Cyrodyl posted a schematic file named FET Dip Meter.PNG (you may have to be logged on and joined to the group to view this), and this is my comment about it. I posted this comment file to the Yahoo group RegenRX2, which is the overflow from the RegenRX group, which deals with regenerative receivers.
Comments on FET Dip Meter.PNG (I own and use a Heathkit FET Dip Meter)
The M1 analog or “wiggle stick” meter is a 50 uA meter movement, with an
internal resistance that is not given. R9 is a 1.2k resistor in parallel with
M1. Assume that the M1 internal resistance is 1.2k, then the meter and R9
require 100 uA for full scale deflection because half of the current will be
shunted through R9.
A few of the requirements for this meter are that it be small, so that the Dip
Meter can be hand held and used in places such as wherever a coil might be
located. The meter does not have to be accurate, because it is only indicating
the relative strength of oscillation, not the absolute value. Thus the numbers
on its scale are relatively meaningless. It does have to indicate small
changes in strength, such as when a low Q tank circuit is being dipped.
Most meters cannot meet these needs. It is uncommon to find a small analog
meter because the meter scale is too small to read accurately. The Heathkit
uses a small edge mounted meter, which are even less common today because
analog meters have been mostly replaced by digital meters.
This seems to indicate that a few LEDs could be used in place of the meter. I
think 1 or 2 LEDs would not give enough range and still be able to indicate
small changes. 3 LEDs or more might be suitable, with the proper circuit. The
LEDs would use more current, but this is not a problem because the Dip Meter is
used only a few minutes a month or even less and the battery should last a long
time. It would need 1 or 2 transistors to boost the 50 to 100 uA up to enough
current to drive the LEDs.
Another option is to use a digital meter. The 50m uA passing through the 1.2k
resistor causes 60 millivolts drop across the resistor. The cheap DMM on the
200 millivolt range could indicate 0 to 60 millivolts. I’m unsure if it would
be good at indicating the small changes that the analog meter can show. It
would be interesting to try it and see.
Lately I haven’t had any time to do experimenting.