Design Notes: Water Level Motor Controller
Source Requirements Extracted
- Sensor: LM31-00000F-002PG liquid level sensor.
- Sensor output: 0.5-4.5 V ratiometric with 5 V supply over 0-2 psi / 0-0.20 bar range.
- Project range: 0-1 m water level, corresponding to about 0.5-2.5 V sensor output.
- Key points from project brief: 10% level about 0.70 V, 90% level about 2.30 V, 100% level about 2.50 V.
- Motor: PEL00884 brushed DC motor, 6 V rated.
- PEL00884 datasheet points: no load 2800 RPM at 0.02 A; max efficiency 2300 RPM at 0.075 A with 10 g-cm torque and 0.24 W output; stall 0 RPM at 0.33 A.
Important Inconsistency
The uploaded student report text states max-efficiency operation at 2300 RPM and 0.24 A, but the PEL00884 datasheet table shows 2300 RPM at 0.075 A and 0.24 W output. The assignment statement also asks for minimum speed at maximum current of 300 mA. The schematic was built to demonstrate the requested analog current-control method, but the target current should be confirmed before final hardware.
Current Sense Choice
Selected sense resistor: R2 = 5 ohm.
If motor current target is 0.24-0.30 A:
- 0.24 A gives Vsense = 0.24 A * 5 ohm = 1.20 V.
- 0.30 A gives Vsense = 0.30 A * 5 ohm = 1.50 V.
- R2 dissipation at 0.30 A is I^2R = 0.45 W, so use at least a 1 W part.
Linear Command Relationship
For a 0.24-0.30 A current target over sensor voltage 0.70-2.30 V:
- Slope = (0.30 - 0.24) / (2.30 - 0.70) = 0.0375 A/V.
- Desired current command: Imotor = 0.0375 * Vsensor + 0.21375 A.
- With R2 = 5 ohm, sense command voltage is Vctrl = 5 * Imotor = 0.1875 * Vsensor + 1.06875 V.
At 10% level / 0.70 V: Vctrl = 1.20 V.
At 90% level / 2.30 V: Vctrl = 1.50 V.
Full-Level Threshold
A 10 k / 10 k divider from 5 V generates 2.5 V. U1D compares the buffered LM31 voltage to this threshold. When the buffered sensor voltage exceeds 2.5 V, U1D output is intended to go low, turning on LED1 through R7 and clamping Q1 base drive via D2.
Thermal Notes
Linear motor current control can dissipate substantial heat. Approximate Q1 dissipation depends on motor voltage, motor back-EMF/load, and current. Before PCB layout, verify Q1 SOA and heat sinking/copper area for the intended motor operating point.
Validation Status
- LM31 and PEL00884 datasheets were read from the uploaded PDFs.
- Schematic was created and ERC checked: no ERC error/warning matches were returned.
- DRC airwire errors are expected because PCB layout/routing has not been started.
- The current-command resistor network is approximate and should be simulated/tuned before physical build.