MC145018

5

Motorola Sensor Device Data

DEVICE OPERATION

TIMING

The internal oscillator of the MC145018 operates with a

period of 1.65 seconds during no–smoke conditions. Each

1.65 seconds, internal power is applied to the entire IC and a

check is made for smoke, except during LED pulse, Low Bat-

tery Alarm Chirp, or Horn Modulation (in smoke). Every 24

clock cycles a check is made for low battery by comparing

are used in the oscillator, the oscillator capacitor should be of

a low leakage type.

DETECT CIRCUITRY

If smoke is detected, the oscillator period becomes

41.67 ms and the piezoelectric horn oscillator circuit is

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 volt-

age without loading the ionization chamber. The active guard

op amp is not power strobed and thus gives constant protec-

tion from surface leakage currents. Pin 15 (the Detect input)

has internal diode protection against static damage.

INTERCONNECT

connect 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 volt-

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

up, to eliminate false alarming of remote units when the bat-

tery 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

can be altered by external resistors connected from pins 3 or

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

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

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 dur-

ing 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.

Figure 5. Typical Application as Ionization Smoke Detector

TO

OTHER

UNITS

MC145018

116

2

3

4

5

6

7

8

15

14

13

12

11

10

9

330

Ω

8.2 M

Ω

+

9 V

0.1

µF

1.5 M

Ω*

0.001

µF

220 k

Ω*

0.1

µF

1 M

1 M

TEST

*NOTE: Component values may

change depending on type of

piezoelectric horn used.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com