ELM337
Light Switch
Description
Features
The ELM337 provides a convenient means to
• Low power CMOS design - typically 1mA at 5V
• Operates from 3V to 5.5V
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.
• No external timing elements required
• Low parts count
• Digitally selected delays of up to 10 minutes
• Pulsed or continuous outputs
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.
• Internal line frequency filtering
• High current drive outputs - up to 25 mA
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.
Connection Diagram
PDIP and SOIC
(top view)
1
2
3
4
8
7
6
5
VDD
M2
M1
M0
VSS
Applications
Light
Dark
LDR
• Automatic lighting controls
• Security light beam monitoring
• Position sensing
• Light controlled counters
• Time delay circuits
Block Diagram
Line
Frequency
Detector
7
6
Light
Dark
Output
Logic
Programmable
Timer
5
LDR
4
2
3
M2 M1 M0
ELM337DSA
Elm Electronics – Circuits for the Hobbyist
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