Method and Apparatus for Detecting Smoke in an ION Chamber

An apparatus for detecting smoke, comprising:

• an ionization chamber having a radiation source and comprising first and second chambers with a conductive screen therebetween, wherein the first chamber is open to smoke ingress and the second chamber is closed to smoke ingress;
• a voltage comparator having a first input coupled to the conductive screen and a second input coupled to a voltage reference;
• a capacitor coupled between the first and second inputs of the voltage comparator;
• a flip-flop having a D-input coupled to an output of the voltage comparator and a clock input coupled to a clock generator, wherein each time a clock signal is received from the clock generator a logic value at the D-input is transferred to a Q-output of the flip-flop;
• a feedback resistor coupled between the Q-output of the flip-flop and the first input of the voltage comparator for charging and discharging the capacitor;
• wherein when a voltage on the first input of the voltage comparator is greater than a voltage from the voltage reference the output of the voltage comparator is at a logic low and the capacitor is discharged, and when the voltage on the first input of the voltage comparator is less than the voltage from the voltage reference the output of the voltage comparator is at a logic high and the capacitor is charged;
• a first counter for counting a first number of clock pulses from the clock generator when the Q-output of the flip-flop is at a logic high during a certain time period;
• and
• a second counter for counting a second number of clock pulses from the clock generator during a certain time period;
• wherein when the first number changes a certain amount within the certain time period a presence of smoke is detected within the first chamber.