Conductivity sensor circuit

Date of Award


Degree Name

M.S. in Electrical Engineering


Department of Electrical and Computer Engineering


Advisor: John S. Loomis


The M11 sterilizer unit currently in production at Midmark Corp. is used to disinfect and sanitize the dental equipment used during standard dental procedures. The sterilizer unit has a filter that keeps the water, which is used for the steam that cleans the dental equipment, purified of unwanted particles. There is currently no indicator to tell the user when the filter needs to be replaced. The project presented will resolve this problem by developing a conductivity sensor circuit to measure the conductivity of the water in order to determine the water purity from the number of ions present. Research is performed to decide which conductivity sensor will best fit the electrical and mechanical specifications of the M11 sterilizer. Cost is also a factor in selecting the sensor since the circuit and sensor will be included in every unit and therefore increase the price of the sterilizer. The bulk of the project however is the simulation, schematic drawing, and PCB layout. Simulation is performed in LTspice, which offers the chance to determine if the circuit will work in theory before attempting experimental tests. In this stage it is determined that a 555 timer is to be used to create a square wave for the input of the conductivity sensor. The input to the sensor is held at a constant current so that a voltage measurement is used to determine the conductivity. The square wave output of the sensor is put through an RMS to DC converter, LTC1966. Finally a low pass filter is used to riddle out any unwanted high frequencies. The schematic for the circuit is drawn using the software PADS, in which the proper components are selected and connected according to the pinouts of those components. PADS is also used for the layout of the PCB, in which the components are laid out in agreement with the design rules that are regulated by Midmark Corp. and their PCB manufacturers. The circuit design proves to be effective in both simulation and experimental testing. Depending on the range of conductivity accepted by Midmark Corp. and regulatory standards for the sterilizer water tank, the circuit is adjustable by multiple switches to amend to the customer. Although modifications will need to be made to implement the circuit into the sterilizer system, the research has proved that the conductivity can be accurately measured and output to the user determining when the filter needs to be replaced. The circuit will likely be used in the next generation of sterilizers at Midmark Corp.


Water Purification Filtration, Dental instruments and apparatus Sterilization, Detectors Design and construction, Detectors Design and construction Simulation methods, Filters and filtration, Electrical Engineering, Engineering, Dental Care, conductivity, sensor, Midmark, sterilizer, filter, constant current, M11, circuit design, LTspice

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