vgs (Called Drive voltage in that table) well below the supply voltage.
HKJ-lygte:
You need p-Channel and a Vgs (Called Drive voltage in that table) well below the supply voltage.
Actually, that is not quite correct.
You need a low RDS(on) specified for a VGS which is well below the supply voltage.
The IRF640 is pretty useless in this regard with a VGS of 10V. 
Paul__B:
Actually, that is not quite correct.You need a low RDS(on) specified for a VGS which is well below the supply voltage.
A low Rds on for that small camera? That is not very important.
Best to give advice that is reliable when used in as many circumstances as possible.
Putting a few Ohms in series with the power supply to any given device may or may not be such a good idea. 
Paul__B:
Best to give advice that is reliable when used in as many circumstances as possible.Putting a few Ohms in series with the power supply to any given device may or may not be such a good idea.
The problem with giving general advise is that you need to include many more parameters. Here I know the voltages will be in 3.3V to 12V range, i.e. no reason to worry about max gate voltage, the current will be fairly low, i.e. no reason to worry much about Rdson or power in the mosfet. There is no need for fast switching, i.e. no raeson to worry about gate capacitance or current drive of the processor or pullup resistor.