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Liquid Level Detector

IP.com Disclosure Number: IPCOM000079628D
Original Publication Date: 1973-Aug-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 30K

Publishing Venue

IBM

Related People

Carmichael, JM: AUTHOR [+2]

Abstract

The level of a liquid, such as ink, is detected using the equivalent capacitance of the fluid in a capacitor divider network excited with an AC signal, applied to the input of the circuit shown in Fig. 1. Two sensor probes P1 and P2 shown in Fig. 2 are inserted into the liquid.

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Liquid Level Detector

The level of a liquid, such as ink, is detected using the equivalent capacitance of the fluid in a capacitor divider network excited with an AC signal, applied to the input of the circuit shown in Fig. 1. Two sensor probes P1 and P2 shown in Fig. 2 are inserted into the liquid.

Circuit operation is as follows. Resistors R1, R2, R3 and transistor T1 receive the AC input signal, resulting in T1 switching on and off. This signal is applied to the capacitor divider network comprising C1 and C2, Fig. 1. C1 is the equivalent capacitance of the liquid. C1 is large when the ink covers the probes and small when the ink is not adequate to cover the probes. The equivalent capacitance C1 has an equivalent resistor R6 in parallel with it which is small. When the ink supply is adequate, covering the probes, the large C1 and small R6 (equivalent resistance) causes nearly all the AC voltage to be present at C2, which is filtered and rectified by R4, C3 and D1. This results in transistor T2 turning on, and the output voltage is low.

When the ink goes below the probes, almost no signal is present across C2 due to the impedance change of C1 (lack of ink results in high impedance). This results in T2 being shut off and the output goes high.

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