Photoelectric Smoke Detector

with Interconnect, Timer, and Latching Alarm Indicator

A5303

6

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Pin and Circuit Description

(In Typical Application)

C1 Pin

A capacitor connected to this pin determines the gain of the

chamber monitor test. A typical capacitor value for this high-

gain (supervisory) mode is 0.047 μF, but it should be selected

based on the photochamber background reflections reaching the

should not exceed 0.1 μF. Coupling of other signals to the C1,

C2, and DETECT inputs must be minimized.

C2 Pin

A capacitor connected to this pin determines the gain of the

this low-gain mode is 4700 pF, but it should be selected based

on a specific photochamber and the desired level of sensitiv-

increases by a nominal 45% after a local alarm is detected (three

consecutive detections). A resistor must be installed in series

with the C2 capacitor.

DETECT Pin

This is the input to the photoamplifier and is connected to the

cathode of the photodiode. The photodiode is operated at zero

bias and should have low dark leakage current and low capaci-

tance. A shunt resistor must be installed in parallel with the

photodiode.

STROBE Pin

The minus side of all internal and external photoamplifier cir-

cuitry is referenced to this pin.

VDD Pin

This pin is connected to the positive supply potential,

typically 3 V.

LVSET Pin

This pin allows the user to externally adjust the low-battery

alarm threshold. To increase the threshold, a resistor can be con-

nected between LVSET and VDD. To decrease the threshold, a

resistor can be connected between LVSET and VSS.

IRED Pin

This output provides a pulsed drive current for the external IR

emitter. To minimize noise impact, the IRED is not active when

the visible LED output is active.

ISET Pin

This pin allows the user to externally set the IRED current by

connecting a resistor between it and VSS. The IRED current

controls the amount of light generated by the IR LED in the

chamber. The IRED current, in mA, can be approximated using

the following equation:

(1)

The chosen resistor should set a maximum of 300 mA (typically

a minimum of 1Ω).

I/O Pin

A connection at this pin allows multiple smoke detectors to be

interconnected. If any single unit detects smoke, its I/O pin is

driven high, and all connected units will sound their associated

alarm indicators. As an input, this pin is sampled every 4 clock

cycles (nominally 43 ms) during standby, and two consecutive

samples and one additional clock with I/O high are required

before signaling an alarm. If the I/O line goes low at all during

the 96.8 ms, the remote alarm is not enabled, providing sig-

nificant immunity to I/O noise and other pulses on the I/O line

which are shorter than 9 clock cycles. The LED is suppressed

when an alarm is signaled from an interconnected unit, and

any local-alarm condition causes this pin to be ignored as an

input. An internal NMOS device acts as a charge dump to aid

in applications involving a large (distributed) capacitance, and

is activated at the end of a local or test alarm. This pin has an

on-chip pull-down device and must be left unconnected if not

used. In the application, there should be a series current-limiting

resistor to other smoke alarms.

SOUT0, SOUT1, SOUT2 Pins

These pins provide push-pull CMOS logic outputs to control an

external sound IC. The outputs indicate the state of the device as

follows:

Condition

SOUT2

SOUT1

SOUT0

Standby

L

L

L

Local Alarm

L

L

H

Remote Alarm

L

H

L

Push-Test Passed

L

H

H

Push-Test Failed

H

L

L

Low-Battery

H

L

H

Degraded Chamber

H

H

L