Photoelectric Smoke Detector

with Interconnect, Timer, and Latching Alarm Indicator

A5303

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

HORN Pin

The HORN pin is a logic output provided to enable an optional,

external boost converter that can drive a piezoelectric (piezo)

horn. Using a boost converter to drive a piezo horn allows alarms

to generate high SPL levels from low supply voltages. HORN will

be driven high to enable the boost converter. The output of the

boost converter will be connected to the piezo horn such that the

horn will sound when the converter is enabled. If a boost con-

verter and horn are not used, this pin should be left open.

BLINK Pin

This logic input determines the LED operation while the device is

in standby. If BLINK is connected to VDD the device will blink

once every approximately 43 s in standby. If BLINK is connected

to VSS the device will not blink in standby. If a low-battery or

degraded-chamber condition exists while the device is in standby,

the LED will blink as described in the Alarm Indications section,

regardless of the state of the BLINK pin. The BLINK pin has no

effect when the device is in local, remote, or test alarm.

LED Pin

This open-drain NMOS output is used to directly drive a visible

LED. The LED indicates detector status as follows (with compo-

nent values as in the typical application, all times nominal):

Condition

Pulse Occurrence

Standby, BLINK = VDD

Every 43.0 s

Standby, BLINK = VSS

No LED pulses

Local Smoke

Every 0.5 s

Remote Alarm

No pulses

Test Mode

Every 0.5 s

Timer (Hush) Mode

Every 10.8 s

OSC CAP (Oscillator Capacitor) Pin

A capacitor between this pin and VDD, along with a parallel

resistor, forms part of a two-terminal oscillator and sets the inter-

nal clock low time. With component values shown, this nominal

time is 10.4 ms and essentially the oscillator period, which is also

the STROBE pulse width. The internal clock low time can be

calculated by:

(2)

TRES (Timing Resistor) Pin

A resistor between this pin and OSC CAP is part of the two-termi-

nal oscillator and sets the internal clock high time, which is also

the IRED pulse width. With component values shown, this time is

nominally 105 μs. The internal clock high time can be calculated

by:

(3)

VSS Pin

This pin is connected to the negative supply potential (usually

ground).

HUSH Pin

This input pin serves two purposes in standby mode. It serves to

enable/disable entering the internal 10-minute (nominal) “hush”

timer mode, and also as the reference for the smoke comparator

during timer mode. Timer mode allows the user to temporarily

hush alarms caused by nuisance smoke or steam (such as from

cooking).

When the voltage on this pin is greater than approximately

50 mV, entering timer mode is enabled, and a high-to-low transi-

tion on the TEST pin resets and starts timer mode. If use of timer

mode is not desired this pin must be connected to VSS, and timer

mode is disabled.

During timer mode the smoke comparator reference is established

externally by a resistive divider (R1 and R2) between VDD and

is internally reduced to about 55% that during the normal-gain

conditions allow the detector to operate with reduced sensitivity

during timer mode. If the level of smoke increases such that the

temporary alarm threshold is reached, a local alarm will sound. If

the HUSH pin is connected directly to STROBE without using a

resistor divider, then a local alarm will never occur during timer

mode, regardless of the smoke level. When not in timer mode,

the smoke comparator reference is set internally to approximately

The resistor dividers formed by the adjustable photoamp-divider

and the HUSH divider (R1 + R2, if timer mode is used) should

be chosen so that the load on STROBE does not exceed 500 μA.

Thus, the photoamp-divider (8 kΩ in the typical application) in

parallel with the HUSH divider (R1 + R2) shall be no

less than 4 kΩ.

TEST Pin

This pin has an internal pulldown device and is used to manually

invoke push-to-test mode and timer mode. Push-to-test mode is

on this pin (usually the closure of a normally open push-button

switch to VDD). After one oscillator cycle, the amplifier gain is