I have a very old garage door opener remote. I would like another one, haven't looked for a genaric replacment yet, but the "build one" idea came to mind when I saw how basic it looks inside.

I have limited RF experiance. I love radio, but haven't got to building any radio circuits yet.

Questions - Is this flat out dumb idea thats just going to be frusterating? Or is it doable?

What freq. do these things generaly operate at? The PCB is kinda sloppy compared to current standards. Can I assume its somwhat low freq. from that? Khz? low MHz?

What are the things that look like diodes wrapped in fine wire? (in picture 3 they are below the big silver coil)

Lastly - how does this thing work? Don't most modern remotes use a code of some sort? This looks like a crude transmitter that is tuned with an adjustible inductor?

Anyway, just looking for comments/musings.

Attached Images

	rf11.jpg (77.5 KB, 29 views)
	rf22.jpg (83.5 KB, 30 views)
	rf33.jpg (53.9 KB, 43 views)

 

Actually this is a very do-able project. The remote usually includes only two basic stages, a code generator (or tone generator), and a simple RF oscillator. The RF oscillator usually operates at about 315 Mhz but I have seen a few in other bands. Yours looks like the 315 Mhz type, based on that little coil in the corner. These oscillators are easily tuned to frequency by bending the oscillator coil (squeezing turns together or spreading them apart. In the ones that I use, the oscillator is turned on and off in sequence by a simple digital serial code generator (in other words, they use Pulse Modulation). This code generator, usually an IC, uses the code setting on a multiple DIP switch to generate the serial code. Looking at the photos of your unit, I don't see the usual digital code chip and dip switches. The one in the photo may be modulated with a simple audio tone or something like that. I'm puzzled about the purpose of the large tunable inductor, but it appears to be coupled with a polystyrene capacitor so I'm guessing this is the tone generator and it is tuned by adjusting that inductor (or is it some sort of variable resistor?). Perhaps the tone is in the ultrasonic range, which keeps the L or C values smaller.

Although the circuit is simple, the problem you might have is that you need some way of testing that it is operating, some other way than the garage door receiver itself. RF guys usually use instruments like spectrum analyzers or test receivers for this kind of thing. The best second choice is to have an FM or AM receiver that can tune to 315MHz. Can you borrow something like that? It is frustrating trying to make RF circuits without some sort of way to look into the circuit, and often an oscilloscope just won't do. The spectrum analyzer is the tool of choice.

Those little things with the fine wire are RF chokes, basically just inductors with fairly high values of inductance, sometimes built around a resistor to de-Q the choke.

I agree, this is the crudest one I've seen. Check out that on/off pushbutton switch!

__________________
RadioRon

 

Thanks for your reply Ron! You must be the only RF guy here

I did a little poking at it with my scope the other night. On one of the leads coming off the big tunable indutor, I got a nice clean 15 kHz sinewave. I'm guessing that would be the tone? 15khz?

My scope only handles 60mhz, and I don't have access to a spectrum analyzer or any reciver that will recive 315mhz.

I get an odd looking trace in other parts of the circuit with my scope. Maybe due to the lack of bandwidth?

I'm still trying to come up with some type of tunable reciver. I hate to just give up. It looks like it could be a fun project. What is the 315 mhz range alloted for in the US? I just googled, but I can't fint any band plan chart or anything.

Would a wide band reciver, somthing like this -
http://cgi.ebay.com/New-Icom-IC-R5-W...QQcmdZViewItem
work?

I wouldn't mind adding one to my radio collection somtime in the future

 

That band is used for short-range control. There are some limitations on the duration of the transmission and of course power. While a person operating it does not need a license, the transmitter must be licensed. I have no idea how strict the enforcement is.

Several frequencies are used, with 315 MHz being common for garage doors. Other openers do use other frequencies, however. See this link for what some other brands use: http://www.aaaremotes.com/ John

Edit: Here's another link: http://www.rentron.com/remote.htm

 

Frequency counters are quite cheap and as most work up to 1.2GHz they are perfect for tuning transmitters.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

It just occurred to me that your transmitter doesn't have any code setting switches. It is probably just CW with maybe a continuous modulation (15 KHz???) to keep your door from going up and down everytime someone else operates theirs.

Reynolds sells a nice receiver (or use the one in your garage). You can put your scope on the demodulation pin and see what's going on. You can also use that to tune, to an extent.

I know southern MN is a pretty safe place, but I wouldn't trust a non-coded garage opener. Just leave the door open...except when it snows. Have you had a freeze yet?
John

 

To be strictly correct, the transmitter does not need to be licensed, it needs to be either verified or certified (different procedures) under part 15 as an intentional radiator (in the USA that is. The specifics are unique to each country). Once this is done, licensing is not required. You had the right idea, but the jargon is important to get right because the terms mean very different things under FCC rules. The enforcement for this kind of thing is non-existant and there is zero chance of getting into any trouble by fooling around making your own garage door opener unless you interfere rather badly with someone else's life or business.

__________________
RadioRon

 

It can indeed be very confusing trying to see RF waveforms on a low bandwidth scope and I guess that you are suffering from this. That little IC-R5 looks like just the ticket. The key things include that it has to be able to receive any frequency around 315 Mhz, you have to be able to select either AM or FM on any of the frequencies. It is also very very handy if you can choose the IF bandwidth of the receiver no matter whether you've chosen AM or FM mode. It can be very handy to have a very wide receiver even when using AM, for demodulating high data rate signals without too much distortion.

This receiver seems like an expensive thing to buy just for this project, but if you have other reasons to get one, they can be a lot of fun.

You can tell a lot about the oscillator that you build by listening to the oscillator signal through a receiver. For example, if you built a really good clean quiet oscillator, you could tune it in with the FM mode and you should hear nothing, no hiss, no noise, just absolute quiet, and it woudl only be quiet at your oscillator frequency. If your oscillator isn't perfect, you may hear buzzing, or hum, or other crud, and your ear can help you try to identify the crud which gives you clues about where the crud is coming from. If you oscillator drifts too much, it will be difficult to keep the radio tuned into its signal. This is all useful stuff.

__________________
RadioRon

 

Let's see, it is an important semantic detail for a rule that is not enforced. Is that a joke?

Not to beleager the semantics, but here are two additional terms in the US that have specific legal definitions and are often also referred to in casual speak as "licensed," but are not: "cleared" and "approved."

The point I was making was that once a device is verified or certified under Part 15 by the manufacturer, any modficiation to it by the user, voids that verification of certification. It matters little whether it is verified, certified, cleared, approved, licensed, or in any other legal speak one may want to add, but simply to be aware that it is a regulated device. John

 

Andy - my old Heathkit dip meter also behaves as a tuned RF detector and I think it goes up that high in range. While the calibration is poor you might be able to sense the presence of RF or not presence, so to speak. You might also build an RF probe though in a circuit like this it would seem that it could severely detune the circuit. Still - it's little more than a capacitor, diode and resistor or two that would rectify the RF and provide DC. Sensing the RF even without knowing frequency might be of some help.

__________________
stevez

 

A long time ago I found a really easy way to detect RF output from a garage door opener or RC transmitter with my 35 MHz scope. Just clip the scope probe's ground lead to the tip of the probe forming a one turn loop.

It can detect RF frequencies way beyond the advertised frequency of the scope. Just bring the loop close to the portable device, or better, around the antenna.

Bob