I have this pool alarm http://www.diycontrols.com/p-6711-sensor-espio-inground-pool-alarm-safety-item.aspx. I would like to log whenever the alarm goes off so I know if it is tripping while I am gone very often. How can I find out when a signal is sent? I don't want to hack into the remote alarm, but actually detect the radio signal.
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4 Answers
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Looking at the manual, this is FCC certified. That means you can find out a lot about it.
I went to here and searched by the parent company's name, MG International, bringing up 4 applications. One of these is the Sensor Espio.
From there, I can see it works on 433.92 MHz - this is probably the most common band for wireless devices. The schematic is available - it uses a TI CC1100. The operational description says it uses OOK with a 16 bit security word and 8 bit command word.
Most of these 433MHz devices use fairly simple Manchester encoding, sometimes using infra-red remote protocols over RF. I've not used a CC1100 before, but have used a CC1110, which is fundamentally the same. They have a lot of fancy features for FSK and MSK, but are quite basic and hard work for ASK/OOK. I think this means that the protocol is essentially free choice, but most of these will be a long preamble of 1010101010 or similar, followed by the security/ID and then command word repeated several times.
You can use any of the simple 433MHz receivers to pick these raw signals up, but then you will need to use a microcontroller or similar to wait for the preamble and listen to the words. The website JeeLabs has a large number of tools to help you connect a 433Mhz receiver to an Arduino and decode it.
By logging - do you mean simply recording? Any microcontroller can do this. Or do you mean alerting you via SMS or email?
answered Apr 24, 2012 at 11:54
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\$\begingroup\$ Wow, this is great info. For now I am going to just log (most likely to a pc), but in the future I may write an Android app and setup a server so I can see it real time. \$\endgroup\$ Commented Apr 25, 2012 at 5:12
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\$\begingroup\$ As I am US based I was thinking of picking up this sparkfun.com/products/11018 it looks like it should work. \$\endgroup\$ Commented Apr 25, 2012 at 6:10
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\$\begingroup\$ I'd avoid the RFM22 modules. They have a microcontroller inside them and it is hard to force them away from using their own internal state machine to deal with reception. They are great modules - I have about 20 in my house reporting various things to a central node. \$\endgroup\$ Commented Apr 25, 2012 at 8:53
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\$\begingroup\$ I used things like these seeedstudio.com/depot/… which just have a digital out and deal with the automatic gain control themselves. You just need to listen out for the preamble to work out when it is gained up and showing you noise or receiving an actual signal. \$\endgroup\$ Commented Apr 25, 2012 at 8:56
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You probably won't have much luck just detecting the radio signal. There is noise on every frequency, so your alarm listener would probably constantly detect a signal. What you would need to do is listen for a valid wireless message. But you will need to do some major reverse-engineering to figure out what constitutes as a valid message. Much better to hack into the remote which is already programmed to receive an alarm message. I would probably just add a mosfet to one of the alarm outputs which activates your own circuit to perform logging.
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\$\begingroup\$ This is definitely the problem and unfortuanlty you will need to hack into to the receiving device and add something to it. Unless you can find the exact frequency and protocol.. its a needle in a haystack \$\endgroup\$ Commented Apr 24, 2012 at 10:46
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You can just attach a latching device so when it gets switched on, it stays on. Search for SCR (thrysistor). You can attach one to an LED, and if the LED is on, then you will know. Attach this to the remote.
To log, I know of a clever instrument. You can have a photoresistor attached to a 555 timer circuit. Attach output to tape recorder. As you play back the recorded tape, if there are any frequency changes, you know that it has gone off. Point the photoresistor at the indicator lights.
Or you can just place a tape recorder next to the alarm. Attach mike. Point mike at speaker of the alarm!
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If you can hear the alarm, you dont need the RF signal, just filter the audio tone with a good BPF and a cheap mic and an event monitor perhaps on USB with suitable software or use an auto-dialer to your desk at work.
BPF of my choice is a high Q Sallen & Keys with adjustable F and adjustable Q to tune to Pool alarm. Or higher Q with State Vbl BPF adjusting Q with Ra.
Choose Q>100
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\$\begingroup\$ Great idea thinking "outside the box". But I am interested in learning about RF and this is an opportunity to do so. \$\endgroup\$ Commented Apr 25, 2012 at 5:15