I want to design a circuit that will close a relay once a transmitter is within a certain range. (a couple of yards <10yrds). So if a circuit is set up on a pedestal and i am holding a second transmitter/receiver and i get close to the pedestal the relay will close. Can someone tell me where to start my research on something like this.

 

Did you just want a circuit to detect your proximity or a circuit with the ability to control one or several relays remotely? One, two, three and four channel remote relay cards controlled by a key fob can be had inexpensive. The reason I suggest buy over build is because making a board for remote RF is not an easy task, especially with parts placement and how things need not only laid out but distances between even the runs on a printed circuit board.

Here is an example, 433mhz 12V DC 6 Channel Wireless Remote Control Relay Switch 12V Remote Relay ON Off Transmitter and Receiver. A Google of "wireless relay card" will bring up a few dozen examples. You can also Google just for example "how does my truck know when the FOB is close proximity"?

Really for what you seem to be after as to research, how does your car or truck know when the correct coded FOB is in close proximity?

Ron

 

There are a number of approaches to consider, with lots of pros and cons depending on what is important.

How accurate does the distance at which the relay trigger need to be? If it is 10 yd now and 4 yd fifteen minutes from now and 12 yd tomorrow and 2 yd next week, is that okay? Or does it need to be a repeatable distance within a few inches? Very different problems.

Is this transmitter going to be transmitting continuously, 24/7? Or does it need to be silent unless it hears some kind of a beacon from where the relay is?

 

I am trying to imagine the purpose of such a scheme and nothing comes to mind except a booby trap setup.

 

I am trying to imagine the purpose of such a scheme and nothing comes to mind except a booby trap setup.

I don't know. I used my truck as an example. It knows when the correct key fob with the correct code is in range. Hardley a booby trap. I can likely come up with other examples but the key fob in close proximity to the vehicle is an example.

Ron

 

I don't know. I used my truck as an example. It knows when the correct key fob with the correct code is in range. Hardley a booby trap. I can likely come up with other examples but the key fob in close proximity to the vehicle is an example.

Ron

The key fob thing is a special case and it does not send out a signal until it gets a signal. If it transmitted constantly the batterylife would be short..

 

I am trying to imagine the purpose of such a scheme and nothing comes to mind except a booby trap setup.

I can think of a few benign applications -- a system that automatically opens a gate when you pull up to it, or something that performs a task when an animal wearing a transmitter collar gets near (not necessarily a trap of some kind, perhaps a gate or a food dispenser). Lots of issues that will drive various decisions, including battery life of both ends.

 

What kind of radio transmitter is this?
What is the frequency and power of the transmitter?

 

Im trying to make something that when my dog gets within a certain range i make a relay come on. So if i put a small fob on his collar and he gets within 5ft or so i can trigger a device.

 

I am trying to imagine the purpose of such a scheme and nothing comes to mind except a booby trap setup.

First thing I thought of after reading the title and before reading the thread was "Bomb". I'm sure that's not the case. And several posts have pointed out other potential uses. For instance, my wife comes home at night. The porch light comes on because it detected motion. As she approaches the door it unlocks and she can quickly get in. But a potential problem I see with that is if someone is following her then she has to get more than 10 feet away from the TX/RX unit so the door can again be locked.

I've been looking at remote door locks for the house. They're expensive. But you can open them with your cell phone and an app. Some require the phone to be held against a reader pane before the door will unlock. Can we ask what the purpose of your "trigger"?

 

First thing I thought of after reading the title and before reading the thread was "Bomb". I'm sure that's not the case. And several posts have pointed out other potential uses. For instance, my wife comes home at night. The porch light comes on because it detected motion. As she approaches the door it unlocks and she can quickly get in. But a potential problem I see with that is if someone is following her then she has to get more than 10 feet away from the TX/RX unit so the door can again be locked.

I can see a few ways to deal with this situation. The simplest would be to have a timeout such that if the door is unlocked with the remote, once the door is opened it will ignore the signal from the remote for some reasonable amount of time.

