Electronics - Ignition Board
The Ignition Board (previously called the "rocket ready" board) now handles determining if the rocket is ready to launch, if the shore power through the umbilical cord has been cut, and has the ignition battery running through it with charging features.
- This will be a 5v system - power fed from the power system board.
- The LiPo battery pack for ignition (14.8v) will mounted next to the ignition board .
- Ignition relay will be on the board but will be SPDT so in its normally closed position it will charge the battery.
- Note: Data sheets in the GIT repository "launch-tower"
- Inverter Schmitt Buffer --> p/n: CD74HC14M96 --> Digikey --> (IC surface mount with 6 invertors)
- AND Gate --> p/n: SN74AHCT1G08DBVR --> Digikey --> (IC surface mount with 1 gate)
- Optoisolator --> p/n: 4N35-300E --> Digikey --> (IC surface mount with 1)
- N-MOSFET --> p/n: part changed - need new part #
- Relay --> p/n: G5LA_E_0911 --> Digikey (through-hole)
- TVS Protection --> p/n: SN65220DBV
- LEDs are Dialight 550-xx04 which try to draw 20mA but we are instead limiting them to 5mA. These should be bright enough to see in desert sun.
- Circuit paths are double-sided where possible to allow ample current flow.
- There is a top and bottom ground-plane.
Some of the decisions on this portion of the project were debated - here's the reasoning:
Analog vs. Digital Circuit
The original circuit had an IC programmed with a few lines of code to give a known state (state machine). That worked really good in the past but complexity was higher. The original design also had some complex logic about voltage level for the state machine. The original wasn't necessarily used.
- Wouldn't it be cool if we designed an analog circuit to do the same thing? Harkens back to the era of Apollo or something like that.
- An analog circuit then can have its timing done by just modifying a resistor and capacitor.
Ultimately it didn't matter which way we went - just thought it would be cool to do it this way.
LIPO Batteries for launch
We've decided to try using a LiPo (Lithium-ion polymer) battery pack like the ones on the rocket for the ignition source. This will lessen our load of batteries and be cool (enough said).
On-board or off-board chargers will be used and the power fed to it will come through the Phidget relay board. This will allow us to select from ground control if we want to charge the batteries or not. Additionally, this battery has a Phidget voltage monitor on it so we'll know its charge level.
Optoisolator on rocket ready signal
The optoisolator on the rocket ready signal is converting a voltage source into a current source (or visa-versa)
- We don't have on one the V-Sample or Shore Sample because the signal coming in is already what we need to monitor
Design Reference - July 23, 2012
Here are the current schematic and layout designs as of July 23rd, 2012. This design was submitted as the final production run. Major changes have occurred on the board compared to June 18 version and final description of board and choices are above. Eagle files available in the GIT repository "launch-tower".
Design Reference - June 18, 2012
Here are the current schematic and layout designs as of June 18th, 2012. Left here as a reference comparison to final version.