Poorman's Space Program
The BalloonSat Easy Flight Computer
Part 1. Assembling the Computer
L. Paul Verhage
Introduction
There are 27 parts in the programmable BalloonSat Easy Flight Computer. The heart of this flight computer is a PICAXE-18X with 2,048 bytes of memory, or enough memory to store between 600 and 1,000 lines of code. Data collected during the mission is stored in a separate 24LCxx family memory chip. Depending on the specific memory chip used, over ten thousand mission readings can be stored.
Although a separate memory chip stores mission data, this doesn't preclude the PICAXE-18X from also storing data. A single nine-volt battery is all it takes to operate the flight computer, but there is a separate 3 AAA battery pack in case a servo is part of the BalloonSat. The weight of the BalloonSat Easy and its batteries is only 125 grams. That leaves a lot of available weight for the BalloonSat airframe and its sensors.
Figure 1. The BalloonSat Easy Flight Computer.
Parts List
Look at the top of the printed circuit board (PCB) in Fig. 1 and you'll see white lettering to indicate the placement and orientation of the individual electronic components. Below is a list of the components and their PCB reference.
C1 22uF tantalum capacitor
D1 1N4001 diode
D2 Light-emitting diode (T1-3/4)
J1 3- pin straight header
J2 2-pin right angle header
L1 6V reed relay
R1 10k ohm resistor (brown, black, orange, gold)
R2 22k ohm resistor (red, red, orange, gold)
R3 4.7k ohm resistor (yellow, violet, red, gold)
R4 4.7k ohm resistor (yellow, violet, red, gold)
R5 4.7k ohm resistor (yellow, violet, red, gold)
R6 1k ohm resistor (brown, black, red, gold)
R7 10k ohm resistor (brown, black, orange, gold)
R8 330 ohm resistor (orange, orange, brown, gold)
U1 PICAXE-18X
U2 24LCxx memory IC
U3 LM2940 +5 volt regulator (TO-92)
Eleven additional items are required to complete the BalloonSat Easy, but they don't have a reference on the PCB.
3 by 3 female receptacle
1 by 3 male header
18-pin IC socket
8-pin IC socket
Wire (#24 AWG)
Nine volt battery snap
3 AAA cell holder
Toggle switch (two)
Shorting block
Printed Circuit Board
Heat shrink tubing (two inches long)
Theory of Operation
The voltage regulator (LP2950) converts the slowly declining voltage of the nine-volt transistor battery into a constant five volts that the PICAXE prefers. The 22 uF capacitor next to the voltage regulator acts as a temporary battery and helps the voltage regulator maintain a more constant five volt output. The main power toggle switch connects the nine volt transistor battery to the voltage regulator, allowing the control of the power from outside of the BalloonSat.
Figure 2. BalloonSat Easy Flight Computer schematic.
The LED's only function is to light up when five volts is present. Therefore the LED is just a power indicator (there is no other way to see that the BalloonSat Easy is operating). The 1k resistor connected to the LED limits the current flowing through the LED so it and the voltage regulator are protected from excessive current.
The program header is a three straight pin header that connects a PC to the PICAXE so the PICAXE Editor can download its program into the BalloonSat Easy. The 22k resistor limits the amount of current flowing between the PC serial port and the PICAXE during the programming process, and the 10k resistor is a pull down resistor that ensures that the PICAXE won't detect false data.when no programming instructions are flowing between the PC and PICAXE.
The commit header is a two-pin right angle pin header. One pin connects to the PICAXE and to five volts via a 10k resistor (called a pull up resistor). The other pin in the header connects to ground, or zero volts. When the shorting block is not on the commit header pins, the PICAXE detects the five volt signal from the pull up resistor. When the shorting block is on the commit header pins, the PICAXE detects zero volts, because current from the pull up resistor bypasses the PICAXE and travels to ground. The program in the PICAXE monitors the removal of the shorting block before it begins operating experiments and recording data. This means the BalloonSat Easy can operate for hours before it begins its mission.
The 24LCxx is a family of eight-pin I2C EEPROM memory ICs. The 24LCxx receives data sent from the PICAXE and stores it in its nonvolatile memory. Since it is EEPROM, data can be stored for years without backup battery power. The two 4,700 ohm resistors between the PICAXE and the memory are pull-up resistors. They ensure a +5 volt signal is always present on the communication lines between the PICAXE and memory IC when no data is sent between them.
The three input-output (I/O) ports provide every experiment plugged into them with five volts, ground (or zero volts), and a unique connection to the PICAXE. Therefore, every experiment receives power when it is plugged into an I/O port. In return, the experiment provides data to the PICAXE for recording. The data from sensors can be a voltage that varies by magnitude in response to a particular environmental condition, a voltage that either is on or off based on conditions, or digital data that sends meaningful pulses. The program in the PICAXE analyzes the output from sensors and records the results in the 24LCxx memory for downloading after recovery.
