Hi John,


       I have made a few minor additions to your circuit for testing the damping.

       The 0.1 mu F ceramic capacitor on the input is fairly effective in removing

any RF pickup.

       The two 4.7 K Ohm resistors mimic the input of the amplifier and trim the

damping slightly.

       I am not very happy with the output drive capabilities of the MAX420 or MAX430,

or their price. The LTC1150 has the same supply ratings, better drive and is cheaper.

       I used a couple of 9V NiMh batteries to power the amplifier. You don't really

need to carry a power supply around?

       The output RC filter removes hash noise from the opamp.

       I mounted a relay coil horizontally on the axis of the sense coil, facing one

of the magnet poles. I then used a 1.5V Battery to drive the coil and watched the damped

trace on a 'scope. I could equally well have recorded a trace with an ADC.


       Chris Chapman