Pulsed Current Switch Input Circuit for Wet-Condition Detection
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Solution Overview
Problem
Existing safety circuits in tractors, grass mowing machines, and utility vehicles face issues with water ingress into low-cost, unsealed switches, leading to false readings in microcontrollers due to high impedance inputs, excessive heat dissipation, and incorrect sensor readings at low battery voltages, necessitating an improved discretionary current input circuit that can accurately read switch states in wet conditions.
Innovation Solution
An improved discretionary current input circuit using a normally off power transistor connected to open body switches, which draws a nominal current most of the time and a higher threshold current for short intervals, controlled by a microcontroller to accurately read switch states and disable vehicle functions if necessary, utilizing a smaller resistor to manage power and voltage thresholds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a microcontroller with high impedance inputs is used to read switch states, then the circuit complexity is reduced and cost is lowered, but water in the switch may cause false readings due to insufficient current to overcome water resistance
Solution Approach 1:
The patent implements periodic current pulses through the switch using a timer-controlled transistor circuit. The transistor switches on periodically to deliver current bursts (e.g., 100-500 mA for 1-10 milliseconds) through the switch to the microcontroller input, then switches off. This periodic action cleans water or contaminants from the switch contacts during the off periods while maintaining reliable signal detection during the on periods, resolving the contradiction between simple high-impedance inputs and reliable operation in wet conditions.
2Reliability
If continuous high current is drawn through the switch to overcome water resistance, then reliable switch state detection is achieved, but excessive heat is dissipated inside the control enclosure
Solution Approach 1:
The circuit uses periodic current pulses rather than continuous current flow. The transistor is controlled by a timer to switch on only during brief intervals (1-10 milliseconds) at regular periods, delivering high current (100-500 mA) only when needed for reading the switch state. Between pulses, the current drops to nominal levels. This periodic operation achieves reliable switch detection while dramatically reducing average heat dissipation in the control enclosure, resolving the contradiction between detection reliability and energy loss.
3Adaptability or versatility
If the voltage threshold for the microcontroller input is lowered to work at low battery voltages, then the microcontroller can operate during cold cranking, but the input becomes more sensitive to water in the switch causing false readings
Solution Approach 1:
The periodic current pulse circuit actively drives current through the switch during each pulse interval, maintaining a strong signal at the microcontroller input regardless of the input voltage threshold setting. This allows the microcontroller to use lower voltage thresholds for operation during cold cranking while the periodic current pulses ensure that water or contaminants do not cause false readings, as the active current drive overcomes the resistive effect of water on the contacts.
4Use of energy by moving object
If a larger resistor is used to limit current during normal operation, then power consumption is reduced, but the resistor must dissipate more power during current pulses and becomes bulkier
Solution Approach 1:
The circuit uses periodic current pulses with carefully selected resistance values that limit power dissipation during both the pulse and non-pulse periods. The resistor is sized to handle the peak pulse current while dissipating acceptable power during continuous operation. The periodic nature of the current flow allows the use of smaller, more compact resistors compared to continuous current designs, as the average power requirement is much lower even though peak currents are higher during pulses.
Data Source
AI summary
An improved discretionary current input circuit includes a switch sensing an operating condition of a tractor and having a first state if the operating condition is not satisfied and a second state if the operating condition is satisfied, and a microcontroller that deactivates at least one function of the tractor if the switch is in the first state. The switch draws a nominal current except during specified time intervals that are shorter than the time for drawing the nominal current. A power transistor is connected through a diode and resistor to the switch. The power transistor normally is in an off condition, and is powered during the specified time intervals to an on condition to increase current above the nominal current to a threshold through the switch. The microcontroller reads if the switch is in the first state or the second state only during the specified time intervals, and uses a timer circuit to determine a duration of each of the specified time intervals such that the power transistor is turned off before the end of a task interval of the microcontroller.