Fluid Detector

Fluid-Detedor Switch Parts List

Quantity	Part	Description
1	R1	470 MΩ, ¼W resistor
1	R2	2 KΩ, ¼W resistor
1	C1	0.001 μF, 25V capacitor (disk)
1	C2	0.05 μF, 25V capacitor (disk)
1	C3	20 μF, 25V electrolytic capacitor
1	D1	1N914
1	T1	2N2222 PNP transistor
1	U1	LM1830 (National Semiconductor)
1	S1	Metal container with polar liquid
1	RL-1	9-12Vdc SPST relay

Fluid Detector With Sound Output Parts List

Quantity	Part	Description
1	R1	1.5 KΩ, ¼W resistor
1	C1	0.001 μF, 25V capacitor (disk)
1	U1	LM1830 (National Semiconductor)
1	S1	MRD 300 phototransistor or thermistor
1	SPKR	8 Ω Speaker

The National LM1830 fluid detector detects the Presence, absence, or level of a polar liquid such as water. The LM1830 is a linear bipolar integrated circuit that operates from a wide range of operating voltages. The fluid detector shown in Fig. 1 determines the presence of a fluid by comparing the resistance of the fluid between the probes and the internal resistance of the IC. An ac signal is used to overcome the plating problems associated with a dc voltage source. Provisions are available for connecting an external resistance in applications where thy fluid impedance is of a different magnitude than an internal resistor in the IC. As the probe resistance increases above a preset value, the internal capacitor produces a signal.

The internal oscillator is coupled to the base of an open collector transistor so that the output signal can drive an LED a speaker, or low-current relay. A 0.001μF timing capacitor between pins 1 and 7 produces a 6 kHz signal on pin 5 of the fluid detector. A filter capacitor on pin 9 enables the fluid detector to operate at a constant output. Removing this filter allows a 50% duty cycle. When used with a speaker, the output will cycle on and off.

Although the LM1830 detector was primarily designed for sensing conductive liquids, it can also be used in the direct-coupled mode. A variable resistance sensing device such as a light-dependent resistor or a thermistor, could be used as the sensing device, as shown in Fig. 2 The fluid detector is a complete, low-cost level-detector system that can be used in many different sensing applications The detector can drive a low-current relay as a local alarm. The relay contacts could be wired to other sensors, and the multiple sensors could activate a central alarm panel.

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