I'm trying to replicate your sim Ian, but I get the following error message:







I'm using the 741 model that @Papabravo so generously posted a while ago. And placed it in the same directory as my Draft, per his suggestion.

 

Fixed it! ... I just used an older version of LTsipce ... now I can start playing with this thing, see if I can tweak it a little

 

Fixed it! ... I just used an older version of LTsipce ... now I can start playing with this thing, see if I can tweak it a little

See if you can get the right op-amp model. Even SPICE knows a 741 won't work at 3.3V.
My simulation was done with an MCP6071. For some reason I can't explain, whenever I use the MCP6071 model it gets labelled "ua741".

 

Well, the sim is working as it is. With the model I downloaded from Papabravo's post:

​

 

I found an affordable chip capable of generating a negative voltage. It's the RS6903YF5, do you think it would make things easier?

 

Hello,

The chip is a chargepump capacitor circuit.
It might introduce noise on the negative supply.
The LTC3260 is also a simelar circuit, but with build in LDO regulators.
The LTC3265 has also the capability to do a boost.
See the datasheet for more info.

Bertus

 

Why not connect the pickup coil as a differential input? That will allow not having ant resistors or capacitors in series with the signal. Of course you will still need bias and feedback portions of the circuit.

 

Hello,

The chip is a chargepump capacitor circuit.
It might introduce noise on the negative supply.
The LTC3260 is also a simelar circuit, but with build in LDO regulators.
The LTC3265 has also the capability to do a boost.
See the datasheet for more info.

Bertus

Thanks bertus, but the price of that chip is beyond prohibitive ... and thanks for the warning about the noise.

 

Why not connect the pickup coil as a differential input? That will allow not having ant resistors or capacitors in series with the signal. Of course you will still need bias and feedback portions of the circuit.

Very interesting suggestion, Bill ... I'm experimenting with it right now ... see what I can get. This signal looks usable, although it's inverted because I removed C2. But it doesn't matter for my purposes. I wonder if noise would be an issue, though.

​

 

Theoretically, this would work beautifully. But something tells me I'm committing a cardinal sin, noise-wise.

 

BINGO! ... this is the exact behavior that I want from my circuit. I don't know if it will work in real life, though. And I guess there's only one way to find out:

​

 

BINGO! ... this is the exact behavior that I want from my circuit. I don't know if it will work in real life, though. And I guess there's only one way to find out:

View attachment 363210​

It's badly distorted.

 

It's badly distorted.

You mean the waveform at Out1 ? ... that's not relevant for my purposes. What is relevant is the High/Low voltage at Out2. That is the result I was aiming for from the beginning. I'm just hoping that two things don't happen when I build it: first, that the device picks up too much ambient noise and randomly triggers Out2, and second, that a second stage will be needed in case the waveform at Out1 is too small.

 

What about noise, Ian? ... do you think the way the pickup is connected could make it sensitive to EMI ?

 

For noise immunity….Perhaps some hysteresis?
Let me think something.

 

For noise immunity….Perhaps some hysteresis?
Let me think something.

I was thinking about a humbucker technique

 

I would have deigned it with a linear amplifier, followed by a comparator with a little hysteresis, and kept the amplifier biassed at 1.8V. That way, as the signal varies, the average DC output of the amplifier stays the same.

 

I would have deigned it with a linear amplifier, followed by a comparator with a little hysteresis, and kept the amplifier biassed at 1.8V. That way, as the signal varies, the average DC output of the amplifier stays the same.

Can you make a quick sketch of your idea? ...

 

Also, what are your thoughts on the TL071 Op Amp ?
EDIT: Never mind, I've just noticed that the device works with a minimum of 4.5V, so it doesn't meet my requirements.

 

Also, what are your thoughts on the TL071 Op Amp ?
EDIT: Never mind, I've just noticed that the device works with a minimum of 4.5V, so it doesn't meet my requirements.

You can try to use OPAx340 (opa340 in LTspice) device:
(10pcs) New OPA2340U $ 0.99, shipping $ 3.36
Features:
• Rail-to-Rail Input
• Rail-to-Rail Output (Within 1 mV)
• MicroSize Packages
• Wide Bandwidth: 5.5 MHz
• High Slew Rate: 6 V/μs
• Low THD + Noise: 0.0007% (f = 1 kHz)
• Low Quiescent Current: 750 μA/Channel
• Single, Dual, and Quad Versions
• Supply voltage from 2.5 V to 5 V