gkasprow
commented
5 years ago Frequency stability can be improved by adding crystal generator. But popular low cost oscillators are 25, 48, 50 or 100MHz. And still they cost at least 1$ per piece. But we need plenty of that sensors.
We already have hex inverter IC, so let's try to build oscillator out of gates.
Here is example circuit
borrowed from here.
But still, there are not such crystals that run at 80MHz. We need to run ordinary crystal at third overtone.
This is example configuration that would do the job
. I borrowed it from here
In an overtone mode, an additional inductor LI and capacitance CC is required to select the 3rd-Overtone mode, while suppressing or rejecting the fundamental mode.
The Lc tank may be located at either input or output of the inverter. However, the Lc tank at the out put is referred, because it helps to clean up all unwanted modes before signal goes through the crystal.
We cannot use Schmidt type negator, we need to have linearised inverter which acts as linear amplifier.
Finally we end up with something like this:
C13 and L2 are tuned at 81MHz, C5 and C12 give together 19pF which is close to 18pF required by the crystal.
jordens
muz0k
ikotzig
petergzmo
ollyboy
delta77101
Some day the water distribution failed - one of the tubes slipped off the filter terminal and plants were not watered for a few days. Another time, I shut down the main water valve by chance and left home for one week. Plants hardly survived. This was my inspiration to build distributed soil humidity monitor. I want to have following features:
So i built simple circuit to check the proof of principle. The schematic is simple:
The generator built on the Schmidt inverter generates ~50MHz square wave which is fed to the divider that consists of the capacitance probe and R9. Resulting voltage is rectified and fed to the uPC ADC input.
The sensor is piece of FR4 with two long strips, covered by plastic sleeve.
This is very simple but has some issues: