Induction coil interference from an electromagnet

There is a little amplifier I am very fond of which I have used with dynamic mics. Since your coil situation is much like the one faced by someone trying to amplify a dynamic mic, I encourage you to try this circuit and see if produces a usable signal. Just replace the "microphone" (speaker) with your coil.

http://electrosuite.com/audio/bc547-dynamic-microphone-amplifier.html

The lower transistor acts as a common base amplifier, which is fairly unusual. It has a current gain of less than 1, but it does provide significant voltage gain. The upper transistor is in a common collector configuration, so it has essentially unity voltage gain, but the current gain is good. In short, this first amplifies the voltage, then the current, to give you a strong output signal. Although you will likely need a second stage after this, that should be much easier.

EDIT:

Concerning your noise issue, do you know where the noise is coming from? Sometimes I just have a power supply that's super noisy and ruining everything. I like low budget solutions, because of, well, my budget, so I usually test this by running the circuit on batteries. If things start magically working, then it's time to work on the power supply.

The mistake is assuming the metal core will be saturating. This might be the case should the core form a complete toroid but it doesn't - the major influence on the flux density is the air - think of the ferrite core being in series with a longer air gap - magnetic flux has to travel through the core then thru a much longer length of air before returning to the core. The core may not be anywhere near saturation.

The formula for the attractive force between an electromagnet and a ferrous material is: -

Force = \$\dfrac{F_M^2\cdot \mu_0\cdot A}{2\cdot g^2}\$

• where Fm is the ampere-turns of the coil,
• \$\mu_0\$ is 4\$\pi\times 10^{-7}\$,
• A is area of cross section of solenoid and
• g is the gap to the piece of metal.

Maybe this formula will help - I'd imagine the force would be twice that given in the formula due to two coils interacting.

EDIT: -

If the mean length of the H field is about 2m for each coil, with 70 amps and 200 turns the H field is 14,000 ampere-turns per metre. To calculate B use the formula B = \$\mu_0\cdot\$ H. Because the field is 99%+ dictated by the air gap on each solenoid \$\mu_0\$ is largely valid. B, flux density then works out as 17.6 mT i.e. nowhere near saturation. Of course the mean length of the magnetic field could be much smaller than 2 metres but it's unlikely, given your quoted dimension of diameter to be much below 0.5 metres and this would make B 4 times bigger at 70mT but still not close to saturation.