ELM ELM337SM, ELM337P Datasheet

ELM337

1234876

5

Light Switch

Description

576

4M03M12

Features

The ELM337 provides a convenient means to
interface standard photocells or light dependent
resistors (LDRs) to digital logic circuits. It provides
an input comparator with hysteresis, line frequency
filtering, digital delays, and high current output
drivers all within the one 8 pin package.

Three mode setting pins configure the ELM337
for pulse or continuous output and for a delay on
change of from 1 msec to 10 minutes. The transition
threshold is set with a single external resistor,
simplifying designs and minimizing costs.

Applications need not be limited to those
employing photocells, as the Example Applications
section shows. The Schmitt trigger input provides a
convenient way to interface to virtually any input
signal, whether from digital logic or from a slowly
varying analog source. The remainder of the
ELM337 can simply be thought of as a time delay on
pickup and dropout type circuit - very useful for
sequencing processes.

Applications

• Automatic lighting controls

• Security light beam monitoring

• Position sensing

• Low power CMOS design - typically 1mA at 5V

• Operates from 3V to 5.5V

• No external timing elements required

• Low parts count

• Digitally selected delays of up to 10 minutes

• Pulsed or continuous outputs

• Internal line frequency filtering

• High current drive outputs - up to 25 mA

Connection Diagram

PDIP and SOIC

(top view)

VDD VSS

LightM2
M1
M0

Dark

LDR

• Light controlled counters

• Time delay circuits

Block Diagram

ELM337DSA

LDR

Line

Frequency

Detector

Programmable

Timer

M2

Output

Logic

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Light
Dark

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ELM337

Pin Descriptions

VDD (pin 1)

This pin is the positive supply pin, and should
always be the most positive point in the circuit.
Internal circuitry connected to this pin is used to
provide power on reset of the microprocessor, so
an external reset signal is not required. Refer to the
Electrical Characteristics section for further
information.

M2 (pin 2), M1 (pin 3) and M0 (pin 4)

The digital levels at these pins determine the mode
of operation, as shown in Table 1 below. Levels are
read whenever the LDR input signal changes, so
that modes can be changed ‘on the fly’.

frequency filtering so that slowly varying and
pulsating signals will not cause circuit instability. A
logic low level on this pin will be interpreted as the
presence of light, while a logic high results in a dark
input. See the Example Applications section for
typical connections.

Dark (pin 6) and Light (pin 7)

These are the two active high circuit outputs.
Depending on the mode selected, they will either
remain continually active, or provide a 50 msec
pulse output on change from light to dark (Dark
output) or dark to light (Light output).

LDR (pin 5)

This is the voltage sensing input pin. It uses Schmitt
trigger circuitry, and a peak detecting circuit for line

Mode Inputs Delay on Transition to

M2

L
L
L

L
H
H
H
H

M1

L

L
H
H

L

L
H
H

M0

L
H
L
H
L
H
L
H

VSS (pin 8)

Circuit common is connected to this pin. This is the
most negative point in the circuit.

Type of

Light

1 msec
50 msec
50 msec

50 msec
10 sec
10 sec
10 min
10 min

Dark

50 msec
1 msec
50 msec
50 msec
10 sec
10 sec
10 min
10 min

Output

Continuous
Continuous

Pulse

Continuous

Pulse

Continuous

Pulse

Continuous

Table 1. Modes of Operation

Ordering Information

These integrated circuits are available in either the 300 mil plastic DIP format, or in the 200 mil SOIC surface

mount type of package. To order, add the appropriate suffix to the part number:

All rights reserved. Copyright ©2000 Elm Electronics.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be

given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.

ELM337DSA

300 mil Plastic DIP............................... ELM337P 200 mil SOIC.....................................ELM337SM

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