The relay connects to the PICAXE and triggers a camera shutter when commanded. When the PICAXE energizes the coil inside the relay, a magnetically activated switch inside the relay closes and triggers the camera shutter attached to the BalloonSat Easy. When the relay's coil is de-energized, the collapsing magnetic field of the coil induces a backwards flowing current towards the PICAXE. The PICAXE protected from this back current by a diode. By orienting the diode in the proper direction, the back current routes safely to ground and away from the PICAXE.
The three-pin header next to the I/O ports is where a servo plugs into the BalloonSat Easy. A separate battery pack of three AAA cells (or 4.5 volts) operates the servos and is controlled by the Servo Switch. The servo's separate battery pack prevents electrical noise caused by the servos interfering with the programming running in the PICAXE.
Assembling the BalloonSat Easy
Figure 3 shows the placement of components on the BalloonSat Easy PCB. Read each step carefully and check them off after you complete them.
Figure 3. Parts Layout for the BalloonSat Easy Flight Computer.
1. Resistors
Form (bend) the resistor leads before inserting them into the PCB. Each resistor's position is indicated by its R-number.
_ R1 10 k-ohms (brown, black, orange, gold)
_ R2 22 k-ohms (red, red, orange, gold)
_ R3 4.7 k-ohms (yellow, violet, red, gold)
_ R4 4.7 k-ohms (yellow, violet, red, gold)
_ R5 4.7 k-ohms (yellow, violet, red, gold)
_ R6 1 k-ohms (brown, black, red, gold)
_ R7 10 k-ohms (brown, black, orange, gold)
2. Diode
The diode has its name (1N4001) printed on it and a white stripe near one end. Orient the diode's stripe according to the diagram above. If it's backwards, the relay will never trigger the camera and the PICAXE could be damaged.
_ D1 1N4001
3. Commit Header
Insert the short leads into the PCB and leave the longer pair hanging over the edge of the PCB.
_ J1 2-pin right angle header
4. Power Cables
There are two power connections to the BalloonSat Easy, the nine volt battery snap and the 3 AAA battery holder. Each has one red wire and one black wire. The red wire is positive voltage and the black is ground. These must be soldered to the proper pads in the PCB or the flight computer will not power up and the servo will not rotate.
Note there are large diameter holes near the edge of the PCB and smaller holes inside them. The large holes are the strain relief that prevents normal usage from breaking the wires off the PCB. Push each wire up through the strain relief hole from the underside of the PCB and then bend each wire down and into its pad.
Figure 4. Example of a wire using a strain relief hole.
Push the wires through the pads until their insulation is flush with the PCB and there's no bare wire exposed above the PCB as shown in the illustration above. Then solder the wire and trim the end.
_ Battery Snap
_ Battery Holder
Part 2 and a BalloonSat Update
In Part 2, L. Paul Verhage will describe how to make the connections to the BalloonSat Easy Flight Computer. Paul's series on near space experiments using BalloonSats is among the most important collection of amateur science articles to appear in The Citizen Scientist. Paul will eventually publish the series as a book. Meanwhile, be sure to stay tuned to Paul's latest installments in the series. If you have missed any of his articles, you can find them in the list below. For an excellent presentation on the importance of BalloonSats, be sure to check out Paul's BalloonSat Principia . Editor.
Poorman's Space Program (07 September 2007).
Federal Regulations Regarding Near Space Flights (05 October 2007).
What and Where is Near Space? (02 November 2007).
What you can Expect at your BalloonSat Launch (07 December 2007).
The Thermal Test Chamber (TTC) for Near Space Instruments (01 February 2008).
Designing and Constructing BalloonSat Airframes (Part 1) (04 April 2008).
Designing and Constructing BalloonSat Airframes (Part 2) (02 May 2008)
Designing Near Space Experiments (Part 1) (04 July 2008)
Designing Near Space Experiments (Part 2) (01 August 2008)
Designing Near Space Experiments (Part 3) (05 September 2008)
Designing Near Space Experiments (Part 4) (03 October 2008)
Testing BalloonSats (Part 1) (07 November 2008)
Testing BalloonSats (Part 2) (05 December 2008)
Processing Near Space Data. Part 1. Creating Workbooks Using the Excel Spreadsheet Program (02 January 2009)
Processing Near Space Data. Part 2. Creating Charts in Excel (06 February 2009)
Processing Near Space Data. Analyzing Automatic Position Reporting System (APRS) Reports (06 March 2009)
The BalloonSat Mini (03 April 2009)
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