Building a ResistiveWave ProbeYour Wave ProbeVoltage DividerWave Probe Electronics SchematicCheck the Math!Guidelines to BuildingHooking up the ampsMassachusetts Institute of Technology 12.097Building a ResistiveWave ProbeMassachusetts Institute of Technology 12.097V+water levelComputerElectronics AssemblyV-Start loggingDownload dataElectronics Assembly:Data logger and microcontrollerPower amp for bridgeAmp for level shiftingBatteriesMassachusetts Institute of Technology 12.097Resistance R = Volts loss per Ampere driven through the “device”R is nota property of bulk material!Conductance = 1 / ResistanceConductivity of seawater: K ~ 4 S/m, where1 S = 1 Siemen = 1 Ampere per Volt:One Volt across opposing plates of area 1m2drives one Ampere1 S/m means one Volt drives one Ampere, when the distance between electrodes is one meterConductivity is a property of bulk material!Resistivity = 1 / ConductivityEquivalent resistance through seawater:R = distance / [ area x K ]V+ V-R (Ohms, Ω)V+V-Massachusetts Institute of Technology 12.097Your Wave Probed = wire diameterz = immersion depthD = separation distanceTwo wires of immersed surface area πdz: Then R = D / [ π d z K ]~ 0.10m / [ π x 0.001m x 1m x 4S/m ] = 8ΩDdV+V-Massachusetts Institute of Technology 12.097Voltage DividerTT8 digital i/oV1LM324N ampOPA551PA ampTT8 analog inRb(fixed)V2= V1Rp/ (Rb+Rp)Rp(WATER)Massachusetts Institute of Technology 12.097Wave Probe Electronics Schematic+9V-9V+9VGND__++9VGNDLM324NOPA551200Ω70kΩ8kΩ+400mVV1(+5V)V2protoboardTT8 D3 (pin A8)Probe HOTTT8 AD0 (pin B18)Massachusetts Institute of Technology 12.097Check the Math!• Output of OPA551 is zero (bridge not energized) or five Volts (energized). • The voltage divider splits the fixed 200Ω with about 10Ωin the water, so V2~250mV.• The TT8 AD0 sees about:(70kW+8kΩ)/8kΩ*400mV – 70kΩ/8kΩ*V2 ~ 3.9V – 8.8 V2~ 1.7V if V2= 250mV OKOKMassachusetts Institute of Technology 12.097Guidelines to Building• Run the TT8 with a 12V battery – always double check polarity! RED IS POSITIVE• You’ll run the program sclogger; the output will always be written to output.dat. Use ren output.dat myfile.dat to rename the file.• Tie the analog and digital grounds of the TT8 to the protoboard.• Tie the middle of the two 9V cells to ground; the batteries supply +9V and -9V.• Start by building up the OPA551; confirm that V1is zero when input is zero, and V1is 5V when input is 5V. • When the circuit is complete, use a resistor of around 10Ω in place of the probe to check the LM324N output. It should be about +1.5V to +2V. • When you can log the test circuit output using sclogger, you’re ready to go in the water!• When you calibrate, the curve should have the form V = k1+ k2 / z• TT8 can read 0 to +4095mV, in units of one milliVoltMassachusetts Institute of Technology 12.097Hooking up the ampsOAP551 LM324NHigh Voltage Low-Power High Current QuadOUTPUT A 1-IN A 2+IN A 3B+9VN/C 1-IN 2+IN 3-9V 4 5 N/C6 OUTPUT7 +9V8 N/CDGNDC8-DIP (dual in-line