Introduction
This short blog update continues from Part 4 of the ‘Wi-Fi-controlled rocket launcher’. A prototype design featuring 18650 batteries with the ESP modules was tested.
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| Rocket Launcher 3D Model |
Design Change
As suggested in Part 3 of the blog, the design was updated to integrate two 18650 batteries onto the rocket launcher circuit board (PCB). The battery configuration can be set on the PCB to be either series or parallel using jumper resistors.
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| Dual Battery Series or Parallel Option |
The linear regulator on the Adafruit ESP32 Huzzah board can accept up to an input of 6 V DC, meaning that an additional regulator was not required, depending on the battery configuration.
A fully charged 18650 battery is rated at 4.2 V, so the series battery resistor jumper option should be used with the optional DC-DC regulator. Using the DC-DC regulator should also allow very close to the full capacity of the battery to be utilised.
On the ESP32 Huzzah, the input to the linear is protected by a series diode. This series diode reduces the effective operating voltage of the battery by approximately 0.28 V. Depending on the battery manufacturer and the load current, the cell voltage could be around 3.4 V when the State of Charge (SoC) is between 0% to 10%. The dropout voltage of the ESP32 board may prevent full use of the cell capacity.
A pre-wired Multicomp part MCR13-36A2-11 was selected to replace the previous power switch.
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| Replacement Switch |
Testing
The board was populated with the two output drivers, ESP mounting headers and the dual battery holder.
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| Bottom Side of Launcher PCB |
The jumper resistors were configured for parallel operation. The optional 3.3 V DC-DC converter was not fitted. Since there were only small design changes, the board was powered first with a 3.3 V supply, then with a charged 1650 battery.
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| Top Side of Launcher PCB |
It was noted that during testing with a fully charged battery, the ESP32 did not boot correctly. This issue was caused by GPIO 15. The Status LED is connected to GPIO 15, which is connected to the 3.3 V supply through a current-limiting resistor. A pull-up on GPIO 15 changes the ESP behaviour. To resolve the issue, the LED will be driven to 0 V on the revised design. A pulldown resistor already exists on the ESP board.
Other Changes
To measure the 18650 voltage, the onboard voltage divider values were adjusted to meet the ESP's maximum 1 V limit. The resistor footprint was standardised to 0603. This feature will be tested with the igniter in the next post.
