ELM337

Elm Electronics – Circuits for the Hobbyist

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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

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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.

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).

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

M1

L

L

H

H

L

H

L

H

M2

L

L

L

L

H

H

H

H

Dark

50 msec

1 msec

50 msec

50 msec

10 sec

10 sec

10 min

10 min

M0

L

H

L

H

L

H

L

H

Type of

Output

Pulse

Continuous

Continuous

Continuous

Pulse

Continuous

Pulse

Continuous

Light

50 msec

1 msec

50 msec

50 msec

10 sec

10 sec

10 min

10 min