Project #27 -- Part 2

 PROTO BOARDS:

Here are some views of the working prototypes.  They were hand stitched on a proto board using point-to-point wiring.  I used enameled 22AWG magnet wire.  The connections are made by "soldering through" the enamel. 

 The output filter coils were made with the type of same wire 18 AWG forming a low-pass filter.  It is not a very good filter but does knock down the some of the out of band harmonics somewhat, especially the big harmonic spike at 54MHz. Although they probably would not pass for a product under FCC Part 15 - it should be fine for testing. 

The temperature sensor is a small 10K thermistor connected to the two small white wires on the right-hand side of the board controls the temperature-to-frequency oscillator.  This is calibrated to the ensure the oscillation is in the audio frequency range especially at the coldest temperatures expected. The altitude sensor is not yet connected to the circuit, instead the cycle timer rate is set by the small potentiometer on the left-hand side of the board.  It will be calibrated to have the longest transmitter key-on time at the highest altitude so the circuits will hopefully stay warmer to hopefully continue to oscillate and we may get more accurate readings.  Short key-on time at ground level may help the battery stay alive longer to aid in recovery. 

MKIII Proto

MKIII Point-to-point Wires

Altitude Sensor Proto:

I repurposed a small syringe that holds a small amount of air after the cap is removed and replaced at ground level. In theory the air will expand in relation to the decreasing ambient atmosphere as the altitude increases and move the plunger to change the resistance. With such a small movement we may actually need a partial vacuum to cover the highest altitudes.  I doubt that it will stand up to the altitude and temperatures would be encountered at altitudes expected of a balloon flight.  Maybe some insulation or even a small light bulb may be needed to keep the plunger from sticking or freezing. 

Air pressure moves the slide pot. 

The transmitter works in AM at 27.145MHz.  The thermistor controls the frequency of the audio range oscillator formed by a couple of transistors, and the altitude controls another multivibrator which keys the transmitter on and off at a rate proportional to the position of the slide pot.  From these two modifications of the transmitted signal the temperature and altitude may be inferred.  

It is Working! 

Receivers capable of receiving in the band can be programmed to receive the signal.   Here is my old PSR-500 handheld scanner set up to receive the signal. 

Receiver settings for Test

Here is a short (really short) video of the transmitter in operation!  The audio is from the scanner receiving the signal set up as shown. 

More to Follow.

Test results and calibration soon...

Stay Tuned!