SparkFun Forums

[slide]

Ok, so here is my problem:

I have two components, a base station and a key. I want the base station to be able to excite a voltage on the key without the key having to use any power of its own. So, I am using the Friis Transmission Equation to compute the power on the receiving antenna and then converting that to a voltage. Here is my matlab file and the results,

Code: Select all

% General
freq = 125e3;                  % Hz
C = 299792458;                 % m/s
D = 2.0;                       % m

% Transmission Power
Pt = 1e-3;                     % W, = 0dBm

% Transmission Antenna
Gt = 1.2;                      % W, = 0.8dBi
Rt = 50;                       % Ohm

% Receiving Antenna
Gr = 1.2;                      % W, = 0.8dBi
Rr = 50;                       % Ohm

% Receiving Power
Pr = ( Pt * Gt * Gr * ( C / freq )^2 ) / ( 4 * pi() * D )^2

% Voltage In
sqrt( Pr * Rr )

Code: Select all

Pr = 13.1131
ans = 25.6057

So, does that mean that 25V will be received? That cant be handled by a pic so some conditioning of the signal must be done, like stepping it down to 5V and then rectifying it?

Im using the nRF24L01 and the chip antennas on both sides.

Thanks,
Brandon

[blitzr34]

First of all, its seems like you are trying to transfer power at 125KHz.
Do you have any idea how big of an antenna you need to transmit at that frequency? if you use a dipole structure, it'll be 100cm long. if you try to compact the antenna by using meandering techniques, you will cause a impedance miss match, therefore penalizing the gain. Thats just a feasibility study. As for the math part, did you realize you ended up with more power received than transmitted?

Also, nRF24L01 works at 2.45GHz. In your matlab code, you have 125KHz
(powering device using RF signal also requires some sort of voltage doubler , using schottky diodes, its not easy as you think.)

[busonerd]

125khz sounds suspiciously like RFID - which actually uses magnetic coupling.

If you're just wanting a 5-10cm range, the standard RFID approach works fine for powering stuff.

Cheers,
--David Carne

[slide]

I need approx 6ft range, but yes I am doing something similar to rfid, i guess just not to spec or anything. I don't care what frequency that it uses it just has to wake up the key from about 6ft

[riden]

What does the key do when it 'wakes up'? Not only do you need to factor how much voltage can be induced, you need to factor in the current flow in the key. In other words, I can induce a voltage in the thousand of volts by walking across the carpet, but the current that I can get to flow is very, very small. Your earlier post indicated that you were considering the nRF24L01, which requires in the area of 12 ma when it operates.

[slide]

The circuit has a coin cell battery to use once its awake, it just needs to get woken up somehow. Actually, ignore the part about the nRF24L01 and the chip antennas, those are for when its awake (they operate on 2.4GHz not 125KHz)

[brennen]

Could you not leave the 24L01 on to monitor the air and put the processor to sleep in software? You could easily use the 24L01's IRQ pin to wake up your microprocessor when you receive a packet (assuming your processor has facilities for wake up on pin change).

[slide]

The nRF24L01 uses 12.3mA in RX mode, with a 150mAh coin cell, that would last like (150mAh)/(12.3mA) = 12h, which isn't even close to how long it needs to last, which is at least a year.

[saipan59]

First of all, its seems like you are trying to transfer power at 125KHz.
Do you have any idea how big of an antenna you need to transmit at that frequency? if you use a dipole structure, it'll be 100cm long.

How do you get 100cm? I'm thinking a dipole for 125 Khz is *2400 meters*. In any case, I'm sure the realistic solution involves a flat-wound inductor that resonates at 125 Khz, but it would be very short range (like RFid).

I need approx 6ft range, but yes I am doing something similar to rfid, i guess just not to spec or anything. I don't care what frequency that it uses it just has to wake up the key from about 6ft

I'm guessing that 125 Khz at a distance of 6 ft is not practical. The transmitter would probably have to put out a huge amount of power, because the overall efficiency would be so low.

Further guessing: Your mathematical approach is admirable, but 'intuitively' I doubt that it's even close to predicting any sort of reality at such a low frequency with such a small antenna.

Microchip has an app note on their web site that describes a system like what you're trying to do, using a combo of 125 Khz and 413 Mhz, as I recall. It's the type of thing used for Remote Keyless Entry for cars and such. If you don't mind spending the $$, Microchip has a complete development kit with all the required parts and SW ready-to-go.

Pete

[riden]

The circuit has a coin cell battery to use once its awake, it just needs to get woken up somehow. Actually, ignore the part about the nRF24L01 and the chip antennas, those are for when its awake (they operate on 2.4GHz not 125KHz)

This is more like the active RFID readers used on the tollways or on the Mobil Speedpass that attaches to your car. I think those are 13.5Mhz units and have a range of 4-6 feet. Anyhow, if you do have a coin cell, that opens up possibilities. You could go to low power sleep mode (current draw in the microamps) and wake up for a small fraction of a second. The other idea is to build a small resonant circuit tuned to your transmitter, use a couple of diodes as a voltage doubler, and have the output trigger your processor via an interrupt. A zener diode on the output would limit the output voltage to a safe level. You won't need much in the way of current (~1-10ua depending upon the processor) as the coin cell is used to power the processor in sleep mode. The trigger circuit would be entirely passive. I'd give this approach some thought, and maybe prototype the trigger circuit and see what results you get.

[michaelgerhard]

Try the PIC16F639. This has a wake-up filter which responds to a preset sequence such as high for 2ms then low for 2ms or various combinations.

Its designed for 125kHz. We use this chip in one of our products so I know quite a bit about it. I wish I could tell you a really clever way of tuning the tank circuit but unfortunately its top secret.

I would be surprised if you could induce a big enough voltage to power the chip at that distance although you never know. You can certainly light an LED using RFID for up to 100cms - that requires some careful work though!

To get the best possible range make a big coil and obviously make the power factor high (Q) and keep the wires tidy to reduce the capacitance. That means thick wire. To give you an idea our hands free system has a coil the shape of a DVD case (thats what the prototypes were!). Its quite hard to get the tank tuned perfectly in those ranges because you must use high voltage components. Keep ferous material away from it too.

A big coil on the transponder end will help if you want to power something but a standard RFID coil will be OK if you use the PIC16F639 with thewake up filters.

I know our hands free system will work over the distance of about 3 or 4 car lengths which is more than 6ft. But remember that is if the coils are aligned.

I've said too much already so experiment yourself!

[DarioG]

The nRF24L01 uses 12.3mA in RX mode, with a 150mAh coin cell, that would last like (150mAh)/(12.3mA) = 12h, which isn't even close to how long it needs to last, which is at least a year.

I've been doing something similar on 2401, and I put the processor to Sleep, then woke up once in a while (say 2seconds), send out a "I'm here" packet, so that Base Station (Master) will now that it has some mSec to send out any command it has in store.

But... I also seem to remember that 24L01 should have a "IRQ" pin that goes up from Sleep mode, when some special packet is received (something like PANID on CC2420). I'm not sure though.