remote key finder using rf

[Alila]

I am doing a project on how to use RF to implement a remote key finder. The remote should be able to locate the key within the radius of at most 50metres. What should be my working frequency and what is the easiest and cheapest way to implement this for a developing country economy?

 

[davenn]

433MHz is as standard allocated frequency for these sorts of devices --- google ISM frequencies for a full list

 

[Alila]

433MHz is as standard allocated frequency for these sorts of devices --- google ISM frequencies for a full list

Could you recommend a viable implementation module for the same?

 

[betwixt]

Try looking at www.rfsolutions.co.uk for ideas. There are many devices you can use, your problem will be uniquely identifying a single key rather than the wireless link.

Brian.

 

[biff44]

easy to do if you use an RFID chip with a battery attached to it. I would use the 880 or 915 MHz band for easy antenna size.

Passively finding it...would be pretty hard at that range.

 

[FvM]

easy to do if you use an RFID chip with a battery attached to it. I would use the 880 or 915 MHz band for easy antenna size.

Passively finding it...would be pretty hard at that range.

Your consideration shows that the idea has little chance of success. Passive RFIF with 50 m range is more than hard to implement, it's simply impossible using reasonable RF power levels. So it can't work without an active receiver. What kind of battery do you suggest for long term operation?

 

[biff44]

I would not say it was impossible. For instance, there could be a retrreflector type setup, where a very low DC power was used to continuoutly vary round trip phase modulation....something like a fet phase shifter that only took picoamps...a small lithium battery might last 5 to 10 years. You would need a very sensitive receiver.

But 3 to 5 years battery life is currently very doable with a receive only RFID setup, where the transmitter ONLY turns on when interrogated. One company makes a chip with a coding scheme where 1 mW transmit power will easily do 50 meters.

I am hearing reports of "miles" range with 10 mW transmit power and this heavy coding scheme

 

[betwixt]

I would rule RFID out completely. It hasn't got the range and would consume too much from the battery. My preference would be a passive or superregenerative receiver with a micro to decode the data. The micro could be in 'sleep' mode until a signal arrived. There is no need to work bidirectionally like RFID, some audible/visible indication at the receiver (the lost key) when stimulated by the transmitter is all it needs.

I ran a test rig at 433.92MHz a while ago which transferred over a million data packets without error over about 100m distance and ran from two AAA cells for several months. It can be done.

Brian.

 

[FvM]

O.K., I agree it can work with a sophisticated design. Operating the receiver with a few µW quiescent power is the basic point.

I'm still sceptic regarding the marginal conditions metioned in the initial post.

 

[jiripolivka]

O.K., I agree it can work with a sophisticated design. Operating the receiver with a few µW quiescent power is the basic point.

I'm still sceptic regarding the marginal conditions metioned in the initial post.

Acoustic key finders are nice in that they need a low power to "listen" and respond to whistling.

Using RF would require a lot of interrogating power to trigger a response.
There is, however, a simpler solution. Ornithologists use pulsed transmitters in nests or on birds, and directional antennas with sensitive receivers to locate the transmitters.
One-transistor transmitter operates in super-regenerative mode to emit short pulses, are quartz-controlled and using a tiny watch battery, their active life is several months.

Google for details, I like the idea, you will too.

 

[biff44]

I would base the handheld "finder" on this transceiver. With its -133 dBm receiver sensitivity, a directional antenna, you would not need that much transmit power at the "lost" key.

https://www.silabs.com/Support Documents/TechnicalDocs/Si4468-7.pdf

 

[FvM]

Both solutions should work, and result in a roughly similar average power consumption,
- Sending periodically at a very low duty cycle
- Enabling a low power receiver with fast preample detection at low duty cycle

In the second scenario, the interrogator must send an extra long preamble to give a chance for the target to detect it. As there's no actual bidirectional communication required, the "send only" variant will achieve the purpose with less effort, but also create an ubiquitary RF pollution, is used by many people. It has also the disadvantage of disclosing information to the public, e.g. burglars.

Sending on demand can be protected with a taget ID and a rolling code for the interrogator.