Archive for October, 2009

Moisture Meter for Plants

My wife suggested to me the idea of a visual indication for the soil moisture level in a houseplant (that is, before the brown leaves indication). We hadn’t seen anything like it in stores. So I made a prototype which worked quite nicely, and then we found that they were already available online through retailers like Amazon. Here’s the prototype I made:

Moisture 016

It has two probes at one end that get inserted into the soil, and the LED then flashes either red, yellow, or green depending on the moisture content of the soil. It all runs on one AAA battery for about a month, but that could be improved by reducing the flash time and ADC sampling. The code for the prototype flashes the LED every second or two and samples every time.


Saturday, October 10th, 2009 Electronics 11 Comments

Strain-Guage based Pressure Sensor

I just finished building a self-contained pressure sensing device using a MEMS strain-guage type sensor from Measurement Specialties. Single phase AC goes in one side, and a 0.5V-4.5V signal proportional to pressure input comes out the other side. The sensor has a rated max of 5″ of water.

Pressure 001

The circuit is simple enough, the AC goes through a fuse and into a AC-DC converter. The output is regulated by a 5V regulator, which powers an op-amp and the sensor. The op-amp buffers the output of the sensor. I used a MAX4242 op-amp, as it can run on less than 5V, and has rail to rail inputs and outputs. This keeps my signal in the CMR range.
I have used products from Measurement Specialties in the past, and they seem to be very good products for the money. I have some of their button load cells which were very affordable and also have about twice the output signal (mV/V) of similar load cells from Omega or Futek.

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Saturday, October 10th, 2009 Electronics 2 Comments