Radio Controlled Propellers

Philldapill

I currentl·have a propeller opening and closing my electronic gate for my driveway. I can purchase a RF·remote control for the gate controller, but they are about $40 each and they get lost frequently. This is why I am using the propeller. I have an IR detector mounted on the controller and use a TV remote to trigger it. The propeller sees the pulses from the IR detector, and if the code recieved is one programmed in it, it triggers the gate. This method works fine, but I have to be within 15 feet for it to trigger.

I want to build a RF controller.

I think this would be fairly simple to do. What I plan, is a simple transmitter that discharges a HV capacitor into an inductor so that it produces a large EM spike. Hopefully, this EM spike will be able to propagate enough(50') to a receiver that consists of an inductor with many turns. This inductor would then be connected to an opamp that would sense a smalle spike and cause the output to go high. I would set the opamp up so that I could adjust the sensitivity. The output of the opamp would then be fed to the input of the propeller, which would have a program running, monitoring this pin.

The program would consist of this - watch the pin using waitpeq, and figure the length of time between pin changes. The spikes would usually be about the same length, but the length between spikes would determine what was being sent. I figure I would have to send, I dunno, a few(3 or more) consecutive spikes so that the gate doesn't open when lightning strikes a single time. If the propeller "sees" some spikes at fairly close intervals, as oppossed to random spikes(lighting or other EM pollution), it will trigger the gate.

Remeber, this is all hypothetical and probably wouldn't work due to a number of factors, but any help from you expert guys is always appreciated.

Mike Green

I think you will find that your scheme is not very workable primarily because there's little to distinguish electrical noise from your EM pulse. You would do much better using a modulated CW (continuous wave) signal with some kind of unique code used to identify the signal. Use Parallax's 433MHz transmitter and receiver as your model, even if you eventually make your own transmitter and receiver.

With a properly tuned transmitter and receiver and a good antenna, you really need very little power. 50mW has been used to send Morse code for thousands of miles. Some Family Radio Service walkie talkies (en.wikipedia.org/wiki/Family_Radio_Service) have tone generators for signalling. You could easily use a tone decoder chip to detect a single tone or DTMF tone pair (Touchtone). RadioShack sells a pair of these for $20 with a 5 mile range.

JMLStamp2p

Philldapill,
I agree with Mike, I just completed a RF project (with help from a couple of guys on the forum). You can view the code on under "Remote Control Props" if you like. I am using Maxstream's RF transceivers because I needed some long range capabilitys, they are
very reliable and give the best distance for power output that I have found. I am using the 1-Watt, 900 MHZ spread spectrum, frequency hopping model.
As far as the code is concerned the 2-Props are transmitting Global Timer varibles back and forth, then displaying them on a 4 by 20 matrix LCD's by Matrix Oribital "bought from parallax". They then turn on a corresponding output for relays to operate a large water pump. My code a little rough looking, I don't pretend to be a great programmer but it might help you out some :>)
JMLStamp2p

JMLStamp2p

I am also using a 4 by 4 matrix Keypad to enter the Timers values which is running in a seperate Cog.
JMLStamp2p

Paul Baker

Im with Mike and JML, an RF pair is the way to go. I would even go a step further and put Keeloq on the system. These are the rolling code chips used in remote keyless entry systems, widely used in cars ad garage doors. It's inexpensive and adds moderate amount of security to your system. Without it all someone would have to do to break into your system is to listen to the RF traffic when you activate your remote. Keeloq uses the secret ID as a seed for an unique 1-time use value. So someone who listens to your RF traffic would get the one time value, but because it was already used it is no longer valid.

Now some industrious individuals may note "but Keeloq has been cracked, the brute force method was posted in 2007 and the side channel attack was published last month". But the brute force method requires 2 weeks of interogating the device,·and requires the exclusive access to the system (iow if you use the gate during that time, they have to start over). And the side channel attack requires them to monitor the current consumption of the keeloq chip and 50 dual core pentium machines 2 days while constantly interrogating the device (again difficult to hide 50 computers near your setup).

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Ken Peterson

I have successfully transmitted audio to an FM radio using the Prop before. Only hardware required was a piece of wire. I have been thinking of how one might detect and decode a radio signal with the Prop and minimal external components, but I don't see how it can be done without at least some external filters and amplifiers. One approach I was thinking about is to generate an IF frequency with the PLL that would be heterodyned with the incoming signal using an XOR gate. However, I haven't thought this through in detail or built any prototypes yet.

I have 315MHz transmitter/receiver set from sparkfun.com that seems to be able to transmit some distance. You need to get fancy with the protocol, however because you receive a lot of garbage.

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The more I know, the more I know I don't know.· Is this what they call Wisdom?

Phil Pilgrim (PhiPi)

Micrel's QwikRadio chips may be of some help here. They require almost no external components to transmit or receive.

-Phil