SparkFun Forums

[wklose99]

So I am using an ID-12 to read a passive tag then output this in inverted TTL to a RF Link(http://www.sparkfun.com/commerce/produc ... ts_id=7816) trans/receiver pair.

I first inverted the TTL output from the ID-12 by using a 2N2222A NPN transistor(using the RTL converter circuit here: http://www.play-hookey.com/digital/expe ... erter.html). That way the input to the transmitter is not inverted. The circuit works and was checked with an oscilloscope.

I then fed the output of this transistor inverter circuit into the data in of the RF transmitter. The signal is then sent to the receiver and reception can be seen with an oscilloscope so I know everything is set up correctly. However, I am having a big problem with noise. I have tried numerous different capacitor coupling on the data output ports of the receiver as well as on the antenna port and the grounded/Vcc input ports. The only capacitors that make a difference are the ones on the antenna and data out ports. However, The capacitors seem to distort the output signal from a square wave to a spiked(triangle) waveform. I haven't tried putting it through a max232 and then looking at it on hyper terminal but I am fairly certain it is too distorted to still be read as serial at 4800 baud(which is what it needs to be read at). I was wondering if anyone knows why I am getting so much distortion on the receiver side(this is an ASK transmitter, so some distortion is expected I guess) and if there is a known fix for this?

Thanks much, -William Kohse

[Philba]

I don't think you want to cap couple the receiver. You could try a low pass filter but I think that won't remove the noise.

Basially, you appear to be trying to use the rf link as a wire replacement. It doesn't work that way. I would approach this problem differently - use a micro to create a packet that contains the RFID data as well as a check-sum for error checking. I'd also send multiple copies so that at least one might get through. If the interference is bad, I'd look at error correction codes of some sort. Without a micro, you will only get one shot at receiving the tag data - kind of iffy over rf.

[wklose99]

According to the spec: "Quite simply, it just sends
your data out the transmitter and the receiver grabs it, acting as if you had a wired
serial connection between them, minus the wire!"

When I couple the antenna with a cap I can see the receiver grabbing it and it appears similar to what is being inputted to the transmitter but it is not a perfect square wave.

How would I go about using a PIC for packet sending?

Thanks for your help, -WK

[wklose99]

So we got it to work amazingly well using a low pass filter on the receiver side, cap filtering the input voltage on the transmitter side, and using a 40cm antenna on the receiver side. Works just like a wire!

-WK

[Philba]

glad to hear it but you will be surprised some day when there is interference.