Circuit explaination.

[behrendfry]

I am trying to make a simple door that opens when my pet is within a certain distance from the door. They make doors like this but they are expensive. So I started searching for RF circuits and infrared devices that would allow me too attach a "tag" (transmitter) on my pet and have it activate a small dc motor that will open a door. I stumbled across this circuit:
**broken link removed**

Does anyone have suggestions on this idea, would it work for what I am describing or is there and easier way to accomplish what I am trying to do?

 

[Russlk]

The transmitter puts out a carrier that produces DC in the receiver. There is no signal to pass thru the transformer X2. Since it is an FM receiver, you need some FM modulation of the transmitter.

 

[MikeHibbett]

ah man, I had a good laugh at the circuit diagram. The transmitter circuit says the power source is a 12V car battery ) I can just imagine your cat with one of those round it's neck.

Personally, my years of professional embedded design work have left me with a deep distrust of the reliablity of electronic circuits, especially in cases like this. Why not use a magnet on your pet and a hall effect switch at the door? You will never need to change the batteries...

I have just finished a project using a small 400MHz FM transmitter, from RF Solutions in the UK. I used a PIC12F628 to generate a 1KHz burst every few seconds, the rest of the time the circuit is in a low power mode drawing a few 100uA. The circuit fits in the handle of a tennis bat with a 3v battery. I would still go with the magnet on my cat though.


Regards,

Mike.

 

[audioguru]

I think the simple FM circuit works in an opposite way.
Interstation noise passes through the transformer and is rectified, keeping the relay activated and the alarm off. A nearby FM transmitter without modulation causes quieting in the FM radio and the relay is relaxed, setting off the alarm (or motor for your door).

Little FM transmitters can be designed to operate from a lithium coin cell or a couple of alkaline button cells as its battery, but not for very long.
Simple FM transmitter circuits such as this one aren't stable, they change frequency with temp change, battery voltage change and anything conductive that comes near them.

 

[mstechca]

Little FM transmitters can be designed to operate from a lithium coin cell or a couple of alkaline button cells as its battery, but not for very long.

Keep in mind that the length of operation is (battery's mAH rating / circuit consumption in mA) hour(s).

Simple FM transmitter circuits such as this one aren't stable, they change frequency with temp change, battery voltage change and anything conductive that comes near them.

They say that transmitters having crystals in them are quite stable, but my transmitter that I posted here seems to be in good competiton. In fact, Audioguru loved it, and another user reported that (s)he got a 100 foot range. Also, it only operates on 3V lithium cell.

But a huge number of transmitters (especially with the antenna connected to the tank circuit) change frequency when your hand or (whatever signals can't go through) is close by.

 

[akg]

Keep in mind that the length of operation is (battery's mAH rating / circuit consumption in mA) hour(s).

A passive rf tag ?? ..or ckts based on electric field change??

 

[mstechca]

I'm referring to the entire battery life! In other words, how long the circuit will work before it stops working due to a dead battery.

 

[behrendfry]

The passive rf tag is how i want to go about making this device but i was looking at other ideas. I do not know enough in this field and i have been looking for information on how to create a passive rf tag and a receiver. This would be ideal because i want to incorporate it into other devices such as the cat box and food dish. This way the same tag will work ann all 3 devices. It will keep my dog out of the cats food if i have a lid that opens only when the cat is around along with the cat box. Anyway, any helpful information on how to create a passive rf tag and receiver would be great. Thanks.

 

[akg]

The passive rf tag is how i want to go about making this device but i was looking at other ideas. I do not know enough in this field and i have been looking for information on how to create a passive rf tag and a receiver. This would be ideal because i want to incorporate it into other devices such as the cat box and food dish. This way the same tag will work ann all 3 devices. It will keep my dog out of the cats food if i have a lid that opens only when the cat is around along with the cat box. Anyway, any helpful information on how to create a passive rf tag and receiver would be great. Thanks.

i feel it is difficult to build a practical passive rf module using discrete components . since the power needed for functioning of the module is transmitted, not much power will be available for the tag for operation. also after receiving the power it shld keep that power for some time and retransmit back. need more resources :roll:

 

[Sebi]

For small distances (up to 15-25cm) not too difficult to build a device for a passive RF-tag. The antishoplifting RF-systems work with same method with processor(s).

 

[akg]

For small distances (up to 15-25cm) not too difficult to build a device for a passive RF-tag. The antishoplifting RF-systems work with same method with processor(s).

possible(???) with descrete elements ?? have anyone tried that ?