Hi All, I am building a system to send data using a magnetic field over a range of about 1 - 2 meters max. I have a transmitter coil resonating at 140KHz approx 30cm diameter, built using H-bridge driver and micro-controller. The receiver coil is a 15mH wire wound SMD inductor, with parallel cap to tune it to 140KHz. I have a resistor in parallel too to act as a small test load of 500K ohm.
Using the oscilloscope I can detect the clean 140KHz sine wave easily around 1m away which is around 20mV pk-pk. Getting closer to the transmitter the amplitude shoots up to around 2V pk-pk.
So far so good. My next step is to amplify this signal, which is where I am struggling. The receiver needs to operate at 3.3V, and I need to amplify the signal to ideally a on/off square wave. Then I can easily filter and modulate the 140KHz to transmit some data.
I have tried a op-amp microphone circuit, but it didn't work. It needs a high input impedance of around 500K to 1M ohm so as not to dissipate the received energy. C2 couples the signal to the +ve input via a voltage bias since it is AC.
With a 1NF cap between R4 and ground I can almost get the input signal out, but it has a small negative gain. I need to try and make this positive. The IC is wired up correctly I have checked many times.
Any ideas would be welcome. Even if I can get the amplifier working, I will prob then need a comparator stage to get the desired on/off signal out.
Cheers
Phil
Using the oscilloscope I can detect the clean 140KHz sine wave easily around 1m away which is around 20mV pk-pk. Getting closer to the transmitter the amplitude shoots up to around 2V pk-pk.
So far so good. My next step is to amplify this signal, which is where I am struggling. The receiver needs to operate at 3.3V, and I need to amplify the signal to ideally a on/off square wave. Then I can easily filter and modulate the 140KHz to transmit some data.
I have tried a op-amp microphone circuit, but it didn't work. It needs a high input impedance of around 500K to 1M ohm so as not to dissipate the received energy. C2 couples the signal to the +ve input via a voltage bias since it is AC.
With a 1NF cap between R4 and ground I can almost get the input signal out, but it has a small negative gain. I need to try and make this positive. The IC is wired up correctly I have checked many times.
Any ideas would be welcome. Even if I can get the amplifier working, I will prob then need a comparator stage to get the desired on/off signal out.
Cheers
Phil