RF remote signaling

[analogee]

I've got a friend with a long driveway, and he'd like a signal to tell him, in the house, when someone has entered the driveway. It'd be a pain to run a wire down there, so he'd like it to be a wireless transmitter that is battery powered (could even be a car battery he charges every few weeks or so).<p>Anybody know of a simple system (assuming we work out some kind of motion detector or optical sensor to detect the car) that can send a very reliable, very simple but unmistakable digital message (a bit more sophisticated than carrier exists at a certain frequency, but not much more) over a 1000 ft. distance through a couple trees? A good feature would be if it were technically legal! I'm figuring some kind of semi-sophisticated antenna will be needed, but if not, all the better.<p>Something like a wireless doorbell would be ideal, but I don't think any of those have the range we need.<p>Regards,
Todd

[Chris Smith]

Lasers are the easiest, cheapest, and simplest. You send the ir beam down the drive way, hit a 45 degree mirror which crosses over the drive way, and another 45 degree mirror brings it home. Then like a light beam in the 7/11 when the beam breaks it goes off. Its good for a mile or so depending on your laser and lens. Use front surface mirrors. The reciever is just a hold on relay that fails and set off the bell when no light strikes it.

[Dean Huster]

Consider an el cheapo 40MHz walkie talkie set. Wire an oscillator to the microphone input and let the electronics of the sensor close the power switch. At the house, the companion receiver can be run from a wall wart and a PLL can react to the tone that's transmitted. Legal? Modulation-wise, maybe not. But it's a pretty public band with all kinds of crap on it including baby monitors, so what's the big deal considering a baby monitor may be transmitting 24/7? Besides, your tone would be a momentary transmission. And a quick check using two people will tell you if the talkies have the range.<p>Dean

[analogee]

Thanks for the ideas. I especially like the walkie talkie one. The laser idea seems workable in general, but it seems I would need exceptional mechanical stability (not my forte) in the mirrors, since probably a small fraction of a degree of error would cause the light to miss its target. And, really, there is not a clear line-of-sight shot from the driveway entrance to the house.<p>Anything simpler than the LM567 these days to detect a tone? Can you even get an LM567; National says discontinued.<p>Regards,
Todd

[Riptide]

The walkie talkie is a good idea and RS has one that has a orange button on the side that will transmit a tone when pressed, eliminating some tone generating circuit.Simply short the contacts then hook up as Dean said.<p>I think Ramsey still sells the 567 tone kits that would make this project very simple.<p>http://www.ramseyelectronics.com/cgi-bi ... on&key=TD1<p>RT<p>[ July 11, 2003: Message edited by: Riptide ]</p>

[Ron H]

According to National's web site, LM567C is in full production in both SOIC and DIP.<p>Dean, I've been casting about for a simple way to control a damper (with an on-off thermostat) on the AC duct to the bonus room above my garage. The damper is in the garage, and adding wires is pretty much out of the question. I've been thinking RF, but I'm too lazy to design and build a transmitter, receiver, and decoder. I hadn't thought of the walkie-talkies. I have a couple that I just recently replaced with FRS/GMRS radios. You may have solved my problem!<p>Ron

[desterline]

Walkie Talkie's are an OK solution, I used them in the past for simmilar projects; however, nowadays I'd look into cordless phones. I've seen 900 Mhz ones for les than $25 and I'ev seen them transmit reliably more than a 1/4 mile.<p>They already have DMTF encoders built into them, and decoders are redily available for resonable prices. As far as FCC regs is concerened, as long as you don't change the output power, you should be fine. And of course, it doesn't need to be connected to the phone line.<p>The detection circuit would probably need to be a little more complicated, because I'm not sure you could get by with somthing as simple as holding down the tone button and pulsing the power. (you could with walkie talkies)<p>You would probably need to sequence the unit, power on, wait, send tone. But I guess that would depend on the make / model of the unit you use.<p>Good Luck
Denny

[Calcbert]

You might want to look into these RF modules from Laipac. They seem to be a lot like how garage door openers work, where you'd have a dip switch to set a unique code. I haven't actually worked with them, though I want to. They don't seem to have any distributors, but they do sound willing to ship to the US (they based in Canada). See their products at http://www.laipac.com/products/wireless.htm

[Calcbert]

One more easy idea, about 30 seconds later. I've had great success in modifying cheap wireless doorbell sets to do my bidding. I've operated one for a while as a remote control SLR camera trigger. My Nikon N-70 uses an electrical bulb-type control, and you definitely don't need to buy the official SPST switch from Nikon to make it work. I digress...<p>You may have to test a few doorbell sets to find one with enough power to reach the house, but that should be easier without walls and such to go through.<p>The best way to make this work is to get out a voltmeter, connect one side to the negative of the batteries, and probe around the circuit while it's ringing, looking for a good digital type signal. Obviously pins of ICs are the best to check on, so long as you don't get a all-in-one-chip-under-an-epoxy-blob circuit. <p>Just connect an NPN transistor base to this point, run the collector to the power supply, and the emitter through a relay or other device to ground, and you're set. Or just use the bell sound, since that's the kind of detection you want.<p>As for the transmitter side, it's just a switch in the circuit. You could probably hook up a commercial reflector beam sensor, or if you're feeling adventerous, rig up an ultrasonic sensor (there are some that work with Basic Stamps for example) to detect an object within N feet.

[Calcbert]

OK, I missed the part about the 1000 foot range. Most doorbell sets have DIP/jumper settable codes to avoid interference. You could make relay stations perhaps?