MC145012 Datasheet, PDF (9/12 Pages) Freescale Semiconductor, Inc – Photoelectric Smoke Detector IC with I/O and Temporal Pattern Horn Driver

V_DD (Pin 5)
This pin is connected to the positive supply potential and
may range from + 6 to + 12 V with respect to V_SS
CAUTION: In battery–powered applications, reverse–polar-
ity protection must be provided externally.
IRED (Pin 6)
This output provides pulsed base current for external NPN
transistor Q1 used as the infrared emitter driver. Q1 must
have β ≥ 100. At 10 mA, the temperature coefficient of the
output voltage is typically + 0.5%/°C from – 10° to 60°C. The
supply–voltage coefficient (line regulation) is ± 0.2%/V maxi-
mum from 6 to 12 V. The IRED pulse width (active–high) is
determined by external components R1 and C3. With a
100 kΩ/1500 pF combination, the nominal width is 105 µs.
To minimize noise impact, IRED is not active when the vis-
ible LED and horn outputs are active. IRED is active near the
end of strobe pulses for smoke tests, chamber sensitivity
test, and pushbutton test.
I/O (Pin 7)
This pin can be used to connect up to 40 units together in
a wired–OR configuration for common signaling. V_SS is used
as the return. An on–chip current sink minimizes noise pick
up during non–smoke conditions and eliminates the need for
an external pull–down resistor to complete the wired–OR.
Remote units at lower supply voltages do not draw excessive
current from a sending unit at a higher supply voltage.
I/O can also be used to activate escape lights, auxiliary
alarms, remote alarms, and/or auto–dialers.
As an input, this pin feeds a positive–edge–triggered flip–
flop whose output is sampled nominally every 1 second dur-
ing standby (using the recommended component values). A
local–smoke condition or the pushbutton–test mode forces
this current–limited output to source current. All input signals
are ignored when I/O is sourcing current.
I/O is disabled by the on–chip power–on reset to eliminate
nuisance signaling during battery changes or system power–
up.
If unused, I/O must be left unconnected.
BRASS (Pin 8)
This half of the push–pull driver output is connected to the
metal support electrode of a piezoelectric audio transducer
and to the horn–starting resistor. A continuous modulated
tone from the transducer is a smoke alarm indicating either
local or remote smoke. A short beep or chirp is a trouble
alarm indicating a low supply or degraded chamber sensitiv-
ity.
SILVER (Pin 9)
This half of the push–pull driver output is connected to the
ceramic electrode of a piezoelectric transducer and to the
horn–starting capacitor.
FEEDBACK (Pin 10)
This input is connected to both the feedback electrode of a
self–resonating piezoelectric transducer and the horn–start-
ing resistor and capacitor through current–limiting resistor
R4. If unused, this pin must be tied to V_SS or V_DD.
MC145012
LED (Pin 11)
This active–low open–drain output directly drives an exter-
nal visible LED at the pulse rates indicated below. The pulse
width is equal to the OSC period.
The load for the low–supply test is applied by this output.
This low–supply test is non–coincident with the smoke tests,
chamber sensitivity test, pushbutton test, or any alarm sig-
nals.
The LED also provides a visual indication of the detector
status as follows, assuming the component values shown in
Figure 5:
Standby (includes low–supply and chamber sensitivity
tests) — Pulses every 32.4 seconds (typical)
Local Smoke — Pulses every 0.51 seconds (typical)
Remote Smoke — No pulses
Pushbutton Test — Pulses every 0.51 seconds (typical)
OSC (Pin 12)
This pin is used in conjunction with external resistor R2
(7.5 MΩ) to V_DD and external capacitor C3 (1500 pF) to V_DD
to form an oscillator with a nominal period of 7.9 ms (typical).
R1 (Pin 13)
This pin is used in conjunction with resistor R1 (100 kΩ) to
Pin 12 and C3 (1500 pF, see Pin 12 description) to determine
the IRED pulse width. With this RC combination, the nominal
pulse width is 105 µs.
V_SS (Pin 14)
This pin is the negative supply potential and the return for
the I/O pin. Pin 14 is usually tied to ground.
LOW–SUPPLY TRIP (Pin 15)
This pin is connected to an external voltage which deter-
mines the low–supply alarm threshold. The trip voltage is
obtained through a resistor divider connected between the
V_DD and LED pins. The low–supply alarm threshold voltage
(in volts) ≈ (5R7/R6) + 5 where R6 and R7 are in the same
units.
TEST (Pin 16)
This input has an on–chip pull–down device and is used to
manually invoke a test mode.
The Pushbutton Test mode is initiated by a high level at Pin
16 (usually depression of a S.P.S.T. normally–open pushbut-
ton switch to V_DD). After one oscillator cycle, IRED pulses
approximately every 1.0 second, regardless of the presence
of smoke. Additionally, the amplifier gain is increased by
automatic selection of C1. Therefore, the background reflec-
tions in the smoke chamber may be interpreted as smoke,
generating a simulated–smoke condition. After the second
IRED pulse, a successful test activates the horn–driver and
I/O circuits. The active I/O allows remote signaling for system
testing. When the Pushbutton Test switch is released, the
Test input returns to V_SS due to the on–chip pull–down
device. After one oscillator cycle, the amplifier gain returns to
normal, thereby removing the simulated–smoke condition.
After two additional IRED pulses, less than three seconds,
the IC exits the alarm mode and returns to standby timing.
Motorola Sensor Device Data
9