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MC145018

Low-Power CMOS Ionization Smoke Detector IC with Interconnect and Temporal Horn Driver

Freescale Semiconductor 

enabled. The horn output is modulated 500 ms on, 500 ms

off. During the off time, smoke is again checked and will

inhibit further horn output if no smoke is sensed. During local

smoke conditions the low battery alarm is inhibited, but the

LED pulses at a 1.0 Hz rate. In remote smoke, the LED is

inhibited as well.

An active guard is provided on both pins adjacent to the

detect input. The voltage at these pins will be within 100 mV

of the input signal. 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 op amp is not power strobed and thus gives constant

protection from surface leakage currents. Pin 15 (the Detect

input) has internal diode protection against static damage.

Interconnect

The I/O (Pin 2), in combination with VSS, is used to

interconnect up to 40 remote units for common signaling. A

Local Smoke condition activates a current limited output

driver, thereby signaling Remote Smoke to interconnected

units. A small current sink improves noise immunity during

non-smoke conditions. Remote units at lower voltages do not

draw excessive current from a sending unit at a higher

voltage. The I/O is disabled for three oscillator cycles after

power up, to eliminate false alarming of remote units when

the battery is changed.

Sensitivity/Low Battery Thresholds

Both the sensitivity threshold and the low battery voltage

levels are set internally by a common voltage divider (see

Figure 2) connected between VDD and VSS. These voltages

can be altered by external resistors connected from pins 3 or

13 to either VDD or VSS. There will be a slight interaction here

due to the common voltage divider network. The sensitivity

threshold can also be set by adjusting the smoke chamber

ionization source.

Test Mode

Since the internal op amps and comparators are power

strobed, adjustments for sensitivity or low battery level could

be difficult and/or time-consuming. By forcing Pin 12 to VSS,

the power strobing is bypassed and the output, Pin 1,

constantly shows smoke/no smoke. Pin 1 = VDD for smoke.

In this mode and during the 10 ms power strobe, chip current

rises to approximately 50 µA.

LED Pulse

The 9-volt battery level is checked every 40 seconds

during the LED pulse. The battery is loaded via a 10 mA

pulse for 11.6 ms. If the LED is not used, it should be

replaced with an equivalent resistor such that the battery

loading remains at 10 mA.

Hysteresis

When smoke is detected, the resistor/divider network that

sets sensitivity is altered to increase sensitivity. This yields

approximately 100 mV of hysteresis and reduces false

triggering.

To

Other

Units

0.1 µF

330 Ω

8.2 MΩ

+

9V

1

16

MC145018

2

15

3

14

4

13

5

12

6

11

7

10

8

9

1.5 MΩ*

0.001 µF

1M

1M

TEST

0.1 µF

220 kΩ*

*NOTE: Component values may change

depending on type of

piezoelectric horn used.

Figure 6. Typical Application as Ionization Smoke Detector

Sensors

Freescale Semiconductor

MC145018

5

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