Cicuit for multiple sensors on a single line

[Vam]

Hello,

I'm interested in designing/developing (although I don't know much about electronics) by own secuirity system for my house. This will consist of two systems, a wired electromagnetic sensor switches (to detect open/closed doors and windows) and a camera system (approximately 8 cameras, that will feed video to 3 TV's around hte house).

What I would like your help to design the circuit to connect the sensors to a controller unit (the designed circuit) which in-turn will be connected to the PC's USB or serial RS232 port (preferably USB).

Once the PC picks up the signals, I can get the necesary Software written to alert me or do something (maybe activate a few relays on hte same designed circuit to sound an alarm, light etc).

The tricky part is this: I would like to seperate my house into zones, preferably about 6 -8 (although this is not essential). and I would like to connect multiple sensors (e.g. 10) to a single wire pair (or to each zone wire pair)that will run around the house.

So in order for this to work, each sensor has to send a unique signal which the controller unit will identify. This is the reason for the zones, i.e. if the unique signal combination in less than 50, it's better to split the wiring into zones so that you can have x number of unique signals per zone, increasing the coverage. Ofcourse if you can have the number of sensors greater than 50, I suppose we can scrap the zone story.

could this be achieved by a 2 wire wiring, or will you need a 4 wire cable? also another Idea I had was to make small cuitcuits that will be placed in each room of the house, that will have approx. 6 sensors inputs. this will allow the additional wiring to be done only within the room, also it will make the door/window sensors small.

I'll talk about the camera requirement in a seprate posting.

your reply, ideas and solution is greatly appreciated.

 

[Oznog]

It can indeed be done, but it's tricky. Since this is quite useful to anyone wiring a car, house, or building it's always a hot topic.

A straightforward solution is the 1-wire digital protocol. It requires a microcontroller as a master. The microcontroller can then talk to the RS232 port. There's really no good way to do this with just resistors, capacitors, and such. A bunch of simple devices run on 1-wire and you can write microcontroller code to make a slave out of any number of sensors.

There are a number of hacked solutions where a serial protocol (such as RS232) can support multiple transmitters. You can make it so that the computer transmits an address of a slave device and the slave will then take control of the bus and transmit its packet back. You'll probably need to write microcontroller code here too.

 

[Vam]

Hi,

Thanks for the reply, yes the Microcontroller 1-wire solution sounds like a plan. Do you think you can give me some info on the circuit (if you have anything in mind) so that I can look into the possibility.

I'm totally clueless about electronics, but very eager to understand. Some you can help me out a bit with the circuit, or where I can find some help.

 

[Oznog]

The 1-wire is one signal wire and a ground. The signal wire is pulled up to 5v by a resistor. Either the transmitter or receiver can pull it to 0v by connecting it to ground. It can't carry a lot of data because the way it handles data is cumbersome but should be fine for just simple switches and such.

Google for the spec sheet for the DS18S20. That's a 1-wire device, a temp sensor.

You can't handle video with any of this stuff though. Seems unlikely you could combine the data and video on the same wire. You can get cheap little wireless cameras. They've got poor quality CMOS imagers but do ok. Of course anybody else could pick the video up too, so keep that in mind.