ELM337 Ver la hoja de datos (PDF) - Elm Electronics

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ELM337

Light switch.

Elm Electronics 

ELM337

Example Applications

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.

To the

controlled

circuit

8765

337

+5V 1 2 3 4

0.01µF

+5V

22KΩ

start

Figure 2. Time Delay Dropout Switch

+5V

0.01µF

1

8

2

7

3

6

4

5

RT

22KΩ

To the

lighting

circuit

Rprot

(see text)

Rλ

Figure 1. Yard Lighting Controller

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…

ELM337DSA

Elm Electronics – Circuits for the Hobbyist

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

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