Opto Coupled Tunable Coil Circuit - Comments Please

[epsolutions]

The circuit below is a thought exercise. The assumption is that an audio frequency signal is induced from an external source into the right-hand coil. At the same time is applied a 2Hz "gating" pulse at a voltage sufficient to power the LED. The two variable caps interactively tune the coil as well as the primary of an audio coupling transformer. The photo transistor is powered by 12VDC.

I would like to know 1) how the interaction of the two variable caps affects the behavior of the circuit and 2) how this interaction is modified by the opto coupling.

Please overlook any conceptual issues with the circuit itself which is speculative and not a final design. Apologies for the hand drawing. My schematic program refuses to work with my new Linux distro.

 

[kellys_eye]

All you've achieved is a 'buffer' circuit with isolation. Assuming you could get a variable capacitor to tune to 2Hz with the chosen inductor (good luck with that) you can 'peak' the resonance - which isn't really required as the driver transistor doesn't require it.

On the output side you have the same issue - getting a resonant circuit using C as such a low frequency will be impractical. You could achieve the 'boost' by reworking the primary-secondary ratios anyway.

 

[epsolutions]

Thank you for your reply. The coil is intended to detect ambient audio frequency EM fields. The 2Hz is just to power the LED and provide a gating effect on the detected/tuned EM signal.

Do you have any comment on the different action of the two variable caps? The left-hand one tunes both the detector coil and transformer primary, being in parallel with both. The right-hand cap tunes the detector coil with the transformer primary in series. Part of its function is to block the 12VDC from the power supply. There would be an audio amp or LM1875 on the 8R side of the transformer.

Any further insights would be much appreciated.

 

[Alec_t]

The left-hand one tunes both the detector coil and transformer primary, being in parallel with both.

How? The schematic shows it in parallel with one coil only.

 

[epsolutions]

I was assuming that the parallel arrangement between the left coil and cap would also extend through the opto coupler to include the right cap and transformer primary. Please correct me if I am wrong regarding this point, or any other features of the circuit.

 

[Alec_t]

I was assuming that the parallel arrangement between the left coil and cap would also extend through the opto coupler to include the right cap and transformer primary.

I don't see how, if the opto-coupler is a standard type, which has a LED as the input device.

 

[epsolutions]

If I understand your reply correctly, the left coil and cap resonate independently of the transformer primary and its series cap. But the oscillatory signal of the left coil is carried through to the transformer via the opto coupler. Hence the "buffering" effect.

Just one more point please. What effect upon the circuit does the variable cap in series with the transformer primary have? How would this be different if the cap was in parallel with the transformer primary?

 

[AnalogKid]

Many of us here have a pretty good handle on the concept of tuning. BUT -

1. Please explain what *you* mean by "tuning". Are you trying to select or extract or respond to only one frequency (or a narrow band of frequencies) from an audio signal that has many kilohertz of bandwidth?

2. What is it you are trying to achieve?

3. Gating an audio signal on and off can be done without transformers and without an optical isolator. Why do you think you need optical isolation in a circuit that has both input and output transformers?

ak

 

[Alec_t]

What effect upon the circuit does the variable cap in series with the transformer primary have? How would this be different if the cap was in parallel with the transformer primary?

Simulation of your circuit (albeit with guestimated component values) with the value of that cap changed by a factor of 100, suggests the cap value has no detectable effect when the cap is in series with the primary, but increasing the value reduces the output from the secondary when the cap is in parallel with the primary.
Why are you using a step-down transformer if the opto is providing isolation? The signal is likely to be very small initially and probably below the noise level by the time it emerges from the secondary.

 

[AnalogKid]

Why are you using a step-down transformer if the opto is providing isolation? The signal is likely to be very small initially and probably below the noise level by the time it emerges from the secondary.

I understand that transformer. It is an audio output transformer, as in old 6-transistor radios, to step up the current into an 8 ohm speaker.

What I don't understand is the optoisolator. Operating in its linear range, it will severely distort the audio signal (assuming it is not a specialized device).

4. What is the opto part number?

ak

 

[epsolutions]

Thank you for the most recent replies.

The theory of the circuit is as follows. The left coil is intended to couple with a nearby EM field source (not shown) which contains a random mix of frequencies. The cap in parallel resonates the coil to a particular range of these frequencies. The opto isolates the transformer side of the circuit from the detector coil to prevent loading. The transformer is impedance matching to the input of an audio amplifier set at around 50 gain. The second cap, either in series or parallel, with the transformer primary is to control any oscillation.

This is an experimental circuit related to electronic music and I am aware there will be distortion and perhaps some unusual effects.

 

[AnalogKid]

There is no need for impedance matching into a typical audio amplifier. Also, an optocoupler is a very bad choice for a buffer stage because it is so non-linear. I would say an opamp or even a simple emitter follower would be better, but we still so not know the max and min frequencies of interest.

Assuming the input signal is sinusoidal in character, do you want to preserve that waveshape, or convert it to a complex square wave?

ak

 

[epsolutions]

Non-linearity is sort of what I was looking for. The idea of the transformer is both to current limit the photo-transistor and not load the output of the "tuner" with whatever scratch-built amp I might use. I suppose the transformer could instead be reversed so the 8R side faces the tuner and there is a voltage step-up at the secondary. Loading might again be an issue. But I am always interested in alternative designs to try.

 

[AnalogKid]

to current limit the photo-transistor try.

What does this mean?

 

[epsolutions]

Yes, actually the resistor at the collector already does that. I was thinking of an earlier emitter side design.