I've been looking at remote door locks for the house. They're expensive. But you can open them with your cell phone and an app. Some require the phone to be held against a reader pane before the door will unlock.

I would find this more inconvenient than fishing out my keys: Dig out phone. Unlock Phone. Open App. Do whatever the app needs me to do (probably after watching three or four commercials ).

And what happens if the system fails or the power goes out? Even if you could still unlock the door manually with the key, many people would be reliant on the app to unlock their door, and so would stop carrying their key with them.

I wouldn't mind having a keypad-based electronic entry, but the one I would like would be like ones I saw back in the '80s in which the keypad buttons were each a one-digit LED display and each time the pad activated, the numbers were placed in random positions. That made it very difficult for anyone to observe you entering the code and figuring out which code you entered. The buttons were constructed so that you had to be looking straight into the pad to see the digits displayed -- even moving your head to the side much over a foot made them unreadable.

Of course, I realize that this would be overkill, since if the person wants in the house, there's LOTS of easy ways to accomplish that, so it's purely the cool factor.

Can we ask what the purpose of your "trigger"?

Look at the post above yours -- it's for his dog.

 

My sister has just such a system to operate their dog door. It was needed to keep other creatures out of the house and it works very well. It uses a small transponder on the dog collar and it is adjusted to a range of about two feet so that it does not open whenever tha dog walks by.
I can ask about the brand name next time I see them.

 

I would find this more inconvenient than fishing out my keys: Dig out phone. Unlock Phone. Open App. Do whatever the app needs me to do (probably after watching three or four commercials

I understand. I think you might be over-complicating the process. But here's my thinking on this: The door could be unlocked while my wife is still in the car. This would still make her a little more vulnerable but I was just creating a scenario for the sake of making an example. Maybe I was a bit over the top as well.

And what happens if the system fails or the power goes out? Even if you could still unlock the door manually with the key, many people would be reliant on the app to unlock their door, and so would stop carrying their key with them.

I carry my car keys all the time. My fob allows me to unlock the door just by touching the door handle. I can program the car to respond with just unlocking the driver side door or to unlock all doors. I would never depend 100% on technology because we all know tech can let us down at the worst possible moment.

Look at the post above yours -- it's for his dog.

I missed that.

On the subject of unlocking the door, I'd like a system where I'm carrying the key in my pocket and as soon as I touch the knob it unlocks. I don't know how secure that would be but it sure would be convenient.

On the subject of doggy doors, I had one for my cat. When he wore his collar with a magnet hanging on it, the door would unlock and allow him in. He could go out anytime he wanted unless we locked the door, locking him in. If we accidentally locked him out he could still get in with his magnet. The ONE problem we had with that system was that he'd manage to get his collar off. The door consisted of a 9V battery, a reed switch and a solenoid.

 

The system my sister has for her dog is much more sophisticated in that the transponder must reply with the correct digital code to open the door. That would make spoofing attacks rather difficult and quite complex. And a person could easily carry the small fob in a pocket.

 

The system my sister has for her dog is much more sophisticated in that the transponder must reply with the correct digital code to open the door. That would make spoofing attacks rather difficult and quite complex. And a person could easily carry the small fob in a pocket.

That would depend on what size the dog is. A large dog will require a large door, one a person can slip through. On the other end is the TeaCup dog. I wouldn't worry about so much sophistication in that case.

 

I carry my car keys all the time. My fob allows me to unlock the door just by touching the door handle. I can program the car to respond with just unlocking the driver side door or to unlock all doors. I would never depend 100% on technology because we all know tech can let us down at the worst possible moment.

You wouldn't, but the world is filled with people that would and do. Almost every time there's some kind of disruption to the cell network in an area, there are all kinds of stories about people that can't function because they have become entirely reliant on their damn phones. There is also no shortage of systems on the market of all shapes and sizes that don't allow for continued operation in the event of technology failures -- electronic door locks that simply can't be operated manually, for instance.

 

The system that requires getting a digital response is not likely to be spoofed or otherwise compromised. A bad-actor would resort to brute force methods before they could crack the system.