Electronic Keys & LocksSensors - Tranducers Circuits

Pressure Switch Schematic Circuit Diagram

Pressure Sensor Circuit Overview

A straightforward pressure switch within the range of 50 to 350 bar can be created utilizing a pressure sensor. Remarkably, with slightly diminished linearity, the sensor’s usability can extend up to 500 bar. The circuit, depicted in the schematic diagram, is remarkably uncomplicated, primarily comprising components associated with the sensor. D1, R1, C1, and D5 work together to form a basic voltage stabilizer, maintaining a steady supply voltage of 5 V for both the sensor and opamps. Introducing temperature compensation, three diodes in series with the sensor are incorporated, which will be further explained later. The sensor’s differential output signal undergoes a 30× amplification through an instrumentation amplifier fashioned from opamps IC1a, IC1b, and IC1c. If necessary, the amplification factor can be fine-tuned by adjusting the value of R10.

Pressure Switch Schematic Circuit Diagram

Comparing Amplified Output Signal with P1 Wiper Voltage

In this circuit, the amplified output signal is compared to the voltage present on the wiper of P1. If the voltage resulting from the measured pressure falls below the value set by P1, the output of comparator IC1d becomes High, causing LED D4 to illuminate. Furthermore, an external load can be controlled through the open-collector output of T2. The sensor employed here is the Melexis MLX90240 sensor, although alternative sensors like the Exar SM5310-005-G-P or a Motorola type can be used if you do not have access to the specific Melexis sensor. If necessary, the circuit can be modified as described below. Begin with the sensor sensitivity specification from the datasheet, which is approximately 60 mV per bar per volt in this case. Due to the sensor’s supply voltage being 5 V minus 3 diode drops, roughly 3 V, the net sensitivity amounts to 180 mV per bar.

Adjusting Amplification to Match Sensor Range

The sensor’s range spans from 0 to 350 bar, resulting in a maximum output voltage of 63 mV. The subsequent amplifier has a gain of about 30, causing the output signal to vary from 0 to 1.89 V. This voltage is then compared to the voltage present on the wiper of P1, which can be adjusted between 0 and 2.5 V. If the sensitivity deviates from the nominal value, the amplification can be fine-tuned as necessary by using R10.


Temperature Compensation Consideration

Regarding temperature compensation, it’s crucial to note that the sensor utilized in this setup exhibits a temperature coefficient of 2100 ppm/degree. Different sensor types may have slightly varied values (refer to the datasheet for specifics). Consequently, the supply voltage should rise by 2100 ppm of 3 V for every degree, equivalent to 6.3 mV per degree. Silicon diodes experience a voltage drop of approximately 2 mV per degree with rising temperature. This leads to an increase in the sensor’s supply voltage as the temperature climbs, offsetting its reduced sensitivity. For the specified sensor, employing three diodes in series is necessary to almost entirely counterbalance its temperature coefficient. However, only two diodes are required for the previously mentioned Exar sensor.


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