When connecting a sensor to a microcontroller, you often need to adjust voltage levels for compatibility and safety. A common approach is to use a voltage divider, which consists of two resistors (`R1` and `R2`) in series. The output voltage (`Vout`) is taken across `R2`, and is given by the formula:

```
Vout = Vin * (R2 / (R1 + R2))
```

Here, `Vin` is the input voltage from the sensor, and you want `Vout` to match the microcontroller's maximum input voltage. Selecting resistor values that are both practical and available is crucial. The E12 series (values like 1.0kΩ, 1.2kΩ, 1.5kΩ, etc.) is a standard set of resistor values commonly found in electronics. Using standard values ensures your circuit can be built with off-the-shelf components, reducing cost and complexity.

In this challenge, you will design a voltage divider to safely connect a 5V sensor output to a 3.3V microcontroller input. You will calculate resistor values, verify if they are standard E12 values, and adjust if necessary to achieve the closest possible output voltage.


import unittest
import user_code
import ast
import re   
import importlib
import csv
import unittest
import importlib

class TestTask(unittest.TestCase):
    def test_find_closest_e12_exact(self):
        import user_code
        importlib.reload(user_code)
        # Test if exact E12 value is returned
        result = user_code.find_closest_e12(4700)
        _dynamic_test(
            self,
            result == 4700,
            "find_closest_e12 returns the exact value for E12 input (4700)",
            f"Expected 4700, got {result}",
        )

    def test_find_closest_e12_nearest(self):
        import user_code
        importlib.reload(user_code)
        # 5000 is not E12, closest is 4.7k (4700)
        result = user_code.find_closest_e12(5000)
        _dynamic_test(
            self,
            result == 4700 or result == 5600,  # both are valid, check proximity
            "find_closest_e12 returns nearest E12 value for non-E12 input (5000)",
            f"Expected 4700 or 5600, got {result}",
        )

    def test_design_voltage_divider_default(self):
        import user_code
        importlib.reload(user_code)
        # Standard task: 5V to 3.3V, preferred R2 = 10k
        r1, r2, vout = user_code.design_voltage_divider(5.0, 3.3)
        _dynamic_test(
            self,
            r2 in [10000, 12000],
            "R2 is a standard E12 value near 10k",
            f"R2 not standard E12 value: {r2}",
        )
        _dynamic_test(
            self,
            r1 in [4700, 5600],
            "R1 is a standard E12 value near calculated ideal",
            f"R1 not standard E12 value: {r1}",
        )
        v_expected = 5.0 * (r2 / (r1 + r2))
        _dynamic_test(
            self,
            abs(vout - v_expected) < 0.01,
            "Output voltage is correctly calculated",
            f"Expected output voltage {v_expected}, got {vout}",
        )

    def test_design_voltage_divider_other(self):
        import user_code
        importlib.reload(user_code)
        # 12V to 5V, preferred R2 = 4700
        r1, r2, vout = user_code.design_voltage_divider(12.0, 5.0, preferred_r2=4700)
        _dynamic_test(
            self,
            r2 in [4700, 5600],
            "R2 is a standard E12 value near 4700",
            f"R2 not standard E12 value: {r2}",
        )
        # Calculate ideal R1
        r1_ideal = r2 * (12.0 - 5.0) / 5.0
        # The closest E12 value should be selected
        e12_vals = [1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2]
        multipliers = [1, 10, 100, 1000, 10000, 100000, 1000000]
        candidates = [base * m for m in multipliers for base in e12_vals]
        r1_expected = min(candidates, key=lambda x: abs(x - r1_ideal))
        _dynamic_test(
            self,
            r1 == r1_expected,
            "R1 is the closest E12 value to the ideal",
            f"Expected R1 {r1_expected}, got {r1}",
        )
        v_expected = 12.0 * (r2 / (r1 + r2))
        _dynamic_test(
            self,
            abs(vout - v_expected) < 0.01,
            "Output voltage is correctly calculated for 12V to 5V",
            f"Expected output voltage {v_expected}, got {vout}",
        )

    def test_design_voltage_divider_small_values(self):
        import user_code
        importlib.reload(user_code)
        # 3.3V to 1.8V, preferred R2 = 1000
        r1, r2, vout = user_code.design_voltage_divider(3.3, 1.8, preferred_r2=1000)
        _dynamic_test(
            self,
            r2 in [1000, 1200],
            "R2 is a standard E12 value near 1k",
            f"R2 not standard E12 value: {r2}",
        )
        v_expected = 3.3 * (r2 / (r1 + r2))
        _dynamic_test(
            self,
            abs(vout - v_expected) < 0.01,
            "Output voltage is correctly calculated for 3.3V to 1.8V",
            f"Expected output voltage {v_expected}, got {vout}",
        )

def _dynamic_test(test_case, condition, success_message, failure_message):
    if condition:
        test_case._testMethodName = success_message
        test_case.assertTrue(True, success_message)
    else:
        test_case._testMethodName = failure_message
        test_case.fail(failure_message)

def normalize_text(text):
    text = text.lower()
    text = re.sub(r"\\s{2,}", " ", text)
    text = re.sub(r"\\s*([,:?])\\s*", r"\\1 ", text)
    return text.strip()

def change_var(code: str, var_name: str, value: str) -> str:
    tree = ast.parse(code)
    lines = code.splitlines()
    changed = False
    # Collect all assignment nodes to modify
    assign_nodes = [
        (i, node)
        for i, node in enumerate(tree.body)
        if isinstance(node, ast.Assign)
        and any(isinstance(target, ast.Name) and target.id == var_name for target in node.targets)
    ]

    # If nothing to change, return unmodified code
    if not assign_nodes:
        return code

    # Perform replacements for all matching assignments (from last to first to not break line offsets)
    for i, node in reversed(assign_nodes):
        start_line = node.lineno - 1
        line = lines[start_line]
        indent = ' ' * (len(line) - len(line.lstrip()))
        lines[start_line] = f"{indent}{var_name} = {value}"
        next_line = len(lines)
        for next_node in tree.body[i+1:]:
            if hasattr(next_node, 'lineno'):
                next_line = next_node.lineno - 1
                break
        if next_line > start_line + 1:
            lines[start_line+1:next_line] = []
        changed = True

    return '\\n'.join(lines) if changed else code

if __name__ == "__main__":
    unittest.main()


test_main.py

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Challenge: Design a Sensor Interface

解答