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paceman
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Joined: 07/10/2011
Location: Australia
Posts: 1329

Posted: 01:15pm 25 Apr 2012

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Hi All,

I've set up an RFID reader/writer that I want to control with the Maximite - (model RFIDRW-E-232 from Priority One Design here in Melbourne). Having now installed the external coil I wanted to check the frequency which is available as a test point on the RFID board. The frequency should be around 134 KHz which is well within the 200 KHz limit mentioned in the MM manual.

I connected the test point to pin 11 via a 100R resistor (previous posts suggested this was prudent)and used MMIDE to send the commands. I selected MM pin 11 on the "Direct Commands" tab and then "frequency input" from the dropdown box. MMIDE then sent the SETPIN 11,3 command to the MM. When the "read" button was pressed, MMIDE sent the PRINT PIN(11) command which resulted in a figure of around 65000 being returned. Subsequent pressing returned numbers which ranged between 63500 and 66200. Looking at this suggests the frequency measured is about half of that expected.

I'm fairly certain the RFID board is working correctly because I managed to get our cat held close enough to it (feet upwards) to read her implanted tag correctly using Priority's Windows software on my notebook via a serial/USB link. The RFIDRW-E-232 documentation says the frequency test-point signal is: "A square wave between 0-5V output on this test point at the operating frequency of the unit."

What am I missing here?

Greg

Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333

Posted: 03:41pm 25 Apr 2012

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From my experience with RFID i say your coil is not tuned to the circuit correctly.

This will greatly effect your resonance frequency.

Try adding some small capacitors ( small ceramics should do in the PF range) as tuning capacitors, and see if the frequency increases.

I would expect a read range of about 6 inches, and from your test with moggy would imply a far reduced read range, also hinting the coil is not tuned.

It can be a fiddly job to find the sweet spot for the tuning without some good equipment, but when you acheive tuning it will make a hell of a difference.

I had a laugh about the hi tech lab test method....... scruffing the cat and scanning it with all 4 legs in the air.
One hope moggy likes this treatment as it might be needed a lot during tuning........now wheres that bloody cat???

Sometimes it just works

paceman
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Joined: 07/10/2011
Location: Australia
Posts: 1329

Posted: 01:09am 26 Apr 2012

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Hi Downwind,

Yes, she didn't appreciate the indignity - what cat does? Just as well my daughter wasn't home - it's her (err, our) cat! I'll have to put some longer leads on the reader - or maybe just put gloves on her claws!

The reading range is supposed to be about 2" (50mm) but as you say, tuning's important. I reckon her tag was less than 1" from the coil when it was read (assuming the implant was where it should be, under the skin between the shoulder blades). The documentation is pretty good on how to tune the capacitance for resonance frequency and there are connection points on the board for external caps so adjusting it shouldn't be too difficult as long as I can read the frequency - hence my attempts that returned 65000.

I don't think it would read at all though if the frequency was way down there instead of around 134 KHz. The coil I'm using is the standard external one supplied with the board and the board is set up for this coil, but the manual does discuss that correct tuning may be required. I'd planned to have a small MM program read a heap of frequency measurements and then average them to tell me where I was. If, as seems to be the case though, it's reading half of what it should be, then it's already in the ballpark.

I'm wondering if the "pulses to 5v" that are supplied at the test point can be read correctly on pin 11 the way I'm doing it. Maybe I shouldn't have the 100R resistor there or could it be a rising/falling edge thing or maybe a quirk with MMIDE or even MMBasic maybe. Unfortunately I don't have a 'scope or frequency meter so have no idea what the test signal looks like and I'm no expert on electronics.

Thanks for your response - any other ideas gratefully received.

Greg

Edited by paceman 2012-04-27

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