My children (ages 4-11) are terrible at closing doors/gates, turning off lights, etc. Astonishingly bad. When they leave gates open in our horse pasture several times a year, it allows our 3 horses to roam the neighborhood irritating my neighbors, and causing numerous issues. For years, no amount of discipline, reminders or instruction as been able to stop the slow trickle of incidents of near-disaster caused by them leaving important doors and gates open.

My next step is to build "howlers" for the important gates. Essentially a switch that is closed when the safety chain on the gate is removed from it's correct position. Once the switch closes, there will be a 2 minute timer that will monitor the switch, and if it isn't back to the safety-chain-is-attached position by then, a really loud alarm will "howl". (Ideally to remind them to shut it, worst case to attract an adult to fix the issue. ) Added benefit is it (hopefully) will be scary enough to the horses that they won't approach it to sneak out the open gate. I've got no problem with the circuit part.

What I am struggling with is the best switch style to use. For those unfamilaiar, safety chains are the farm gate equivalent of door chains on a hotel door. They provide redundant latching for the rare, but real situations where the normal latch fails. Normally loose, but have to be able to resist powerful yanks without breaking.

Ideally, I don't want a 3rd step to be required when using the gates (Step 1 = normal lock, step 2 = safety chain), so I would prefer some way to monitor if the chain is engaged as normal without altering the existing chain anchor points. The anchor points and the chain are all metal, but I believe that both they are also grounded. I'm unsure if there is any useful way to use that (using the chain as a "wire" and testing if they are connected or not) as part of the switch due to the lack of isolation.

Watching the chain anchor side for the chain end is another option, but is also fraught with uncertainty: The chain end-hook fits loosely, so getting it to reliably trip a microswitch or to line up nicely for a photo-interuptor would be dicey. (Have to assume that wind, etc. will regularly jostle the end-hook around inside the anchor and it won't be stationary.) A magnetic switch might work, but the only ones I have used required the magnet to be adjacent to the sensor to trip. I don't know if any work well with tolerance of > 1" between the magnet and sensor.

Suggestions? What might be the best way to monitor if the chain end-hook is inside the chain anchor? What will perform best with the loose-tolerances or rattling, wind-blown farm gates?

False positive need to be limited. If these things start succesfully catch 57 of the next 2 times my kids leave a gate open, they'll be too annoying to use. (By design, momentary false positives don't matter. Only false positive lasting long enough for the alarm to sound.)

(FYI, this needs to be done on the safety chain, not on alternate things like the main lock or using hinge deflection. I don't care if the main lock fails if the safety chain is on, and I most certainly DO care if the gate is closed but not locked, since a horse can open it and be gone long before the alarm would sound.)

 

back in the day they would just fix that problem with a good "spanking"

 

I don't know if I can be of much help, but maybe I can light the way for others.

I'm going to take a flying leap and assume most here, like myself, are city folk and have no idea what a safety chain for a farm gate looks like. Could you post some pictures of your door showing the safety chain? This would help immensely and could spark some other ideas.

You mentioned both sides of the chain may be grounded. Can you verify this with a meter? The simplest way would be perform a resistance test between the chain and holder both while connected and while disconnected. This would tell you a) if the resistance is high enough between the two when disconnected to act as an effective open switch and b) if the resistance is low enough between the two when in contact to be an effective closed switch. Using the chain as your switch would be your best option if it works.

I'll wait to "see" your door before making other suggestions.

 

I don't know if I can be of much help, but maybe I can light the way for others.

I'm going to take a flying leap and assume most here, like myself, are city folk and have no idea what a safety chain for a farm gate looks like. Could you post some pictures of your door showing the safety chain?

Let me see if I can help. There are several gates to the pasture, and the safety chains are all hand-made like this: Chain ends with an S-hook with one end crimped shut to keep it on the chain. The s-hook goes through an eye-bolt to "latch"

Images of components for reference: (The eye-bolts are all between 1/2" and 3/4" ID)

You mentioned both sides of the chain may be grounded. Can you verify this with a meter? The simplest way would be perform a resistance test between the chain and holder both while connected and while disconnected. This would tell you a) if the resistance is high enough between the two when disconnected to act as an effective open switch and b) if the resistance is low enough between the two when in contact to be an effective closed switch. Using the chain as your switch would be your best option if it works.

I just multimetered it and it confirmed my suspicions: Resistance value are all over the place with even the slightest motion (roughly 20K - 38M while I was trying to hold it still). And that was on a dry, calm day. Add wind and rain, and the resistance values would be nothing but wildly-fluctuating noise.

There is some chance I could improve thing with a few inches of flexible copper tubing over the s-link and the eye-bolt, soldering an actual wire to the copper and using that, but that'd still be dicey. No guarantee it wouldn't also be ultra-noisy, and it would require a circuit split between the post and the gate. (To complete a loop, I'd need 15-35' of UV resistant and/or underground burial wire per gate, and it would add a lot of potential for wires to get pinched/pulled/cut/etc. to make reliability headaches.)

I'm hoping I can find a solution that is 100% on the eye-bolt side to avoid the issues with making a reliable circuit loop across a 12' gate.

 

This might be a good place for a 'hall effect' type device/circuit. They will detect the presence(or absence) of a magnetic field. Lots of ways to arrange the parts.

 

Okay, the resistance readings confirm the chain cannot be used as switch. I'd still like to see a picture of the gate you're trying to safe guard - much easier to "see" the limitations and sometimes prompt new ideas.

