5355

SMOKE DETECTOR

with INTERCONNECT

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

The A5355CA is a low-current CMOS circuit providing

all of the required features for an ionization-type smoke

detector.

Oscillator. An internal oscillator operates with a period of

1.67 seconds during no-smoke conditions. Every 1.67

seconds, internal power is applied to the entire circuit and

a check is made for smoke. Every 24 clock cycles (40

seconds), the LED is pulsed and a check is made for low

Since very-low currents are used in the device, the

oscillator capacitor at pin 12 should be a low-leakage type

(PTFE, polystyrene, or polypropylene).

Detector Circuitry. An active guard is provided on both

pins adjacent to the detector input (pin 15). The voltage

at pins 14 and 16 will be within 100 mV of the input. This

will keep surface leakage currents to a minimum and

provide a method of measuring the input voltage without

loading the ionization chamber. The active guard ampli-

fier is not power strobed and thus provides constant

protection from surface leakage currents. The detector

input has internal diode protection against static damage.

Alarm Circuitry. If smoke is detected, the oscillator

period changes to 40 ms and the horn is enabled. The

horn output is typically 160 ms ON, 80 ms OFF. During

the OFF time, smoke is again checked and will inhibit

further alarm output if smoke is not sensed. During

smoke conditions the low battery alarm is inhibited and

the LED is driven at a 1 Hz rate.

Sensitivity Adjust. The detector sensitivity must be

externally adjusted to the individual characteristics of the

ionization chamber by connecting resistors between pin

Low Battery. The low battery threshold is set internally

The threshold can be increased by connecting a resistor

battery voltage level is checked every 40 seconds during

the 10 mA, 10 ms LED pulse. If an LED is not used, it

should be replaced with an equivalent resistor (typically

500

Ω to 1000 Ω) such that the battery loading remains at

10 mA.

I/O. A connection is provided at pin 2 to allow multiple

smoke detectors to be commoned. If any single unit

detects smoke (I/O is driven high), all connected units will

sound their associated horns after a nominal 3 second

delay. The LED is suppressed when an alarm is signaled

from an interconnected unit.

Testing. On power up, all internal counters are reset.

Internal test circuitry allows for low battery check by

holding pins 8 and 12 low during power up, then reducing

V

All functional tests

can be accelerated by driving pin 12 with a 2 kHz square

wave. The 10 ms strobe period must be maintained for

proper operation of the comparator circuitry.

CIRCUIT DESCRIPTION