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MC14467-1 Scheda tecnica(PDF) 5 Page - Freescale Semiconductor, Inc |
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MC14467-1 Scheda tecnica(HTML) 5 Page - Freescale Semiconductor, Inc |
5 / 9 page Sensors Freescale Semiconductor 5 MC14467-1 DEVICE OPERATION Timing The internal oscillator of the MC14467-1 operates with a period of 1.67 seconds during no-smoke conditions. Each 1.67 seconds, internal power is applied to the entire IC and a check is made for smoke, except during LED pulse, Low Battery Alarm Chirp, or Horn Modulation (in smoke). Every 24 clock cycles a check is made for low battery by comparing VDD to an internal zener voltage. Since very small currents 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 40 ms and the piezoelectric horn oscillator circuit is enabled. The horn output is modulated 160 ms on, 80 ms off. During the off time, smoke is again checked and will inhibit further horn output if no smoke is sensed. During smoke conditions the low battery alarm is inhibited, but the LED pulses at a 1.0 Hz rate. 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. Sensitivity/Low Battery Thresholds Both the sensitivity threshold and the low battery voltage levels are set internally by a common voltage divider (please 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 outputs, Pins 1 and 4, constantly show smoke/no smoke and good battery/low battery, respectively. Pin 1 = VDD for smoke and Pin 4 = VDD for low battery. 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 10 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 6. Typical Application as Ionization Smoke Detector MC14467-1 1 16 2 3 4 5 6 7 8 15 14 13 12 11 10 9 Test + 9.0 V 8.2 M Ω 330 Ω 1.5 M Ω* 220 k Ω* 0.1 µF 1M 1M 0.1 µF 0.001* µF *NOTE: Component values may change depending on type of piezoelectric horn used. |
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