Introduction
This blog continues from Part 6 of the ‘Wi-Fi-controlled rocket launcher’ with a summary of board changes to suit an off-the-shelf enclosure and the addition to the circuit of a starter continuity check.
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| Updated Launcher PCB |
Hardware Change
A hardware change was made to the output circuit to detect when an unfired starter is connected to the launcher output. One of the output drivers has a built-in diagnostic feature which sends the voltage at the device's output pin to a sense pin when the driver is switched OFF. By adding resistive pull-ups to the output driver, the starter's electrical continuity can be determined in most circumstances.
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| Output Resistive Pull-up Addition |
Shorts across the output, failures in wiring or components on the circuit board are not taken into account.
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| Driver MSENSE Output |
To test the proposed starter continuity solution on the current board, the ESP module was not fitted. The SENSE input on the VN7040 was connected to the onboard 3.3 V supply. This configures the driver in the desired mode.
An external 27 k pull-up resistor was added between the output and the 3.3 V supply. The low-driver input for the starter was then connected to the 3.3 V supply.
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| Testing Starter Continuity Solution |
To validate the MSENSE output voltage, a test wire was connected to the MSENSE output and to a multimeter.
With no Estes starter fitted and the low driver switch ON, the voltage at the MSENSE output was 3.9 V. With a new Estes starter fitted, the voltage at the MSENSE output is close to 0 V. Using the onboard regulator, the voltage range will be suitable for the ESP Huzzah digital levels. Without the onboard 3.3 V regulator, the Huzzah's onboard regulator could be used.
The continuity measurement will be integrated into the code as a continuity check before a launch. It is acknowledged that a starter or shorted output results in the same measurement by the ESP.
For reference, a larger-value pull-up resistor on the output, such as 100 k, does not bias the output driver. No MSENSE output voltage is produced in this instance.
Note on Aerotech Starters
The Aerotech starters were tested on the bench with a power supply.
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| Aerotech Copperhead Igniter (Starter) |
Using a power supply set to 8 V at 2 A, the starters will fire reliably. Applying that requirement to the launcher, two 4.2 V cells could be used in series on the launcher to fire the Aerotech starter. This was not confirmed through testing.
Circuit Board Update
As mentioned in the previous launcher post, an off-the-shelf enclosure will replace the 3D printed case. The Hammond Manufacturing part RP1135C or the Ritec RP1135 were two compatible enclosures. Choosing a wider enclosure provided more circuit board space and new layout options. Most of the surface-mount components were consolidated on one side of the circuit board.
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| Updated Launcher Design |
Switch and External Cable
As two actions (a switch and the website launch button) must be realised for a launch to occur, the existing toggle switch arrangement will remain unchanged. For those desiring safety, a missile-style switch or a suitably rated keyswitch could be used instead. For the external cable interface, a gland will be used to maintain basic ingress protection.
In the next post, the new circuit board build and testing will be covered.
