There are many types of capacitance sensors. I recently come across moisture sensors which measure the dielectric constants of the materials and correlate them to the water content level. It turns out many commercial soil moisture sensors (such as VH400 and SM100) use 80 MHz oscillators, much higher than other lower frequency methods such as those using Timer 555, or simple RC charging/discharging. I am just wondering if anyone knows how to do the high frequency version in an economic way. VH400 sells $30~$40 per piece. All those sensors output a DC voltage proportional to moisture level.
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\$\begingroup\$ Are you asking for a circuit diagram of an 80MHz moisture sensor? \$\endgroup\$– Andy akaCommented Sep 12, 2015 at 9:24
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2 Answers
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Measure the power consumption of an off-the-shelf 80MHz oscillator which is only driving the sensor.
as capacitance increases, the current needed by to run the oscilator will also increase.
answered Sep 12, 2015 at 14:03
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A transistor LC oscillator would be cheap at 80 MHz these days because you wouldn't have to use an exotic transistor. Think about it, 80MHz is less than the LO of a FM broadcast receiver and they are cheap. There are lots of osc circuits to choose from. I would choose something that does not need a tapped or double wound coil. This means that you can use a cheap, off-the-shelf SMD coil. I have used a modified dynatron oscillator but this is really up to you.
Your LC osc output will need to be squared up to drive a microprocessor. I used a simple transistor as a limiter but this is up to you. If 80 MHz is too much for your micro, you can use a divider chip which would also be cheap because 80MHz is nothing these days and the ratio could be a simple binary power like say 64. If you don't want to use a micro then a frequency to voltage converter that could be based on a simple monostable or diode pump should work.
