Gate opener transmitter  - Page 1
Forum section: electronics / beginners

author: marcelamaines
author_rank: Newbie
timestamp: June 05, 2022, 05:06:43 pm
content: Hello there! I am posting here for the first time in this forum.Please apologize if I'm placing at the wrong place.I need to open a gate where I don't have access to the gate opener board itself. Therefore I want to either place another antenna or to install an amplifier that has rp-sma in and out.I was not able to identify where is the antenna or where I could place the amplifier. Could you guys help?Thank you all.

author: golden_labels
author_rank: Super Contributor
timestamp: June 05, 2022, 07:14:14 pm
content: The board employs a simple, very low power SAW oscillator based transmitter. See the image in the attachment, taken from RF Monolithics RO2101 datasheet.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: June 05, 2022, 07:54:26 pm
content: Circuit is designd around microchip HCS201.https://ww1.microchip.com/downloads/en/devicedoc/41098c.pdfApplication note AN728:https://ww1.microchip.com/downloads/en/appnotes/00728b.pdfPower stage is RF transistor BFR92A:https://www.nxp.com/docs/en/data-sheet/BFR92A_N.pdf(Fig. 2 shows basic test-setup)I painted antenna yellow

author: marcelamaines
author_rank: Newbie
timestamp: June 05, 2022, 08:56:25 pm
content: Thank you very much for the explanation!That helped a lot. I now have the following questions:1- If I build an antenna, matched for 433mhz and solder in yellow line you made (the original antenna), will I improve the range?2 - if 1 doesn't work, then do you think if I solder a wire to the RF input of this device could do the job? (https://a.aliexpress.com/_mPCTR3k)Thank you again, and if you have any suggestions that could help me opening the gate from this distance, it'd be greatly appreciated.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: June 05, 2022, 09:33:09 pm
content: Quotei doubt it.Why? It's magic - it's RF.A closer look at the Smith S22 of BFR92 indicates an output imedance approx 4.2 times Zo. Stays in somewhat at 200...240 Ohms with an not neglible portion of -j (capacitive).In easier words: the largest amount of power generated by the output trasistor would be reflected because of the totally mismatch between power stage and antenna. No real improvement.To solve that, an impedance matching network is required. I suggest usage of an miniature Balun 240 ->50Ohm.Can be found in antique TV Antenna splitters.An excellent example is Gunthard Kraus work on an 2m VHF preamplifer:http://www.gunthard-kraus.de/English%20Homepage/2m%20preamp%20with%20low%20noise/2m%20preamp.pdf

author: Uunoctium
author_rank: Frequent Contributor
timestamp: June 05, 2022, 09:59:14 pm
content: I'm not sure, but the reduction of the value of resistor R1 (100Ohms) could be resulting in generate an gain up of some dB's. R1 - feedback resistor - determines the amplification. In the application example they used 18Ohms. You can try 50Ohms by soldering a second 100Ohm direct on top of R1. And leave the rest of the circuit unaltered.If it works, it would be the more elegant solution but with the tiny disadvatage that the battery is discharged much faster.

author: marcelamaines
author_rank: Newbie
timestamp: June 05, 2022, 11:52:45 pm
content: Thank you for your kind answer.I am starting to believe this will be beyond my knowledge.The battery doesn't really matter as I will be using it stationary with a power supply.So in your opinion a 100 Ohm direct on top of R1 will help. I might try that.Thank you again.Marcela.

author: golden_labels
author_rank: Super Contributor
timestamp: June 06, 2022, 03:33:58 am
content: What’s the exact circuit you’re trying to make? It will be easier to suggest something.At this point I can’t find a reason one would want to increase power of the original transmitter itself.

author: marcelamaines
author_rank: Newbie
timestamp: June 06, 2022, 01:58:49 pm
content: Because my house is very far from the gate opener (200+) meters. I don't have access to it and I'm not allowed to make any modifications since it is a condominium. I want to be able to open the gate without having to go near it when a guest arrive.The gate opener uses a rolling code, making impossible to use those aliexpress/ebay long range TX.Any suggestions would be greatly appreciated.Thank you

author: golden_labels
author_rank: Super Contributor
timestamp: June 06, 2022, 04:28:43 pm
content: You have a doorphone system, right? That will require neighbours’ coöperation, but the best option I see is attach the gate to it. You already have all the infrastructure required. Unless this is some weird old system, it’s a matter of programming in one more gate and making a connection to it.The savage way of doing that — putting a receiver next to the key fob and retransmitting signal at much higher power — may work(1), of course. But to get a reliable 200m+ transmission, taking into account obstacles and poor weather, you will have to be blasting the entire neighbourhood in a half kilometer radius each time you open the gate. Radio communications have only one dimension to route signals (frequency) and all people must peacefully share that tiny space. You are also likely to break local radio regulations regarding(1)May as well not, due to reflections, which gate opening systems operating in LPD433 are not expected to deal with.

author: robca
author_rank: Frequent Contributor
timestamp: June 07, 2022, 04:52:37 pm
content: Warning, horrible hack followsIf you have more than one transmitter, you could build a battery powered device sitting halfway between you and the gate or even close to the gate. In that battery powered device you could put a receiver of your choice (LORA) that, when triggered, closes the contact of the rolling code transmitter. Then you would have a powerful transmitter (hopefully using something like LORA that works very long distance without killing everyone else's ability to use the same frequency) to trigger your receiver. LORA is ideal, because it can be made to be secure and can transmit very long distances in the unlicensed spectrum by using clever modulation in exchange for very low date rates. But since your need to only send a secure key to the receiver, that would work well for youDownsides: unless you can secure the box, you risk losing the original transmitter, your LORA receiver and a battery. Battery needs to be charged periodically

author: ledtester
author_rank: Super Contributor
timestamp: June 07, 2022, 04:58:13 pm
content: Another horrible hack...put the key fob on a drone and when you need to open the gate fly the drone out to get it closer to the gate, have it "press" the key fob button and then fly the drone back.

author: Manul
author_rank: Super Contributor
timestamp: June 07, 2022, 05:28:25 pm
content: One more expensive and ugly hack: use passive retransmittion. Take 200 meters of very low loss coax cable (looking at 1000$ or more) and put antennas on both ends. One end near receiver, another near remote. Should work. You can put poles every 10 meters to hold the cable.