Sensors

Freescale Semiconductor

5

MC14568

DEVICE OPERATION

In a NO SMOKE condition (i.e., standby mode with detect

pin 15 higher than internal reference pin 13), the oscillator

operates with a period of 1.67 s and the LED will flash once

every 24-clock periods (or every 40 s).

The smoke detect pin (pin 15) is connected to the

ionization chamber and leads to the inverting input of a

CMOS detect comparator. The voltage on this pin is

compared to an internal reference voltage on the non-

inverting input of the detect comparator set up by a resistor

divider network and a FET switch (this reference voltage can

be increased or decreased by an external resistor from pin 13

to VDD or VSS).

Periodically (every 1.67 s and for a 10 ms window) power

is supplied to the entire IC and a check is made for a SMOKE

condition. During this window the FET switch is turned on and

the detect pin voltage (pin 15) is compared to the internal

reference voltage (pin 13). If the detect pin is lower than the

reference this indicates a SMOKE condition.

A SMOKE condition speeds up the internal oscillator,

which now oscillates with an internal period of 40 ms (the

rising edge of the saw tooth periodic signal on pin 12 remains

the same while the falling edge falls faster in a SMOKE

condition).

Furthermore, a SMOKE condition causes the resistor

divider network that sets the smoke detect reference voltage

to change in such a way as to make the reference voltage

increase by approximately 130 mV (with no external

connections to pin 13 or to pin 3).

This provides hysteresis and reduces false triggering. This

hysteresis is turned off when the IC exits a SMOKE condition

and returns to standby mode.

Once in a SMOKE condition, the LED will flash every 24-

clock cycles (once every second). The horn (piezo electric

element) driver is activated during a smoke condition with a

typical ON/OFF time of 160 ms ON, 80 ms OFF.

During the off time, the IC re-checks to see if a SMOKE

condition still exists. If no smoke is detected the horn driver is

suppressed. During a SMOKE condition the low voltage

battery check is inhibited.

NO SMOKE AND LOW BATTERY CHECK

During a NO SMOKE condition (i.e., standby mode) the

LED is pulsed to load the battery every 24-clock cycles (every

40 seconds) and a fraction of the battery voltage (set

internally by a resistor divider network and pulsed “ON” by a

internal FET) is compared to an internal reference voltage set

by a reversed biased Zener diode by an internal CMOS

comparator. The fraction of the supply voltage (pin 3) to be

compared to the internal reference voltage for low battery

check can be raised or lowered by connecting an external

resistor between pin 3 and VDD or pin 3 and VSS. If low

battery is detected a warning “chirp” will be sounded by the

horn driver once every 24-clock cycles (a low battery

condition does NOT cause the oscillator to speed up as does

a SMOKE condition). Low battery check will still be performed

in reduced sensitivity mode and a low battery chirp will be

TIMER

An internal timer is provided that can be used in various

configurations to allow for a period of reduced smoke

detector sensitivity.