b'FeatureDIY geophysicisttutorial on connecting a geophone to the ADS1115 and alately. The green revolution of solar, wind, and electric vehicles Raspberry Pi to trigger a camera logger (https://core-electronics. is creating a flood of new efficient electric power switching com.au/guides/raspberry-pi/geophone-raspberry-pi/). Needdevices, with many evaluation systems providing ready-made more? If you really need and want 24 bits and sampling todevices for incorporation into new ideas.10 kS/s or higher then I would recommend the ADS131M04 or ADS131M08 devices, providing a programmable amplifierNew power switching devices and the use of microcontrollers input (gain =1 to 128) and 80-to100dB dynamic range. Theseto precisely control the switching provide the geophysicist ICs are designed to perform well on simpler two-layer boardswith new possibilities. Traditionally, we tend to switch electrical and interface via the SPI interface. The data sheet from Texascurrents into a H-bridge (https://en.wikipedia.org/wiki/H-Instruments has pseudo-code (and C code on their supportbridge) that allows efficient bi-directional current through web site) examples on how to program and then set-up of thea load, which in our case is usually the earth (resistive) or a device to access the streaming data. Plus, these are in stockloop (inductive). If we are satisfied with switching on or off and available for less than $10 each by the thousands. To savein less than 200 microseconds and at a rate of less than 10s time you can buy the evaluation board (ADS131M08EVM) andof Hertz then for moderate power (100-1000W) for each with one to three of these boards you have yourself the basisswitch I would use a photovoltaic optocoupler, such as the for a decent (4 ch) MT, TEM system or (8 to 24 ch) engineeringBroadcom ASSR-V621/V622 device (DIP8 package), or Panasonic seismograph system. Hint for happiness, make the connectionAPV1121S (SOP4 package) with a N-channel MOSFET (TO-220 between your microcontroller and the EVM less than 15 cm andor TO247 package, with input capacitance of less than 2000 run the SPI Clock at a frequency just high enough to get thepFhundreds available at various voltage current ratings e.g., data reliably to the microcontroller, say 2MHz. While it mightIRF540). The photovoltaic optocoupler is basically a LED light cost you almost $1000 dollars to purchase a few ADS131M08source coupled to a small photovoltaic diode within a plastic EVM boards and a Portenta H7 embedded system for datapackage. The combined photovoltaic optocoupler and MOSFET logging and communication to your laptop PC (via USB/ makes a solid-state relay that can isolate up to 2500V between Ethernet/Wifi), that cost pales in comparison to the price of athe power switches and the microcontroller electronics. The new geophysical acquisition system, which usually costs manyoptocoupler needs only 5 to 10 mA of current (in the LED tens of thousands of dollars. section) to operate reliably and can be driven with the digital output pins from a microcontroller directly (if you program the Lastly, when measuring analog signals be mindful of what thedigital output to be open collector to sink current). Ready-voltage is with respect to what! This aspect is a little trickymade solid-state relays using this approach can be bought for novices and sometimes people who should know better. Atoo, such as the CPC1779 (good for 2-4 amps at 500 V, and geophone element is a floating voltage source of both positiveshallow resistivity), which are slightly easier to wire but a more and negative polarity so it must be tied to a reference potential,expensive and a difficult-to-find solution. The optocoupler plus either directly or via a resistor. With single ended analog inputsMOSFET solution will suffice for the majority of DIY geophysics (one pin for input as in most microcontrollers) it is best to haveresistivity applications and basic TEM transmitters (equivalent to the sensor reference potential at approximately half of thethe old SIROTEM systems).ADC power supply voltage (e.g., 1.65 volts from 3.3V) for bipolar sensors such as geophones, pick-up coils, or electrode stakesFaster switching with high voltages can be found by utilising in the ground (and most natural signals). The sensor referenceICs intended to drive muti-kW single and 3-phase motors, with voltage can be made via a resistive divider (https://en.wikipedia. the Infineon IM828-XCC a good modern example of switching org/wiki/Voltage_divider), or use a 1.5V alkaline/lithium20 A at 600 V in 100 ns. The lowest frequency of switching is battery (in a holder), or a special rail splitter device such as thedetermined by a bootstrap capacitor that can be large enough TLE2426 (my favourite approach). Tie one end of the geophoneto keep the auxiliary supply powered for 100 ms or longer to the sensor reference potential and the other to the inputto seconds. An evaluation board design is available for pre-using twisted pair wires (as in CAT5/6 ethernet) betweenpackaged convenience and the 3 half-bridge outputs could be geophone and microcontroller. With the differential input of theused with a bit of creativity for resistivity measurement into two ADS131M08 connect a resistor to ADC/microcontroller grounddipoles simultaneously with a shared electrode stake (e.g. with at each geophone output -to ADC input (it has a charge pumpthree pseudorandom binary sequences PRBS into the three circuit to allow measuring small negative voltages with respectdigital control lines to the U,V,W half-bridge-tied load outputs). to ground). With an instrumentation amplifier, such as theOr as a small but powerful CSAMT transmitter. Possibilities are AD623 and others, connect each geophone terminal to sensornow available you didnt think you could buy before, let alone reference potential via a 10k-100k resistor. A buffered voltagefor less than $500.splitter such as the TLE2426 or a resistive divider buffered withAnother approach is to use a Class-D amplifier (designed for a unity gain Op-Amp should be used. Otherwise, confusion willaudio sub-woofers) to drive the resistive or inductive load. The reign supreme as the ADC will likely give a reading of either maxDAC output from many microcontrollers (eg Pyboard v1.1 and or min regardless of sensor input. Portenta H7) can provide an encoded analog signal (0 to 3 Volts) such as a swept sinewave or random signal (uniform distribution Excitation random noise or limited Gaussian noise) to the amplifier. A slightly clumsy alternative is to use a resistive-capacitor low-Many geophysical measurements require changes to electric orpass filter on a digital stream from one of the microcontroller magnetic fields or use electrically controlled (seismic) sourcespins, either pulse-width-modulation or PRBS, can generate the (Dean etal., 2018(a)) and we may use modern developmentssignal (this is the approach recommended on user forums for the in power electronics to our advantage. Over forty years theRP2040, which has no DAC). A nice integrated class D amplifier technology of power switching has evolved considerablyexample is the TPA3255 integrated circuit. Its a fiddly surface from Silicon (Si) transistors to Si MOSFETs (Metal Oxide Semi- mount chip to design and mount directly, but the evaluation conductor Field-Effect Transistors) to SiC and GaN MOSFETboard can either be bought or copied to have a small 300 W 53 PREVIEW DECEMBER 2022'