donnald@visual1.jhuapl.edu (Creig Donnald) (03/22/91)
I have two questions. The first concerns a very simple operation amplifier
circuit that I am attempting to construct. The circuit is supposed to
measure the moisture content of wood (with resistances in the 10's of
Megaohm range). The sample goes in to the R3 position and then R4 is
selected to get an appropriate range. A rough diagram of the circuit
appears below:
o Gnd R1 = ~20K
| R2 = .5K
Z1,C1 R3 = Unknown Sample
18V | R4 = Variable, decade 1K - 1000K
o---R1---+-------M1--+ C1, C2 = .05 uF
| | M1 = 0 - 1 mA meter
| |\ R2 Z1 = 6.2V Zener
+-|+\ |
| >------+
+-|-/ |
| |/ R3
| |
+-----------+
| |
C2 R4
| |
o Gnd o Gnd
My problem is this: In order to test the circuit, I have put 1M resistors
in for R3 and R4. I tweak R2 until the meter has the appropriate reading.
However, after several seconds, the circuit begins to oscillate at about
3 second intervals. If an RC oscillation is being set up some place, I
am at a loss to find it as the capacitors I am currently using don't seem
to be large enough to cause oscillations of that period. As for the op
amp, I have used both a standard 741 as well as a high-impedence one, both
of which came from Radio Shack. Are there any suggestions on what else
I might check or alternative circuits designed to measure high impedances?
The second questions concerns circuit building in general. I am interested
in building an 8-bit device based on an 8085 microprocessor. However, I
have no means of burning ROMs. Is there an inexpensive device that
I can hook up to a Macintosh that will burn EPROMS? I am also interested
in the names of any reputable electronics houses that will sell things
in small quantities to hobbiests.
Please e-mail replies to either question. Thank You in advance.
------------------------------
Creig Donnald
--------------------------------