I'm having a little problem with a circuit and I want to tell you my adventures with it.
A few months ago I found a 125 KHz RFID tag reading circuit on the internet and decided to work with it to learn how these circuits work and to learn more about electronics in general.
All information about the circuit is on the author's website.
The circuit is supplied with 12V, but I decided to eliminate the power circuit and supply it with 5V, from a power supply.
In addition, I decided to improve it a bit (according to my limited knowledge in electronics), and I added decoupling capacitors (C2, C3, C6 and C7) and a pull down resistor (R3) on the digital output that produces the 125KHz wave which makes resonate the circuit. This pull-down resistor was added to eliminate any noise or fluctuation that could occur at that output.
The resultant circuit remains as follows:
I sent it to manufacture at https://www.elecrow.com/ and after soldering it was like this:
I tested it with PVC cards and it worked perfectly. Then I tried it with key fobs and wristbands and it did not work.
I have another reader (a Chinese one, the RDM6300) and all my cards, key fobs and wristbands are read perfectly by this Chinese reader.
Then, I used my logic analyzer and the PulseView software. This software is an open source tool for logic analyzers that has a decoder for the EM4100 protocol (the protocol used by all my RFID tags.) I did this to make sure the digital signal to which goes to the microcontroller is good.
I discovered that the PVC card is decoded perfectly, but the signal of the key fobs and the wristbands did not. So the problem is in the analog signal that comes to my circuit. Specifically, that signal reaches my antenna (air core coil) from the antenna of the RFID tag in question (card, key fob or bracelet).
The problem is that a PVC card has a much larger antenna than a key fob or a wristband.
The antenna of the key fob or the wristbands is a very small antenna, and therefore does not produce an electromagnetic field large enough to generate signals of large amplitudes in the receiver.
To solve this, I must increase the gain in the amplifiers, with a corresponding factor, to be able to increase the signals received.
The Chinese reader (which is a different circuit, schematic) amplifies the signals enough to be able to read the RFID tags of anything: cards, key fobs, wristbands, ...
I would like somebody to help me to solve this problem or at least give me guidance on how to increase the gain in the amplifiers in order to increase the signals received.