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Product Attributes

Introduction

The QT114 QuickLevel™ charge transfer ("QT") sensor IC is specifically designed to detect point liquid levels of liquids and powders. It can project a sensing field through almost any dielectric material (such as glass, plastic, or ceramic) to sense the internal liquid level from outside the container. When used with a two-layer electrode configuration, it has the unique ability to independently sense two trigger points.

The QT114 has no sensing timeout, drift compensation, or other features that would interfere with liquid level sensing. Its thresholds are fixed, and the amount of signal required to exceed a threshold depends on circuit gain, electrode size, and load — all of which are controlled by the designer.

The QT114 requires only a single low-cost capacitor to operate. One or two LEDs can also be added to provide a visual sensing indication.

In most applications, power consumption is below 20mA, allowing years of operation from a lithium battery. In most cases, the power supply requires only minimal regulation.

The QT114 incorporates multiple signal acquisition and processing techniques pioneered by Quantum. No external switches, op-amps, or other analog components are required beyond CS.

A unique feature is the "slosh filter" — a detection integrator that averages detection results over consecutive 15-second intervals before activating or deactivating the OUT pin. This filter allows the QT114 to be used with violently sloshing liquids, such as in moving vehicles, where the output would otherwise toggle between two states.

The device also includes selectable output polarity, allowing the two output lines to be configured as active-high or active-low. It also includes Quantum's pioneered HeartBeat™ signal, which allows a host controller to continuously monitor the status of the QT114 when required. By utilizing the charge transfer principle, this IC performs significantly better than legacy technologies. It is specifically designed to replace electromechanical devices such as float switches, thermistors, and conductance probes.

The QT114 is a digital burst-mode charge transfer (QT) sensor specifically intended for point liquid level sensing; it contains all the hardware and signal processing functions needed to provide stable liquid level sensing under varying conditions. Operation requires only a single external capacitor.

Calibration is accomplished through design by adjusting the electrode size and Cs capacitor. Only in rare cases does the QT114 circuit require individual calibration, and the circuit can be arranged to accommodate this.

The QT114 uses short, low duty-cycle charge transfer burst pulses to acquire signals. Burst mode allows power consumption to remain in the low microamp range, significantly reducing RF emissions, reducing susceptibility to EMI, while enabling excellent response time. Internal signals are digitally processed to generate the required output signals.

Both the QT switching and charge measurement hardware functions are internal to the QT114. A 14-bit single-slope switched-capacitor ADC incorporates the required QT charge and transfer switches in a single configuration, providing direct ADC conversion. The pulse length is inversely proportional to the rate of charge accumulation on Cs, which in turn depends on the values of Cs, Cx, and Vcc. Vcc serves as the charge reference voltage. The larger the Cx value, the faster the charge transferred into Cs accumulates. The trigger point of the sensor can be changed by varying Cs and load capacitance Cx. Therefore, the values of Cs, Cx, and Vcc should be reasonably stable over the expected operating temperature range.

Two fixed thresholds are used — one for low liquid level and one for high liquid level; the design requires adjusting Cs and Cx so that they trigger at the appropriate points, with fine-tuning via adjustment if needed. Figure 1-1 shows optional potentiometers that can be used to fine-tune the positions of these threshold points relative to the signal.

It is not necessary to use both detection threshold points; if only single-point sensing is needed, only the lower threshold and OUT1 need be used, while OUT2 is ignored.

Two option pins allow selection of output polarity and insertion of the "slosh filter" ahead of the OUT pins.

The internal ADC treats Cs as a floating transfer capacitor; therefore, the sensing electrode can be connected to either SNS1 or SNS2 with no difference in performance. During a burst, the polarity of charge accumulation across Cs is identical in both cases. Cs must be within a certain range for proper operation.

Separate Cx and Cx' loads can be connected to SNS1 and SNS2 simultaneously, although the result is no different from connecting the load to SNS2 (or SNS1) alone. It is important to minimize stray capacitance on both terminals, especially if the load Cx is already large — for example, by minimizing trace length and width.

Features

• Detects liquid level of virtually any liquid or powder
• Dual-level liquid sensor — one probe detects both high and low levels
• Single external component — one 5-cent capacitor
• Compatible with internal probe or external electrodes
• Output configurable as active-high or active-low
• Slosh filter averages response to moving liquids
• Both outputs capable of driving LEDs
• 2.5 to 5V, 20μA single-supply operation
• HeartBeat™ health indicator on both outputs

Applications

• Process control
• Vending machines
• Automotive fluid detection
• Consumer appliances
• Medical fluid detection
• Soil moisture detection