Using a Car Antenna as a Door Opener

[hoosiercheetah]

I've read several threads and seen a few websites about this. The mechanicals are pretty straightforward, but I have concerns about the electrical circuit. These are my thoughts.

When you hook a motor up to power, there are two things going on at the same time - volts and amps. (There's actually a lot more to it, but that's all we need to worry about).


The volts will determine how fast the motor operates. More volts = faster motor. The car antenna is designed to run at 12V, obviously. Running it higher or lower won't hurt it, as long as you're within the motor's operating range. In this case, just guessing, +/- 5V would probably be fine, but it will change the motor speed a little bit.


The amp draw of the motor depends on the load. The more load on the motor (heavier coop door), the more amps it will draw. The antenna motor is designed to have a very light load, just an antenna going up. Having it raise a coop door is going to cause it to draw more amps. This is fine, as long as we don't draw too much.


There are three main factors limiting how much amperage we can safely draw. One is the motor itself. If you draw too many amps, you could overheat the motor and possibly melt its internal wiring. The second is the power supply (p/s). If the motor is attempting to draw more amps than the p/s is capable of delivering, you could burn out the p/s. In an extreme case it could catch fire. The third is the wiring. If you draw more amps than the wiring is rated for, you could overheat it, melting the insulation or the wire itself, possibly shorting the circuit to ground. In a dusty coop environment, this could be catastrophic.


Fortunately, it's easy to protect the system from overheating and burning up your chickens - and this is the thing that I have not seen in anyone's design - with a fuse. A fuse is designed to overheat and stop the electricity before the wiring can overheat. Having an appropriately rated fuse is critical. When these antenna are installed in the car, there is a fuse in the circuit. However, we can not simply use the same rating fuse ( I think they are usually 10 or 15 amp) because we are not using the same wiring and we have a heavier load. Our fuse has to be rated for our application, and that's where a little bit of engineering has to happen.


We have to know how many amps our motor is going to draw when lifting our door. The wiring has to be rated to carry more than that many amps. Then we need a fuse that will blow at a lower amperage than the wire's maximum, but a greater amperage than the motor will draw in normal operation. And also, the power supply has to be rated to deliver the amp draw that the motor needs to operate the door.


It sounds to me like so far, everyone who has built one of these has gotten lucky (or did the engineering and just didn't write it all out) but personally, I wouldn't just repeat someone else's build and expect the same results and safe operation, unless I was sure that every detail was identical, and I knew that they had worked through the design to be sure that all of the parts were rated for the job. If any previous builders want to chime in on that, feel free - I'm not attacking anyone's previous work, I'm just asking a fair question in order to ensure everyone's safety in the future.


The good news is, even if it sounds complicated, designing a system like this to be safe is pretty easy. You just have to operate the motor under it's load, measure the amp draw, and make sure everything else can handle it. Leave enough gap between the motor's draw and the wire's rating to add a correctly rated fuse, and you're all set.


It probably sounds like I'm over-thinking this, and maybe re-inventing the wheel. There's a good chance that you can just throw together some parts and make this work, and maybe you'll never have a problem. I just think that a little extra testing and thought going into the project is worth it to ensure the safety of your flock and their home.

That's all I have to say about that. ;-)

 

[Hokum Coco]

https://www.backyardchickens.com/t/...-5-bucks-with-a-radio-antenna-from-a-junkyard

 

[hoosiercheetah]

Yes, I've read through that thread. Maybe I didn't state my point clearly. I'll try again.

I think it's a good design, the only big thing I'd change is it can easily be done with one power supply instead of two.

However, no one in that thread, or anywhere else I've been able to find, has addressed the issue of circuit overload protection.

I think this is a major oversight, considering that most coops are a dusty environment full of highly flammable bedding materials.

The solution to this problem is a simple matter of testing the motor and double-checking the rest of the equipment to make sure all parts of the circuit can handle the current draw. It's really easy.

Probably it will turn out that everything is fine. After all, many people have built this design, and no one so far has admitted to burning down a coop.

However, I would not trust this to chance when it is so easy to make sure everything is safe!

It would cost just a few dollars to add an in-line fuse to protect the system. I think it's worth doing.

 

[mcgolf83]

I think your statements about a fuse are valid, where would you recommend putting the in line fuse? And how would you modify the design to use just one power supply?

Thanks!

 

[hoosiercheetah]

The fuse should go in series in the wire supplying power to the motor. That's the wire that will have the highest amp draw.

The antenna system requires power be supplied to two points - the relay that operates the motor, and the motor itself. All you have to do is run two wires from the positive side of your power supply, one to each of these points. The switch (timer, remote control, or whatever) that will operate the motor's relay goes in series in the wire that powers the relay. If your switch requires power, then you'll have to make provision for that power as well.


I've gotten a lot of questions about this since I made this post. I don't mind helping folks out, but I do think that if you're thinking about building one of these systems, and you don't know enough about electrical circuits to work out the particulars of your set-up, you should probably find a friend who does. When you're MacGuyvering car parts onto a chicken coop, every system is going to be a little different, and you really want someone who knows what they're doing to do the custom design, so that your particular hodge-podge will function correctly.