RE46C143
CMOS Photoelectric Smoke Detector ASIC with Interconnect R&E International
Product Specification A Subsidiary of Microchip Technology Inc.
© 2009 Microchip Technology Inc. DS22178B-page 8
CIRCUIT DESCRIPTION AND APPLICATION NOTES
Note: All timing references are nominal. See electrical characteristics for limits.
Standby Internal Timing – With the external components specified in the typical application figure for ROSC and
COSC the internal oscillator has a nominal period of 10mS. Normally the analog circuitry is powered down to
minimize standby current (typically 4uA at 9V). Once every 10 seconds the detection circuitry (normal gain) is
powered up for 10mS. Prior to completion of the 10mS period the IRED pulse is active for 100uS. At the
conclusion of this 10mS period the photo amplifier is compared to an internal reference to determine the chamber
status and latched. If a smoke condition is present the period to the next detection decreases and additional
checks are made. Three consecutive smoke detections will cause the device to go into alarm and the horn circuit
and interconnect will be active.
Once every 40 seconds the status of the battery voltage is checked. This status is checked and latched at the
conclusion of the LED pulse. In addition, once every 40 seconds the chamber is activated and using the high gain
mode (capacitor C1) a check of the chamber is made by amplifying background reflections. If either the low
battery or the photo chamber test fails the horn will chirp for 10mS every 40 seconds.
The oscillator period is determined by the values of R9, R12 and C5 (see typical application FIG 2). The oscillator
period T=T
R
+ T
F
where T
R
=.6931 * R12 * C5 and T
F
=.6931 * R9 * C5
Smoke Detection Circuitry
– A comparator compares the photo amp output to an internal reference voltage. If the
required number of consecutive smoke conditions is met the device will go into local alarm and the horn will be
active. In local alarm the C2 gain is internally increased by ~10% to provide alarm hysteresis.
Push to Test Operation
– If the TEST input pin is activated (Vih) then, after one internal clock cycle, the smoke
detection rate increases to once every 330mS. In this mode the high gain capacitor C1 is selected and
background reflections are used to simulate a smoke condition. After the required consecutive detections the
device will go into a local alarm condition. When the TEST input is deactivated (Vil) and after one clock cycle the
normal gain capacitor C1 is selected. The detection rate continues at once every 330mS until 3 consecutive no
smoke conditions are detected. At this point the device returns to standby timing.
LED Operation
– In standby the LED is pulsed on for 10mS every 40 seconds. In a local alarm condition or the
push to test alarm the LED pulse frequency is increased to once every 0.5 seconds. In the case of a remote alarm
the LED not active.
Interconnect Operation
– The bidirectional IO pin allows for interconnection of multiple detectors. In a local alarm
condition this pin is driven high immediately through a constant current source. Shorting this output to ground will
not cause excessive current. The IO is ignored as an input during a local alarm.
The IO pin also has an NMOS discharge device that is active for ~1 second after the conclusion of any type of
local alarm. This device helps to quickly discharge any capacitance associated with the interconnect line.
If a remote active high signal is detected the device goes into remote alarm and the horn will be active. Internal
protection circuitry allows for the signaling unit to have a higher supply voltage than the signaled unit without
excessive current draw.
The interconnect input has a 500mS nominal digital filter. This allows for interconnection to other types of alarms
(carbon monoxide for example) that may have a pulsed interconnect signal.
