ELM337
Light Switch
Description
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.
Features
• 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)
Applications
• Automatic lighting controls
• Security light beam monitoring
• Position sensing
• Light controlled counters
• Time delay circuits
V
DD
M2
M1
M0
1
2
3
4
8
7
6
5
V
SS
Light
Dark
LDR
Block Diagram
Line
Frequency
Detector
Programmable
Timer
Output
Logic
7
6
Light
Dark
LDR
5
2
3
4
M2
M1
M0
ELM337DSA
Elm Electronics – Circuits for the Hobbyist
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