Help transmitting a digital on off signal over a distance of 600 to 800 metres

[sobell1980]

Hi all,

Looking for help building a circuit or buying a transmitter / receiver to send a digital "on" and "off" signal about 600 to 800 metres across a housing estate. I am constructing an automatic chicken house door that opens and closes with a light sensor using programmable arduino boards. I want to transmit a very short length signal, to say when the door is open and when the door is closed from my allotment to my house. I am able to program the arduino boards to operate the door but just need some sort of set up to transmit the open or closed signal that is sent from the arduino board to the transmitter. Would be most grateful for any help or input. I live at a very elevated position from my allotment. The signal would need to be sent across the housing estate to my elevated position. Many thanks as always,

Dave.

[Sean Williams]

Dave,

Your best bet would be to go and use one of the license free PMR type radios - cannibalise a pair bringing out the PTT button to a set of contacts - this will easily, and legally work over the range you need.

[paulsherwin]

Assuming you have power at each end, the most pragmatic solution is to use a wifi bridge, with directional aerials at each end. You could recycle an old laptop at the remote end. Once the link was established you could use it to transmit much more than a door open / door closed flag - temperature, for example. You could use it to control heating and lighting remotely, or run monitoring cameras.

You can build effective directional wifi aerials for next to nothing - google 'cantenna'.

[G8HQP Dave]

Another option is to use type-approved short-range telemetry units (on 418MHz?). These appear in PCB form in many electronics catalogues. Bear in mind that any licence-free system is likely to suffer interference so you will need a protocol to cope with this.

[G6Tanuki]

Have you thought of using an old mobile phone ?? Shove in a "pay-as-you-go" SIM from someone like GiffGaff with a fiver's worth of credit. Program one of the phone buttons/memories with your home phone-number. Get the arduino to pull in a solenoid at the appointed time to trigger it for long enough to give, say, five rings then hang up.

On *your* mobile, set up the chicken-coop-phone's number with a specific ringtone. When it goes off you don't answer.

Because the call doesn't connect you don't use up any credit on the PAYG SIM.

Your "missed calls" log will also hold a record of the dates/times the chickens called home.

[paulsherwin]

Originally Posted by G6Tanuki

Get the arduino to pull in a solenoid at the appointed time to trigger it

Or, cannibalise an old mobile, wiring a relay or solid state switch to one of the number contacts. Old mobiles are generally available for the asking if you mention you want one to friends and family. Lots of operators offer non-expiring PAYG SIMs including Asda and Virgin, both of which I use.

A rather more sophisticated approach would be to link a mobile to a laptop using USB or Bluetooth. If you enjoy hardware hacking you could replace the laptop with a Raspberry Pi.

[sobell1980]

Many thanks for all your input on my project. I will have a play with the arduino and mobile phones and get back to you with our progress. Thanks again your all a wealth of knowledge.

Dave.

[TrevorG3VLF]

An alternative is a wireless doorbell. There is a BN wireless doorbell on ebay for £15, this has two push buttons for front and back door, presumably the received signal is different for each. The disadvantage is that the range is only 75m. This could perhaps be extended with a Yagi aerial at each end. I do not know what frequency they use.

[fetteler]

Yes, I was thinking wireless doorbel too, I guess they're 433 mHz. I expect the internal aerials are little things and it might not even be neccesary to use a Yagi, a 1/4 wavelength of wire ought to improve things quite a lot.

Anyone here with a spectrum analyser and a wireless doorbel who'd like to comment??

Cheers,
Steve.

[Nickthedentist]

Don't forget a lot of locals will have wireless doorbells too...

[OscarFoxtrot]

Originally Posted by sobell1980

I am constructing an automatic chicken house door that opens and closes with a light sensor

Do you have any method for counting your chickens out and in, otherwise the door might close locking a chicken out for the night, which presumably is not what is intended?

You could use a IP CCTV camera over 5GHz, most decent cameras have alarm/switch input/outputs so you can wire the door mechanism to a switch, and a PIR detector to an alarm.

I dare say a group A TV antenna would give a bit of gain at 433MHz, use one at each end and 800m would be easy.

[paulsherwin]

Adding a high gain aerial to a wireless doorbell is probably illegal though. There are likely to be many problems, ranging from neighbours' doorbells ringing randomly to out of band interference caused by the very basic transmitters in these things. You won't make any friends if you knock out everybody's Freeview reception whenever the chicken shed door closes. You might even interfere with the police Tetris system, which would certainly result in a visit.

[cmjones01]

Originally Posted by paulsherwin

...You might even interfere with the police Tetris system...

did you mean TETRA? But I do love the idea of a police Tetris system...

Chris

[Radio Wrangler]

433MHz is in an amateur band and you may find that the very basic receivers of doorbells and suchlike will overload easily with any large signal within a few tens of MHz. Expect to find people running 10 to 400W into gainy antennae in this band, which certainly trumps the 100uW or less into a lossy antenna. There isn't really a lot of activity, but when there is, people can easily do 30 minute 'overs'

Amusingly, some 433MHz cordless burglar alarms were programmed to assume that any signal detected that lasts more than 30 seconds was an attempt at jamming, and so the alarms would go off.

The wifi extender sounds a bit better, but how long is a laptop going to last left unsupervised in an allotment shed?

David

[paulsherwin]

Originally Posted by cmjones01

did you mean TETRA? But I do love the idea of a police Tetris system...

Indeed

[pichacker]

Reading this thread I like the idea of using the licence free PMR446 units as they are available in many places and realistically are not that heavily used. Coupled together with CTCSS tones you have a sort of secure link.

However for my house alarm I used one of the GSM mobile phone modules. (Made by RF Devices). This takes a normal SIM card (PAYG) and is communicated to by simple +AT commands at 9600 baud. This should be a walk in the park for the Arduino. I used a PIC on mine.

Obvious advantage is that the range is virtually only limited by your mobile phones signal. Text messages could be sent but of course you'd have to choose a tariff carefully which has inclusive texts. Opens up the possibility of 2 way comms as well.

[llama]

Is it line-of-sight?

A beam of light - maybe a laser?
Graham

[FrankB]

As a wild thought, if your coop and the house are on the same mains feed, you might get by using a "carrier current" transmitter and receiver.

Some of the hams used them here in the states during WW2, and the power companies here also utilize them for data and "remote reading" the new digital power meters.
I have seen several articles on carrier current on the 'net and in old Ham Radio publications of the WW2 era

Not to be confused with 'PLT' ethernet over the mains, all that does is wipe out the shortwave and above. Carrier current uses less than 100kHz (or so).