Soil Meter Info
Design Summary: A simple interface works with a digital volt meter (DVM)
to measure the conductivity of buriedd gypsum blocks. A 555 timer is used
to produce a ~100 Hz bipolar squarish wave and the resulting signal is
sensed by the LM324 dual opamp. Additionally, a simple resistance can be
monitored from a LM35 type IC temperature sensor.
!NOTE! I still have not evaluated the prototype instrument so this design
is preliminary. Please contact me if you can help validate the design and
operation of this instrument. There are several questions that need to be
answered. These are:
- Does this design work, ie. make repeatable measurements related to the
conductivity of the gypsum blocks?
- Will this arrangement (interface box strapped to a DVM) prove workable
for students in the field?
- Are there any temperature or battery voltage related biases to this design?
- Does the temperature measuring circuit work?
- Is there a simpler/easier way to accomplish these goals?
Here is how you can help:
- Contact me with any questions; design an experiment to address a question above.
- Try setting up a controlled situation where you have independent collaboration
of the primary variable.
- The easiest way to do this is to make measurements with a standard soil
meter at the same time and compare the results using a scatter plot.
- Let me know what you find. Contact me at jwash@hwr.arizona.edu
Parts List
- Digital Volt meter
- Mounting box 4x3x1"; plastic
- Perforated circuit board, hook-up wire
- 9V battery and battery clip
- 555 timer (mfc)
- LM 324 opamp
- Resistors: 1.2k, 2.2k, 3.3k, 6.8k, 15k, 68k
- Capacitors: .1uf, 100uf, 1000uf
- Diodes:
- Push-button switch
- Aligator clips or snap connectors
Sensors
- Gypsum block (Forestry Suppliers; Soil Test)
- LM35 precision centigrade temperature sensor (National Semiconductor)
Circuit diagram v. 12/6/96
Voltage-to-Resistance conversion
- I still do not have this for this design, but it should look something like this:
Original design specificationsv. 9/95
UA/GLOBE Home; Last updated: 9/11/97,
Comments? globe@hwr.arizona.edu