ELM337P - ELM Technology

ELM337

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

Connection Diagram

PDIP and SOIC

(top view)

VDD VSS

Light Switch

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.

• 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

• Automatic lighting controls

• Security light beam monitoring

• Position sensing

• Light controlled counters

• Time delay circuits

LDR

Dark

LightM2

Description

Applications

Block Diagram

1 of 4

Features

ELM337DSA

LDR

Programmable

Timer Output

Logic

Light

Dark

Line

Frequency

Detector

ELM337

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

Pin Descriptions

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:

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

2 of 4

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.

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’.

LDR (pin 5)

This is the voltage sensing input pin. It uses Schmitt

trigger circuitry, and a peak detecting circuit for line

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).

VSS (pin 8)

Circuit common is connected to this pin. This is the

most negative point in the circuit.

ELM337DSA

Table 1. Modes of Operation

Mode Inputs Delay on Transition to

Dark

50 msec

1 msec

50 msec

10 sec

10 min

Type of

Output

Pulse

Continuous

Pulse

Continuous

Pulse

Continuous

Light

50 msec

1 msec

50 msec

10 sec

10 min

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

ELM337

Electrical Characteristics

Absolute Maximum Ratings

Storage Temperature.......................-65°C to +150°C

Ambient Temperature with

Power Applied....................................-40°C to +85°C

Voltage on VDD with respect to VSS............0 to +7.5V

Voltage on any other pin with

respect to VSS........................... -0.6V to (VDD + 0.6V)

Note:

Stresses beyond those listed here will likely damage

the device. These values are given as a design

guideline only. The ability to operate to these levels

is neither inferred nor recommended.

3 of 4

All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.

Characteristic Minimum Typical Maximum ConditionsUnits

Supply voltage, VDD 3.0 5.0 5.5 V

VDD rate of rise 0.05 V/ms

Average supply current, IDD 1.0 2.4 mA

Notes: 1. This integrated circuit is produced with a Microchip Technology Inc.’s PIC12C5XX as the core embedded

microcontroller. For further device specifications, and possibly clarification of those given, please refer to the

appropriate Microchip documentation.

2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved

using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as

may be obtained through direct connection to solar cells, or some charge pump circuits.

3. As with all of the delays, the Pulse timing is generated internally, and is affected by both temperature and

supply voltage. Variations are generally less than 5% of this value over the operating range.

Input low voltage VSS 0.15 VDD V

Input high voltage VDD V0.85 VDD

Output low voltage 0.6 V

Output high voltage VVDD - 0.7

Current (sink) = 8.7mA

Current (source) = 5.4mA

see note 2

ELM337DSA

Output pulse width 50 msec

LDR input threshold

voltages 2.8 V

see note 3

4.25 V1.3

to dark

to light 0.75

0.85 VDD

Mode pins

Example Applications

4 of 4

Figure 1 at right shows the ELM337 installed in a

typical lighting control circuit. The mode has been set to

111 (or HHH), resulting in delays of 10 minutes when

recognizing a light or dark condition. In this case, only

the ‘Dark’ output (pin 6) is used to enable (and disable) a

lighting circuit, but other variations on this theme might

use the Light output (pin 7), or possibly use mode 110 to

provide short (50 msec) output pulses to trigger other

circuits.

The threshold setting resistor (22kΩ in Figure 1) is

chosen depending on what resistance it is desired to

have the transitions to light or dark occur at. Using the

typical Schmitt voltage thresholds for a 5 volt supply, it

can be shown that the two resistances that switching will

occur at are 1.33RT (light to dark) and 0.37RT (dark to

light), where RT is the threshold setting resistance. For

the 22kΩ shown, these would be at about 30kΩ and

8kΩ, respectively. The values that you choose would

depend on the LDR used, and your application.

If the LDR (Rλ) is mounted more than a few feet

from the ELM337, the increased exposure could cause

problems due to induced voltages and currents. To

provide protection from these, a resistor (Rprot) should be

added in series with pin 5 as shown at right. Typically

this resistor would be equal in value to the threshold

setting resistor.

ELM337

ELM337DSA Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

Figure 1. Yard Lighting Controller

Figure 2. Time Delay Dropout Switch

The ELM337 is also capable of operating simply as

a delay circuit. By hard-wiring the mode pins for a

desired delay, and applying logic levels to the LDR pin,

symmetrical output delays of up to 10 minutes can be

obtained.

Asymmetrical delays can also be obtained as shown

in Figure 2. This circuit connects the ELM337 to provide

a 50msec delay on pickup and 10 second time delay on

dropout function.

Operation of the circuit can best be explained as

follows. When the ‘start’ pushbutton is pressed, the M2

and M1 pins will both be at a logic low level, while M0 will

be high. The LDR input is also at a low level, simulating

an LDR that is just ‘seeing’ light. Since the mode is 001

or LLH, then from Table 1 (on page 2) the Light output

will go to a high level after 50msec. This delay is

desirable as it ensures that the input is legitimate by

providing some degree of switch debouncing.

As soon as the pushbutton is released, the M2

mode pin returns to a high level, and what appears to be

a dark input will be on the LDR pin. From Table 1, with

mode 101, the output will turn off after 10 seconds.

Certainly several other variations are possible by

connecting the mode pins in different combinations. Why

not try your hand at some…

0.01µF

To the

controlled

circuit

start

337 +5V

+5V

22KΩ

+5V

0.01µF To the

lighting

circuit

Rλ

Rprot

(see text)