Photoelectric Smoke Detector

with Interconnect and Timer

A5366

7

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

the photoamplifier during the push-to-test mode and during 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

the photoamplifier during standby. A typical capacitor value for

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 10% 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 and can

IRED Pin

This output provides a pulsed base current for the external NPN

transistor, which drives the IR emitter. Its beta should be greater

than 100. To minimize noise impact, the IRED output is not

active when the horn and visible LED outputs are active.

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

horns.

As an input, the I/O is sampled every fourth clock cycle (nomi-

nally 42 ms). When the I/O pin is driven high by another device,

three consecutive samples with I/O high plus one additional

cycle (nominally 10.5 ms) are required to cause an alarm. If the

I/O falls below its threshold at any time during those (nomi-

nally) 95 ms, an internal latch is reset and there will not be an

alarm. Thus, depending on when during the (nominally) 42 ms

sample cycle I/O is initially forced high, the I/O must remain

high for a minimum of (nominally) 95 to 137 ms to cause an

alarm. This filtering provides significant immunity to I/O noise.

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 large (distrib-

uted) 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.

HORN1, HORN2, FEEDBACK Pins

These three pins are used with a self-resonating piezoelectric

transducer and horn-starting external passive components. The

output HORN1 is connected to the piezo metal support elec-

trode. The complementary output, HORN2, is connected to the

ceramic electrode. The FEEDBACK input is connected to the

feedback electrode. If the FEEDBACK pin is not used, it must

be connected to VSS.

LED Pin

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

LED. The load for the low-battery test is applied to this output.

If an LED is not used, it should be replaced with an equivalent

resistor (typically 500 to 1000 Ω) such that the battery load-

ing remains about 10 mA. The low-battery test does not occur

coincident with any other test or alarm signal. The LED also

indicates detector status as follows (with component values as in

the typical application, all times nominal):

Condition

Pulse Occurrence

Standby

Every 43 s

Local Smoke

Every 0.5 s

Remote Alarm

No pulses

Test Mode

Every 0.5 s

Timer Mode

Every 10 s