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Wheatstone bridge as Strain Gauge interface This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide. AInc. Pre-presentation Self Assessment Activity Your comfort level with your responses to the following single design challenge should suggest if the presentation that follows will increase you knowledge base on the topic embedded within these questions. Pre-presentation Self Assessment Activity 1) Design a Wheatstone bridge load cell sensor interface circuit that will provided a voltage output that indicates the sensor’s reaction to a change in load (weight on the sensing element). Review Wheatstone bridge circuit If all of the resistance values are equal the voltage between point (a) and (b) is equal to zero. (a) R4 = R3 = R2 = R1 This voltage value is the voltage difference between point (a) and point (b). R3 R1 V + V V=0 R (b) 4 R2 Review Wheatstone bridge circuit If one of the resistance values is different the voltage between point (a) and (b) is not equal to zero. (a) R4 = R3 = R2 = R1 This voltage value is the voltage difference between point (a) and point (b). R3 R1 V + V V=0 R (b) 4 R2 Review Wheatstone bridge circuit If the 4th resistor is a load cell, its resistance value changes with the weight of the load on the strain gauge in the load cell. (a) R4 = R3 = R2 = R1 This voltage value is the voltage difference between point (a) and point (b). R1 R3 V + V V=0 Load cell R (b) 4 R2 Review Wheatstone bridge circuit its resistance value changes If the 4th resistor is a thermistor, load, its resistance value changes with with temperature. weight. This Wheatstone bridge circuit detects this resistance change. (a) This voltage value is the voltage difference between point (a) and point (b). R1 R3 V + V V=0 R4 changes in resistance value as the weight changes and the voltage between points (a) and (b) in the Wheatstone bridge circuit also changes proportionally. (b) Load cell R 4 R2 Post Presentation Self Assessment Activity 1) Design a Wheatstone bridge load cell sensor interface circuit that will provided a voltage output that indicates the sensor’s reaction to a change in load (weight on the sensing element). R Voltage across these terminals indicates if the weight of the object on the load cell has changed. V 3 R 1 + V R 4 R 2 Load cell Note: This comparator configuration for the operational amplifier will drive the output signal to one of the two possible saturation values when the weight on load cell leaves its steady state value. End of Presentation AInc.