Hi folks... I've been developing a homemade EMF meter with LED bargraph display, and am looking for some tips on tweaking the EMF sensing circuit.
I don't have a schematic for my actual circuit handy (circuit's at home, I'm at work), but it's based on this one:
**broken link removed**
For my first couple questions, look at the left side of the circuit (sensing coil and op-amp). When I wired mine the way shown in the schematic, it wouldn't pick up anything. I didn't have a 1mH radial inductor handy, so I fashioned my own using 30ga enamel wire and a round core (100 turns or so, give or take). But, if I disconnect the side of the coil going to the + input on the op amp in the schematic, leaving it free floating, then I can pick up fields. So, in essence, it's acting more as an antenna than an inductor. Any thoughts on this? Maybe I need a better coil?
My 2nd question: the circuit is a LOT more sensitive if I connect it to an earth ground, such as when I connect the grounding clip on my oscilloscope when taking readings. Any thoughts on this? Since I intend this device to be portable, I can't be earthing it all the time.
At present I'm using a TL082 op-amp, but I just got my hands on some LMC6482 CMOS rail-to-rail op amps. Do you think that one would work better for this? I went with the rail-to-rail so I can get a better range in the comparator portion in my circuit, not in the schematic above. I am using a Basic Stamp 2 feeding one side of a comparator with a filtered PWM signal, and the other side is a filtered and amplified version of the EMF signal, and the uC uses successive approximation to determine the signal level and it lights up the bargraph display and beeps a piezo transducer based on its measurements.
I'll post a more complete schematic this weekend when time permits, but thought I'd post what I have in case anyone has suggestions for making it better!
Thanks,
KJP
I don't have a schematic for my actual circuit handy (circuit's at home, I'm at work), but it's based on this one:
**broken link removed**
For my first couple questions, look at the left side of the circuit (sensing coil and op-amp). When I wired mine the way shown in the schematic, it wouldn't pick up anything. I didn't have a 1mH radial inductor handy, so I fashioned my own using 30ga enamel wire and a round core (100 turns or so, give or take). But, if I disconnect the side of the coil going to the + input on the op amp in the schematic, leaving it free floating, then I can pick up fields. So, in essence, it's acting more as an antenna than an inductor. Any thoughts on this? Maybe I need a better coil?
My 2nd question: the circuit is a LOT more sensitive if I connect it to an earth ground, such as when I connect the grounding clip on my oscilloscope when taking readings. Any thoughts on this? Since I intend this device to be portable, I can't be earthing it all the time.
At present I'm using a TL082 op-amp, but I just got my hands on some LMC6482 CMOS rail-to-rail op amps. Do you think that one would work better for this? I went with the rail-to-rail so I can get a better range in the comparator portion in my circuit, not in the schematic above. I am using a Basic Stamp 2 feeding one side of a comparator with a filtered PWM signal, and the other side is a filtered and amplified version of the EMF signal, and the uC uses successive approximation to determine the signal level and it lights up the bargraph display and beeps a piezo transducer based on its measurements.
I'll post a more complete schematic this weekend when time permits, but thought I'd post what I have in case anyone has suggestions for making it better!
Thanks,
KJP