Nuts and Volts - June 2017

shorted (by pushing the button) or not shorted (by removing the shorting block). A low dropout voltage regulator and power indicator LED complete the circuit.

Possible Modifications

One modification to make is to replace the 91K ohm resistor with a resistor of 10M ohms. This increases the sensitivity of the pressure sensor so it can measure changes in the daily air pressure. I’m still investigating this, but it’s possible the circuit will be better with a 4.7M ohm resistor. The reason I say this is that it appears my quadcopter can lift the weather station high enough (400 feet AGL) to drive the pressure sensor to its minimum pressure.

A second modification is to solder the 24LC256 to a separate PCB and plug it into the weather station’s PCB. This modification allows a user to leave the weather station in some location and then switch out and swap the memory chips after the PICAXE has filled them with data.

A final modification (which I have not attempted yet) is to plug an XBee radio into the programming port. This would allow the recording weather station to transmit data from a remote location. A second XBee would be needed along with a way to see the data coming over the XBee link.

Using the Recording Weather Station

After completing the weather station, you need to set up the pressure sensor and calibrate it. The 500 ohm trimmer connected to its + and – pins of the MPS-3138 is used to balance both sides of the Wheatstone bridge.

So, write a short program to digitize the output from the op-amp and debug the results at one second intervals. Then, adjust the trimmer until the voltage is maximized. I I have attempted to calibrate the pressure sensor using a 35 mL syringe to some success, but am not completely happy with the results. Do not calibrate the pressure sensor using air pressure data from your local airport. That air pressure is converted to sea level air pressure so aircraft can set their barometric altimeters.

Some Initial Experimental Results

It’s not near space season in Idaho yet as I write this article. In fact, I’ve been shoveling 24 inches of snow rather than working on near space. Therefore, the results I’ve created so far involve driving the weather station to school, letting the weather station record conditions at the house over 24 hours, and lofting it with my quadcopter. Following are some of the results I’d like to share with you.