Right now I am a little concerned that the output from my cheap automobile backup camera will be strong enough to make it through 150 feet of 24 AWG. It should be OK but only testing will tell. The camera and monitor arrive next week. There is no point in having an ROV if you don’t get back a quality picture, that’s why I am building just a tethered CCTV system first, then comes thruster control.
This suggestion requires a little more work, but have you considered using twisted pair with Ethernet home plug adaptors? You’d want to combine the two conductor with a neutrally buoyant fiber or something for strength if you think you’ll need a manual retrieval safety line. Requires a few things for power management, on-baord main power supply, networking setup for transmission of video and receptions of commands… but tethers and tether management tend to drive requirements on your typical ROV. At depth, tether drag is the largest limiting factor and drives thruster requirements which in turn drives up power requirements, and weight, terrible spiral.
Anyway, just a thought.
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@Richard - can you post a no brainer schematic of your full pulse width circuit. I must be getting old, I still don’t have a clear picture of what you are doing. Are you talking about a DC signal here? The capacitor would appear to be doing nothing but acting as an AC shunt to ground from your description … so that is why I am bugging you for a schematic. You also have line / mutual capacitance with the cable type that you are using … along with a pretty hefty loop resistance.
Question about the CCTV system … what is the signal output for it? If it is a standard 75 Ohm then you are going to have to do some impedance matching since Cat5 is 100 Ohm.
@Richard - just curious … which boat were you a CO on? One of the older ones or the newer toys cruising around?
Real older one, Thomas Edison (SSN 610). She was converted to an SSN as part of the SALT agreement just before I took command. I was CO of the SUBASE Pearl Harbor when I retired in 1990. I’m 74.
As far as the PWM circuit is concerned go to this web site and download the BOE-BOT text. Substitute the pulse width controlled ESC’s that drive the T100’s for the robot servo driven wheels in the text. https://www.parallax.com/downloads/robotics-boe-bot-text
I hadn’t thought about the impedence mismatch in the CCTV circuit. Let’s just see what happens next week with the testing.
I just found this paper which discusses the problem you brought up in going from unbalanced 75 ohm coax to 100 ohm balanced twisted pair. I’ll give it a good study. I guess an UNBAL on one end and a BALUN on the other will work. They look simple enough to make. But like I said, maybe testing will show I can get a way with the mismatch over a short 200 foot run.
Epanorama.net … haven’t been to that site in ages. Lots of old but good stuff there. I have an old tow cable that I test that I need baluns with. It has a weirdo impedance on it. Pain in the butt finding stuff off the shelf so it is wrap your own transformer time.
Video over UTP |
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From the Epanorama.net paper: “The secret to sending signals over UTP is to balance them well in order to limit both radiation and noise pick up. In this balanced mode, the two cores carrying the video signal are balanced to a particular reference point and the cable twists enable a uniform rejection of the interference, effectively cancelling it out. To produce a balanced signal BALUNs are used, standing for BALanced to UNbalanced. For at least 20 years products have been available capable of transmitting video using UTP wire. There are nowadays many adapters (video baluns) that make this possible. Sending AV over CAT5 is an ecellent alternative to using regular coaxial cable; you can run the signal longer and the cable is cheaper, but the baluns also cost some money. Most use of CAT5 cable in CCTV installations involves using equipment called baluns, however it is possible to use CAT5 without a balun on short cable runs although using a balun will always produce superior results.”
Because I will be at least ten miles off the coast using the ROV I don’t have to worry about radiation and noise pickup, only signal strength loss due to the impedence mismatch. Will see.
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@Harold I gave you the wrong reference for the RC time measurement circuit. Here is the correct one (go to page 139):
@Richard - I read the previous document and was trying to figure out what you were talking about. I saw them doing discharge measurements in regards to a photodetector … and I was scratching my end. I will have to checkout the new reference later. Thanks.
@Richard - Ok, read the section covering the RC measurement. I totally understand what they are doing, I just don’t like how they are doing it. They “assume” that the High to Low transisition will always be at 1.4 volts to switch logic states. I guess for what you are doing it is close enough for Government work.
I don’t know how much you are going to have to deal with it and I really doubt any at all … but since you are using a very long cable for your tether line capacitance could come into play. I looked at the Cat5 specification and you might add about 3 to 4,000 picofarads of capacitance but that is an VERY SMALL speed bump addition. If you were going over a shield wire for the measurement, then you could end up with some possible impact because the pF per meter rating would be a whole lot larger.
No matter what, they have a sleezy method for you to measure your pot position.
@Harold- I just ordered these:
@Richard - sweet! Nice find. That should do the trick for you. That company, the seller on Amazon, appears to be just a clearing house for just about anything. You got lucking with that one.