Issue 98, September 1998
Smart Rockets - Data Acquisition in Model Rocketry

SYSTEM CONSTRUCTION

The data-acquisition system was point-to-point wired on a 15?16? ? 3½? piece of pad-per-hole perf board. The PIC chip was mounted in a socket so it could be removed for reprogramming. All other components were soldered directly to the board.

As you see in Photo 2 and Figure 2, three connectors were mounted on the board—two 2-pin connectors to connect to the battery (J3) and to link the trigger lines to the rocket body (J1), and one 3-pin connector for the USART connection (J2).

Photo 2a—The accelerometer is in the TO-100 can at the forward end of the board. Its axis of sensitivity is along the diameter of the can that crosses the tab. The connectors are for the trigger signal (yellow), serial communication (central three-pin connector), and power (green), which is shown connected to the lithium battery. b—The red and green status LEDs, the large black inductor, the mode jumper, and the reset button are visible on the wire side of the board.

The big challenge in laying out this board was minimizing the component heights at the edges to accommodate the cylindrical shape of the payload compartment. Thus the PIC chip, accelerometer, and the large capacitor and inductor of the power supply had to be mounted centrally on the board.

We mounted components on both sides of the board, which had the added bonus of balancing the weight of the payload. Photo 2 shows both sides of the payload circuit board.

A slot was cut in the rear end of the plastic nose cone that acts as a guide to hold the circuit board in the payload section (see Photo 1b). Some material was removed from the plastic bulkhead that forms the bottom of the payload compartment to make a crude battery holder.

Pieces of foam wedged between the battery and the wall of the payload compartment help hold everything in place. The forward nose cone and rear bulkhead connector are taped to the payload tube to prevent the electronics from falling out during recovery.

We soldered wires onto the terminals of the battery and brought them out to a connector on the circuit board. Beware! This is the most dangerous part of building the system.

There is a lot of energy in the lithium cell, and excessive heat can cause it to catch fire or explode! Use a cell that has integral solder tabs, and don’t linger on them with the soldering iron.

One of the more difficult aspects of the design was bringing the trigger lines (RB0 and ground) out of the payload compartment, down the rocket body, and out onto the fin tips. For this we used conductive paint, the kind used to repair windshield defrosters.

An electrical connection was established between the payload compartment and the rocket body by applying two patches of paint on the outside surface of the payload bulkhead connector where it slips into the rocket body, and two corresponding patches of paint inside of the forward end of the body (see Photo 1a).

Conductive lines were painted from these forward patches around the edge of the tube and down the exterior of the rocket to patches of paint at the tips of two of the three fins. These patches contact copper plates on the launch pad that connect to the launch control box (see Photo 1).

The paint patches on the bulkhead connector and the rocket body provide electrical continuity between the payload and the rocket while the model is on the launch pad. This connection is easily broken by the ejection charge.

Connector J1 brings the trigger lines to the paint patches on the bulkhead connector. It isn’t possible to solder wires to the conductive paint, so we essentially pasted the stripped end of each wire to its patch with a thick coating of the conductive paint.

Clearly, conductive paint is not going to hold the wire. To provide strain relief, we glued the insulated portion of the wires to the bulkhead connector using model cement. Still, these connections proved quite delicate and sometimes broke in the field.