Sensors
Freescale Semiconductor 5
MC145010
Table 4. Pin Description
Pin Symbol Description
1 C1 A capacitor connected to this pin, shown in Figure 8, determines the gain of the on-chip photo amplifier during push button
test and chamber sensitivity test (high gain). The capacitor value is chosen such that the alarm is tripped from background
reflections in the chamber during push button test.
A
v
ª 1 + (C1/10) where C1 is in pF. CAUTION: The value of the closed-loop gain should not exceed 10,000.
2 C2 A capacitor connected to this pin as shown in Figure 8 determines the gain of the on-chip photo amplifier except during
push button or chamber sensitivity tests. A
v
≈ 1 + (C2/10) where C2 is in pF. This gain increases about 10% during the
IRED pulse, after two consecutive local smoke detections.
Resistor R14 must be installed in series with C2. R14 ≈ [1/(12√C2)] - 680 where R14 is in ohms and C2 is in farads.
3 DETECT This input to the high-gain pulse amplifier is tied to the cathode of an external photodiodes. The photodiodes should have
low capacitance and low dark leakage current. The diode must be shunted by a load resistor and is operated at zero bias.
The Detect input must be ac/dc decoupled from all other signals, V
DD
, and V
SS
. Lead length and/or foil traces to this pin
must be minimized, also. See Figure 9.
4 STROBE This output provides a strobed, regulated voltage referenced to V
DD
. The temperature coefficient of this voltage is ± 0.2%/
°C maximum from - 10° to 60°C. The supply-voltage coefficient (line regulation) is ± 0.2%/V maximum from 6 to 12 V.
Strobe is tied to external resistor string R8, R9, and R10.
5 V
DD
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-polarity protection must be provided externally.
6 IRED 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 maximum 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 visible LED and horn outputs are active. IRED is active near the end of Strobe pulses
for Smoke Tests, Chamber Sensitivity Test, and Push button Test.
7 I/O 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 625 ms during
standby (using the recommended component values). A local-smoke condition or the push button-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.
8 BRASS 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 sensitivity.
9 SILVER This half of the push-pull driver output is connected to the ceramic electrode of a piezoelectric transducer and to the horn-
starting capacitor.
10 FEEDBA
CK
This input is connected to both the feedback electrode of a self-resonating piezoelectric transducer and the horn-starting
resistor and capacitor through current-limiting resistor R4. If unused, this pin must be tied to V
SS
or V
DD
.
11 LED This active-low open-drain output directly drives an external 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, push button test, or any alarm signals.
The LED also provides a visual indication of the detector status as follows, assuming the component values shown in
Figure 8: Standby (includes low-supply and chamber sensitivity tests) - Pulses every 43 seconds (nominal) Local Smoke
- Pulses every 0.67 seconds (nominal) Remote Smoke - No pulses
Push button Test - Pulses every 0.67 seconds (nominal)
12 OSC This pin is used in conjunction with external resistor R2 (10 MΩ) to V
DD
and external capacitor C3 (1500 pF) to V
DD
to
form an oscillator with a nominal period of 10.5 ms.
13 R1 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.
14 VSS This pin is the negative supply potential and the return for the I/O pin. Pin 14 is usually tied to ground.
15 LOW-
SUPPLY
TRIP
This pin is connected to an external voltage which determines 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.
