Nuts and Volts - March 2009

NEAR SPACE

developing this system, but have recently discovered it is used with other PCBs.

Both Minis use a nine-volt battery snap, so pass the stripped ends of the snap through their strain relief holes and then solder the end of each wire to the PCB. Just watch the polarity of the battery snap when you solder it (the diagram of parts placement is color coded, so you can't mix the wires).

The three remaining wire pairs in the BalloonSat Minis are for the Power Indicator LED, On/Off Switch, and the Commit Pin. Since BalloonSats are usually small in size, the cables can be kept short. Cutting the wires six to eight inches long ought to be good enough. Strip one end of each wire, pass it through its appropriate strain hole, and then solder it to the PCB. I like to color code my wires for the LED so it is easy to identify the correct ones when it comes time to solder the LED to the wires.

After soldering the six wires to the PCB, strip their free ends in preparation for the LED, sub-mini toggle switch, and 1/8 inch phono jack receptacle. The LED cable is stripped back 1/2 inch, twisted tight, and well tinned. Slide thin heat shrink over the tinned leads once the solder has cooled and push them close to the PCB (or else they could prematurely shrink when you attach the LED). Cut back the leads of the LED to 1/2 inch and tin them. Double-check your polarity, then place the proper LED lead flush against the first wire. Heat the LED lead and the wire with a well tinned soldering iron and the two will fuse together. Repeat this for the second lead. Then, slide the heat shrink over the soldered connection and shrink it. The connection will be plenty strong.

The solder tabs of the toggle switch and phono jack have holes to pass the wires through. So, strip an adequate length of insulation from the four remaining wires (one inch of

■ A completed panel for the power switch, indicator LED,

and commit receptacle. Notice that the commit pin is made from a 1/8 inch phono jack.

insulation is about right). The heat shrink for the toggle switch and phono jack receptacle is larger than it is for the LED and needs to be test-fitted before you cut it. As you did for the LED, slide the cut heat shrink up against the PCB so it cannot prematurely shrink. Now pass two-thirds of a bare wire through one hole in a solder tab in the phono jack receptacle and twist the wire tight around itself. Solder the twisted wire with just enough solder to lightly coat the twisted wires. Verify no blob of solder is bridging the gap between the solder tabs, then slide the heat shrink over the connection and heat. Repeat this for the remaining three wires. The BalloonSat Mini 2 has an additional cable for the camera. I strongly recommend the end of this cable be terminated in a connector rather than permanently attached to a camera. If the camera is permanently attached, then eventually the cable between the Mini 2 and camera will break from the weight of the camera. I use either a small Dean's connector or a male header. The termination that you use will depend on how you modify your camera. However, I recommend picking a single type of connector and sticking with it.

TESTING AND PAINTING

At this point, you have a complete BalloonSat Mini. However, before snapping in the PICAXE-08M,

take a moment to check your soldering for unintentional shorts (do we ever have intentional shorts?). Make sure solder did not overflow its pad and that no cut leads are sticking to the bottom of the PCB. After a visual check, get out a multimeter and check for continuity between power and ground in the battery snap. Be sure to check with the power switch both turned off and on. If no short is detected, then attach the nine volt battery to the Mini and flip the power switch. You will see the LED light up, indicating power. Now, set your multimeter to measure voltage.

Between pins 1 and 8 of the PICAXE you should measure five volts — give or take 5% — or between 4. 75 and 5. 25 volts. Then, check the voltage of the I/O ports. To do this, stick cut resistor leads into a +5V and a ground hole of the receptacle to make it accessible to the multimeter. Again, you should measure within 5% of five volts in the receptacle. Of course, if the battery catches fire and sparks fly out of the PCB, disconnect power and look for a short.