Off the top, I'm wondering if you could somehow employ a normally closed switch that is forced open when the gate is closed. Think of a music greeting card or a personal alarm that activates when the strap is removed. You'd add a timer before the alarm of course.

Perhaps a separate pin attached to the end of chain with a rope or smaller chain and once the chain is attached, the pin is placed into box housing the circuit? It would be an extra step, but not a time-consuming one.

If you could change the eye bolt and S-hook with something that goes into hole, latch, etc. where you could have it act as a switch as well, I think you'd have your solution. Something like replace the S-hook on the chain with a threaded eye bolt and find/make a solid block the eye bolt can slide into, perhaps at a 45° angle to help prevent accidental removal. Then add an IR or LDR or even mechanical contact switch to the block.

Another thought is a capacitance sensor that detects the relative capacitance of the eye bolt which will change when the chain is connected and when it is not. Not sure how to go about doing this myself, but it's a thought.

 

Seems more simple to replace the chains etc with a self closing gate. Better, teach the kids to close the darn gate. Seems a week of mucking out the stables or a month of no cell phone would get them to remember to close the gate.

You are going to have problems with powering anything you put out there. Hopefully you have power out there already. If not then you will need to rely on batteries. If you don't want to be changing them all of the time then you will need to install solar panels. And that is a project alone.

 

A better solution might be if something passes through the gate your alarm goes off. I realize it is closing the barn door after the horse has bolted (literally) but it would be far more simple. It could also act as a burglar alarm.

 

This might be a good place for a 'hall effect' type device/circuit. They will detect the presence(or absence) of a magnetic field. Lots of ways to arrange the parts.

Seems like a viable idea.

http://www.jameco.com/1/1/771-ss41-magnetic-field-sensor-hall-effect.html

And they only take 20 ma. I wonder how sensitive they are? The sensor would need to be housed to protect it from the elements. The magnetic field would need to be strong enough to penetrate the housing. I suppose you could encase the sensor in epoxy which should not be too bad..

 

How about something like this:

 

Obtain some sturdy insulation material,make a hook or eyebolt that can be attached to this material,securely mount the insulating material to the post.

The hook/eyebolt is now insulated from the post ,gate etc,when the chain is not used.

You can often pick up quite solid pieces of insulating material free from a plastics supplier's discards,or have a look around your local salvage yard.

 

To expand on Ron's idea, a micro switch would work instead of the photo detector. The micro switch would not take any power like a opto switch, giving the batteries longer life.

The switches are usually water proof so weather wouldn't be a problem. Link to the type of switch - http://www.allelectronics.com/make-a-store/item/SMS-21/MINI-SNAP-ACTION-SWITCH-W/ROLLER-LEVER/1.html

To make the system easier maybe something like this could be hacked. Add a 555 timer to it for the delay form the switch?
http://www.tamperseal.com/travel-door-alarm-p-131.html

 

Obtain some sturdy insulation material,make a hook or eyebolt that can be attached to this material,securely mount the insulating material to the post.

The hook/eyebolt is now insulated from the post ,gate etc,when the chain is not used.

You can often pick up quite solid pieces of insulating material free from a plastics supplier's discards,or have a look around your local salvage yard.

What good would that do? As the OP has discovered, the chain is not a reliable conductor.
Maybe I am missing your point.

 

What good would that do? As the OP has discovered, the chain is not a reliable conductor.
Maybe I am missing your point.

No,I missed where he mentioned the resistance being as high as 38MΩ!

But if the chain is only being used to hold a hook at one end,& not using the links in any way to secure the gate,why not ditch it & replace it with a length of wire rope,which could be a reliable connection?

Obviously,it would be more expensive,but it would simplify things.

 

No,I missed where he mentioned the resistance being as high as 38MΩ!

But if the chain is only being used to hold a hook at one end,& not using the links in any way to secure the gate,why not ditch it & replace it with a length of wire rope,which could be a reliable connection?

Obviously,it would be more expensive,but it would simplify things.

That's a good idea. The hook (or whatever is on the end) would still have to make reliable, robust connection, which may not be simple.

 

How about something like this:

I think this is a pretty good jumping off point for an improved idea. I also appreciate Shortbus's wisdom on using a mechanical switch rather than a photo-interupter for battery life.

Using two anchor-side eye-bolts that would need a bit of alignment would be problematic**, but the idea of an anchor side receiving block with a small enough hole to trip a microswitch has serious potential.


When I get time later today, I'm going to see if I can make some plugs from scrap wood for the existing eye-bolt holes to keep the s-hooks in a small area, rather than rattling around the open center of the eye-bolts. From there, I'll rummage through my parts bin for micro-switches that might work and see if I can find any where I can envision a way to attach them to the plug in a reliable fashion. I doubt I've got enough matching micro-switches, but if I can find one that works, I can order more.

Any good suggestions on mounting the switch reliably (how to position it, how to attach it, how to improve weather-resistance, etc.) would be appreciated.

**Since most of you've never used a safety chain with livestock, you probably don't recognize the pragmatic function of the S-hooks over other types of mechanisms. Primary latches often use thing like hook-and-eye latches, because they're easy, cheap and hold the gate from swaying much. However, if they get bent or misaligned, they end up with weird ways that cause them to pop open. (And horses are great at discovering perfect combinations of push, push, quick pull = FREEDOM!!) That final arch backwards of the S-hook makes it impossible to be unlatched while taught, so when the horse sticks its nose through the gap to play with the safety-chain, they can't dislodge the S-hook.