I need a Magnetic and Photbeam Sensor!

December 3, 2007 11:11 AM

Can you show me a photo beam sensor or a magnetic sensor that is close to the size of my pinky?

My partner and I are designing a cage that is meant to hold a helper monkey. The monkey too smart for a simple lock, so we need to build a system that will ensure that the door is locked when the monkey is in the cage. We decided to use both magnetic and photo beam sensors in specific places of the cage. We need a sensor that is small enough to fit is various spots of the cage. Since I do not know much about sensors, I am not terribly sure what specifics I am looking for. I do know that it needs to be programmable with the rabbit core microprocessor modules (http://www.rabbitsemiconductor.com/products/CoreModules/index.shtml) Any Ideas?

posted by kaozity to technology (8 answers total)

You mean, like the kind included in garage door openers to keep it from closing if there's an object in their path? It's a beam and receiver setup... a little bigger than your pinkie, maybe more the size of two fingers. That might be a good place to start. I gotta believe Radio Shack has some kind of setup that might work as well.
posted by Doohickie at 11:23 AM on December 3, 2007

Here's a magnetic proximity sensor that might do what you're after, though I must admit I don't understand what you're doing with these, or why they're needed to keep a monkey in a cage. What's wrong with a keyed lock, or if you don't want the possibility of a lost key, a numeric keypad?
posted by contraption at 11:39 AM on December 3, 2007

It sounds like you're way overengineering this. Surely the monkey isn't smart enough to pick a combination lock, if it doesn't know the combination? It's also not immediately apparent to me how you plan to use light & magnetic sensors to control the cage lock. And a microprocessor? Wow, that must be one smart simian.

You can get optical and magnetic sensors from Mouser Electronics. The magnetic sensors you're looking for are called Hall-Effect sensors (you could probably also use a reed switch, which is simpler and maybe a little cheaper). These are discrete components; you'll need a bit of electronics knowledge to wire them up properly, but if you can handle the microcontroller, the sensors should be no problem. If you want pre-made modules that hook up to a Rabbit microcontroller, I can't help you (other than to suggest that you surf around the Rabbit site).

Sounds like a fun project, anyway!
posted by spacewrench at 11:47 AM on December 3, 2007

the problem is that a computer needs to manage the system because the people who are using the helper monkeys do not have the capabilities of using any body part from the neck down.
posted by kaozity at 11:52 AM on December 3, 2007

Not to derail, but the oddity of your request really piqued my interest. Knowing a guy who's paralyzed from the neck down, I applaud you for helping. But I have to wonder how the monkey is going to get put back in the cage by a quadriplegic. If it's well-behaved enough to climb back into the cage on command, it seems unnecessary to keep it in a cage.
posted by fogster at 12:25 PM on December 3, 2007

Ah, I see. Sort of. The underlying problem is that of controlling the door lock, but all you need for that is a solenoid and a little bit of mechanical linkage. It'll be like an automatic door lock for a car, and controlled by the microcontroller. Not too difficult.

The second problem is how to control when the door is locked and unlocked. You said you want the door locked when the monkey is in the cage, but there's got to be more to it than that -- if you just wanted the door locked when he's in there, you could just weld the door shut. Done.

What other trigger (besides the monkey being in the cage) will be invoked to set the lock? And what will trigger unlocking it?
posted by spacewrench at 12:37 PM on December 3, 2007

What you really want is a sensor that says yes, the monkey is in the cage, or no, the monkey is not in the cage. Burglar alarms, strangely enough, are where this sort of sensor technology has been refined. You are fortunate because the cage encapsulates the volume of a a simple rectangular prism. Sensors that determine whether or not something is occupying a volume are called volumetric sensors and you can read about them here.

I would probably use passive infrared, taking advantage of the fact that the helper monkey is always going to be a large body radiating at 38 degrees C or so and there won't be anything else that warm in his cage.

I love the idea of a helper monkey, by the way. This was the first I'd heard of them; I'd like to learn more.
posted by ikkyu2 at 8:22 PM on December 3, 2007

Just a few thoughts. If you're using a solenoid to control the door lock, you'll want to design it so that it's locked in the off state so when your system loses power, the door doesn't flop open.

If you need to sense the presence of the monkey in the cage, I agree with ikkyu2 that burglar alarm PIR sensors are a good way to go. If you're really gungho, this infrared b&w camera is a higher-tech alternative. Just store a picture of the inside of the cage empty and compare it with live pictures. Check out the Flo Control project for info.

And I can't recommend Forrest Mims' books highly enough for anyone who wants to build electronic doodads. In your case, check out the sensor mini book.
posted by jdfan at 8:32 AM on December 4, 2007