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Don't know if you have any particular circuit in mind but here is a link where you should start your reading to get "some" understanding in resonant circuits, there might show up better places if you search google.
Oscillators is a huge subject, think you should do some studying by your own before you go further.

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Rl= load resistance.
To build a band pass filter 800-900Mhz is not an easy task I would say that some RF experience is needed.
MP it's a good calculator but as you probably know both C and L involved in a band pass filter like this aren't what the average hobbyist usually deal with, I'm not an Rf expert but to get a proper band pass filter for that frequency sure some experience

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The second part of my post was directed at members who stumbled into this thread by the subject line, "Bandpass Filter"
BTW: I think arsal is going in the right direction to learning all about filters, what he can do and what he cannot do.


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Why use a bandpass filter that will probably be at the wrong frequency?
According to Red's Cell Detector project, when a cell phone is about to ring it transmits a very strong signal to the cell headquarters to tell them it is ready to receive a call. The detector circuit has just a simple coil at its input which detects the transmission no matter which frequency the cell phone is using.

Arsal said he built a detector circuit that did not work. He also posted questions about a Hitachi ordinary 555 but the detector project must use a Cmos 555 because its battery is only 1.5V. Either he should use a Cmos 555 or he should use a 6V battery in that project, then it should work.

If Arsal stayed in the same thread then his ringing cell phone detector project would be working now.

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MP I agree, it's a good way to learn how component values change with frequency and bandwidth  :)
I'm sorry arsal_uet but audioguru is right about the filter, strong nearby signals from other cellular phones or base stations would probably be detected by your circuit. if the frequency would be fixed a narrowband filter could have been a possible solution but then we run into another problem and that is to design and create the filter which need instrumentation that is far beyond what most hobbie electronics can afford.
I do encourage you to continue the project even though it seems to be a real hard thing to solve, don't give up there might be someone lurking around with exactley the right soulution for you, don't forget that you always learn something  ;)

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Sorry I haven't read the other thread arsal_uet wrote.
Audiguru if that's the case a detector is a simple solution but keep in mind that it will also detect every time the cell phone talk with the base don't you think? ;D

MODERATOR EDIT: I have now merged the topics into one 5-3-2006

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It gets more complicated. The detector must discriminate between two cell phones ringing simultaneousy. This is what Arsal wrote:

"assign me to make a circuit that detects the incoming call, but the problem is he has made a condition if two mobile in the reach are being called simultaneously then that device or the circuit should tell that either they are being called from the same bandwith or the differnt ones, that is are the being called by using the same mobile network or the different.
i have seen the circuit here which just tells us the incoming call from any network any i think, but me want ur help  how to make it for atleast two networks and how to make the ciruit sensitive to two seperate networks. and also how to show either using some leds
i want to actuallly catch the incoming call signal for a device to tell me about the incoming call before the mobile rings-
you all might have observed that when u place ur cellphone near ur audia tape or the television or even the mic, a second or so before the call actually appers on the cellphone, ur heard a beep like tic tic from ur audio devices. on the same basis i want to make a circuit that just indicate me by a just light emitting led or thing like this to tel me that a call is comming.
the ciruit whose figure is pasted above, its not working.

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Well that is a hard to tackle circuit I would say, to detect rf is one thing but discriminate between two cell phones make things a lot more difficult.
As I understand (and what audioguru said) every cell phone has its own ID and when the base is calling one of the cell phones it answer, here I am go ahead!
It means that to detect that a incoming call is directed to one of your cell phones you got to have a receiver with same ID otherwise the receiver/detector doesn't know which of them the base is calling.

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in the ciruit suggested to me by someone here;in that circuit the 7555 has one pin empty. i grounded that pin and now the result is that there is a but high chnge in amplitude across the inductor but that is only about few millivolts and they are not being amplified by the transistor, so tel me how to mak the currnet in crease to mak the led turn on.thanks for ur suggestions all u guys.

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Hi Arsal,
Is this the circuit that you made? http://www.electronics-lab.com/projects/misc/010/index.html
Did you use a Cmos 555 IC like it says?
Did you make the coil like it says?
Did you connect the pins of the transistor the correct way?
Did you use a fresh 1.5V battery? It might work better with a 3V battery.

Pin 5 of a 555 IC is its control voltage. Sometimes it has a filter capacitor to ground. If pin 5 is grounded then the 555 IC won't work.

Measure the voltage at pin 2. It should be higher than 1.0V at rest. Then it should drop to close to 0V when the cell phone transmits that it is ready to receive a call. Then pin 2 should go higher than 1.0V again when the cell phone is no longer transmitting and the LED should blink one time.


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if it is aroud distinguhing between two cellponones ringing--RF has nperhpas less to do--all cdma or gsm technology-- setting a bit inyour phone makking it generate (simulate ringing)-- if it is only rquirement-- perhaps one could change the ring tones by selection and crate two different tones  towards ring distingushing--

even if the request is centered around detecting signal from the air-- i fear one may not be able to use simple technics like filters detectors etc,, pardon me --


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hi respected contirbuters.
i got the circuit working today, but not because of the coil that is mentioned in the circuit but rather i took a rf coil from the market of about 10mH same as that is found in our radios. so mak it clear that u use the same as that is used in our radios of about 10mH, because its hard to make a coil the way its described in the specification sheet. I almost spent one and a half week on making it but of no use. now my circuit working and of about range of some a meter or a two.To exaplain to the teacher and my fellow class mates can u plz tel me that how at the pin 2 of the ic 7555 when the cal is incoming.i mean to say what actaully happen at that point. because there are two cirrents coming one from top tp dpen because of the batery 1.5 volts and secondaly the transistor.in other words can u plz exaplin the inner working just at the point pin 2.other i can understand a bit.

Can u tel me how can i measure the frequency that is disturbing the coil just to make an additinal feature to the circuit. and what would be the ic that can be used for this.

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i got the circuit working today
now my circuit working and of about range of some a meter or a two.
Good. ;D

Pin 2 is the 555's Trigger Pin. The 555's datasheet explains how it works. The transistor's collector signal triggers the 555 monostable circuit.

It is very complicated to make a frequency display circuit that operates at the extremely high frequencies used by cell phones. My multimeter displays frequencies up to only 200kHz but my cell phone operates up to 1.8GHz, nearly 10,000 times higher.

Here is a clipped paragraph from the datasheet of a 555:


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another problem.
my teacher has asked me to give him the comlpete design.can anybody help me here now

I already showed a clip from the datasheet of the 555 timer IC that explains how it works.
All that remains is the transistor that amplifies the RF signal and the capacitor that voltage-doubles the supply to a high enough voltage to light the 2V LED.
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