The Multiple Sensor Interface is a simple sensor interface that works with USB, RS485 and GPIO. It allows one to make measurements using a variety of sensors based on the change of inductance, resistance, capacitance, and frequency using the same connector and same electronic interface circuit between the sensor and the microcontroller. The same device also provides some additional connectors for small voltage measurement. Any sensors used for the measurement of distinct phe- nomena can be used if the sensor output is based on inductance, resistance, capacitance or frequency within the measurement range of the device, obtaining a variable precision depending on the used sensor. The device presented is not meant for precise or accurate measurements. It is meant to be a reusable hardware that can be adapted/configured to a varied number of distinct situations, providing, to the user, more freedom in sensor selection as well as more options for device/system maintenance or reuse. Keywords: Sensor systems and applications; Oscillators; Design aiming for reuse, repurpose, repair, customization. 1 Introduction The electronic waste (e-waste) is a modern problem under increasing concern and awareness, there are various possible approaches to reduce and mitigate it, the most obvious is the collection and recycling of discarded devices, however the most ideal is just to make technology that lasts because not only is physically fit by quality design, production, and components; but because its design was intended to be most versatile ensuring the same device can be used and reused in various applications/contexts just by changing connections, jumpers, and firmware configurations. Some design aspects for making a device more reusable are: the use of standard connectors and protocols, think of it as a module to be part of a larger system, minimize barriers for connecting/interfacing components and devices from distinct manufacturers. 1.1 Project objectives and trade-offs This article is focused on the design of a sensor inter- face device with USB and serial(UART,RS-485), aimed to allow the interface to many distinct 2-wire sensors based on the change of inductance, resistance, capacitance, frequency, and also small voltage; sensors that can be interchanged us- ing the same hardware and same port of the device, thus meaning the electronics designed must also be versatile. Obviously providing a versatile device to the users will have its negative trade-offs, like: 1- probably significant lower precision/accuracy; 2- some sensor calibration must be provided/done by the end user after replacing a sensor; 3- the calibration function will not be linear or ’easy’ as de- sired for sensors and its interfaces. However, for some applications the mentioned trade-offs are not necessarily a deal-breaker, like when the user is *Lisboa, Portugal; E-mail: david.n.quelhas@gmail.com https://www.linkedin.com/in/dquelhas ORCID: 0000-0002-0282-0972 https://multiple-sensor-interface.blogspot.comtechnical and is ok with using a device that requires more setup/configuration, some users like devices that are more customizable or repairable. Also is possibly valued a device that if no longer useful for a user, it might still be useful for another user on a different application/context. 1.2 License and context The hardware design here disclosed is distributed under "CERN Open Hardware Licence Version 2 - Weakly Recip- rocal" (CERN-OHL-W), its associated software/firmware under GNU licenses (GPL, LGPL). This article is published under the Creative Commons li- cense Attribution-NonCommercial-ShareAlike 4.0 Interna- tional (CC BY-NC-SA 4.0). This article is about a ’hobby’ project done by the author (David Nuno Quelhas, MSc Electronics Eng, alumni of In- stituto Superior Tecnico, Portugal) with occasional ’work’ between the years 2012 and 2022 on his ’free time’. 1.3 Prior art review The topic and devices commonly described in literature as ’Multiple Sensor Interface’ and also as ’Universal Sen- sor Interface’, commonly fall under 3 distinct categories: a) Device that has a more versatile interface or signal condi- tioning circuit capable of interfacing various sensor types; b) Device that includes various specialized interfaces or signal conditioning circuits for each sensor type, typically built us- ing various PCB boards for the sensors, to connect or stack into a PCB board with a microcontroller that will register and/or transmit the measurements, or alternatively have all these different circuits integrated inside a single integrated circuit (micro-chip); c) Hardware and/or software systems that collect or register sensor data from various distinct sens- ing devices/circuits, that may apply some processing to the raw data for obtaining measurements, then to be transmit- ted to other systems or to a data storage, and so these hard- ware/software systems may also be called/named ’interface’. 1 Multiple Sensor Interface - David Nuno Quelhas - License CC BY-NC-SA 4.0 Figure 1: Diagram of the Multiple Sensor Interface device. The article review presented here will be about ’more ver- satile interface or signal conditioning circuit’ that is the cat- egory most similar to this article. Types of versatile sensor interface found on prior art: 1- Interfacing resistive or capacitive sensors by measuring the charge-discharge time of an RC circuit, or measuring the frequency or PWM from an oscillator whose pace is con- trolled by the speed of a capacitor charge-discharge trough a resistor; for example: [7], [8], [9] . 2- Interfacing sensors based on the variation of impedance (includes sensor based on variation of resistance, capaci- tance or inductance) by an LCR meter, impedance meter, or potentiostat circuit; for example: [10], [11], [12], [13] . 3 - Interfacing a sensor as part of a bridge circuit (ex- ample: resistive sensor on a resistance bridge, capacitive sensor on a capacitance bridge) [14] . The Multiple Sensor Interface presented on this article has a working principle more similar to the circuits mentioned as type 1 (RC time or frequency or PWM of oscillator), how- ever in comparison with the mentioned