Newbie Problem solving. Remote low voltage switch.

[Lab-Chimp]

Hello folks:
im embarking on solving some problems in an ongoing project. I have two underwater video camera housings with adapted Sony FDRax53’s . My friend and i have been using these units for underwater video filming. We have over 70 dives on each unit with no leaks. We would like to have waterproof external monitors. As waterproof external monitors are very expensive, ive looked at making them as opposed to buying them. Waveshare makes a 7” HDMI monitor, that runs on 5volts dc, and has a direct HDMI input to display the video signal from the Sony FDR AX53. I have started figuring out how to construct the waterproof housing for the monitor, laser cut perspex and poly-carbonate. The monitor needs only an ‘on/off’ control. Ive looked at waterproof switches and looking to eliminate water leaks entirely. Ive used a magnetic reed switch to test out an external switch mount on the outside of the monitor housing. I have a very simple flat toggle mechanical switch using a large magnet to activate the reed switch. The monitor housing will be made from perspex and poly-carbonate for the viewing screen. A switch tab will hold a friction switch that holds the magnet.

I can then remove any water leak risks, by having all of the wiring, HDMI signal input, DV voltage supply wiring. I have an electrical gland connector that will carry the silicone hose to the main camera housing, this will carry the wiring to the monitor. I can waterproof the silicone hose to the monitor housing and have the camera housing gland removable for service and assembly.

The Monitor needs only an on off power supply switch.
I have no idea how to construct an internal switch that will be triggered by the magnetic switch, the internal switch must also protect against any shorts or electrical failures. I don’t want that 5volt dc power bank cooking.

Im quite good at mechanical and simple electrical problems, but electronics defies me, and its not an area I’ve encountered before to this degree. Ive got some basic equipment, a multimeter, an old Adcola twin temperature controlled soldering station, (its great i love it), a DC 0-30 volt 2.5 amp electric power supply, a breadboard.

So far we have been extremely successful in modifying old equipment and making to work, making it reliable, and making hot serviceable. The video footage we have gather over the last 12 months has been really outstanding, we are currently working on completing our first short film, with more in the pipeline. With warmer weather just around the corner we need to upgrade our equipment and address the limitations of our current equipment. In spite of these limitations we have collected a great deal very high quality video. Doing a lot with very little.
Any suggestions or practical information would really help us solve this issue.

I hope this post also satisfies the ’Newbie’ requirements.

Chimp out......

[Terry Bites]

A reed switch sounds ideal. They're very reliable. Make an acrylic slider for the magnet, job done.
So you want this switch to control a power switch? See solid state power switch circuit under...
Take care putting it together- a static charge and/ or a reversed S-D can kill a MOSFET. 
To protect your battery pack and MOSFET from a short, add a PTC fuse as shown. PTC fuses reset themselves after the fault has cleared so you won't need to open up your enclosure to fix it. I'd measure the operating current of the monitor and choose a PTC with about 25-30% higher "hold current". The fuse will open when you get to about 2x Ihold. Measure the current draw with your multimeter or a USB widget. "USB Volt Current Doctor" etc on ebay.  You might want (should) to take into account how hot it can get inside your enclosure and de-rate accordingly. Its not complicated or an exact science. 

 https://nz.element14.com/w/c/circuit-protection/thermistors/resettable-fuses/radial-leaded-pptcs?st=ptc%20fuse&gs=true

[Lab-Chimp]

A big thank you to Mr. terry Bites, this is a great help. Ill go over the diagrams and your recommendations, and try and fully understand the design and how to construct it.  I can easily source these parts,thank you for the link. Ive refined my external switch design and im finalising the display housing to incorporate the reed switch. Thanks again for your help its invaluable. Ill give you an update as soon as ive got all the components and tried assembling them.