The L4620 liquid level alarm is designed to operate

with a variety of sensor types which change imped-

ance depending on whether the sensor is above or

below the level of a liquid. If the impedancevariation

of ther liquid itself is sensed, a very simple sensor

(two electrodes) can be used. The output stage

drives directly the alarm indicator with a 300mArec-

tangular wave signal, the duty cycle of which is pro-

grammable.

SENSOR INTERFACE.

As shownin the applicationcircuit, the sensoris con-

nected so that it varies the attenuation of a square-

wave signal between pin 2 and pin 3 where its posi-

tive half cycle is comparedwiththe referencethresh-

old (with hysteresis).

This frequency, generated internally by a 50% duty

cycle oscillator, is 50Hz in the typical application

The threshold of the sensor input is a function of the

outputvoltageat pin 2. The hysteresisis provided by

a Schmitt trigger comparator. As shown in figure 1,

this gives hysteresis with either threshold polarity

selected.

The AC driving of the level sensor allows the use of

which acts as a bandpass filter at the frequency

the sensor input stage. In this way the interference

problems typical of automotive applications are re-

duced considerably. If, however, it is not necessary

to decouple and filter the sensor a simple resistive

network may be used, eliminating the capacitors.

SPURIOUS INDICATION PROTECTION.

To preventspurious alarm signals when the liquid is

agitated or in the presence of interference, the de-

vice includes two protection mechanism :

Firstly, the sensor interface which samples the posi-

tive halfcycleof the sensor signalactivatesits output

only if there are four consecutive alarm conditionin-

dications. Secondly, the alarm output stage is only

activated after an externally programmable delay.

During this delay if the alarm condition ceases the

alarm output will not be activated.

Ω,

which give a typical oscillator frequency of 1.6KHz,

delaysof about 10 s (programmingpin 7 low)or 20s.

INTERNAL MEMORY.

When the alarm output has been activated an inter-

nal latch holds it in the active state until the power

supply is removed. This feature ensures that the

alarm will not be interrupted if the sensor connection

breaks.

OUTPUT STAGE.

Throughpin 8 it is possibleto programthe duty cycle

of the alarm signal waveform (see figure 2). When

pin 8is high the outputsignal has a dutycycle of 50%

; if pin 8 is low the duty cycle is 1 : 64. The period of

the output signal is always 320ms using the compo-

nent values indicated in block diagram.

The outputstagecan deliver up to 300mAand is pro-

tected internally against overvoltages (by a zener).

A thermal shutdown circuit provides additional pro-

tection.

CIRCUIT OPERATION

L4620

5/9