Simple Power Line Communication

Question

I'm working on a remote (wirless) door opener and I need to install a device (just in the opener) at the entrance of a building where there is an electric opener. The problem is i can not install wiring, and i need to power my circuit. To do this, the only way that I thought without installing wiring or use batteries, is to use the wiring that reaches the lock to open the door. In this case what I do is hijack the original system cables and use them to carry the voltage to the device. I want to make it an universal device, and usually, the electric opoeners works with 12 V dc, 24 V dc or 24 V ac, so my device must work with these voltages.

The problem is that my device should not override the previous system, ie, with the old system i must still can open the door. But because i'm using the drive cables i need another way to open the door with the old system.

What I need is:

• A solution as simple and cheap as possible
• Use only two wires to power and transmit data
• Can send a single bit of data over the power line.
• Make it safe.

The solution to which I have come is to send information (a single bit of information, on or off) that run through the power line. I've been reading about the PLC but is too complicated for my project, too expensive and much more than what I want.

This is the scheme of my solution:

I decide to make two devices, one located where the old system is, and another next to my device in the door opener. The first device generates the power (12v ac or 24v ac or dc) the power my device and the door opener, and also listen when the old system want to open the door. When the old system want to open the door it ativate a frecuency generator (for the prototype i'm using a 555) to modulate a ac signal over the power line.

In the receiver side i filter the signal and transale it to a on/off signal, when there is ac signal modulated over the power line is on, and when not is off. This signal goes to the microcontroller and then it opens the door or not.

For cupling the ac signal (with a frequency of 10 kHz) over the power signal i'm using a coil (10mH) in the power line and a capacitor (0.1uF) between the frequency generator and the power line.

In the receiver side i'm using the next circuit:

I am a newbie to electronics so forgive me if I made a huge failure. By the way, i proved it in a proto borad and it works, but i think that this is not the right way to do it.

I want to know if this is safe, usefull and how to improve it, or if there is a better and cheaper solution. If you need more details pls ask me.

Thanks for your help, and sorry for my english.

Answer 1

If the old system works on either 12VDC or 24VDC, how about using 12VDC then wire a 24VDC relay at the opener circuit. Then when you want to send the open signal you would temporarily increase the line voltage to 24VDC, (at 24VDC the relay closes). You would have the relay contacts switch the same line voltage over to the opener circuit. If the relay partially closes or remains closed at 12VDC you might add a high power Zener diode near the relay coil to improve the operation.

As an alternate to this a simple voltage detector could be designed. A stable reference voltage could be generated and the high or low DC line voltage compared to this. As with the relay idea, a simple high voltage detection signals the opener to operate.