Photoelectric smoke detector with count based A/D and D/A converter

A smoke detector including a digitally stored alarm threshold and comprising:

• a photo-emitter and a photo-sensor, said photo-emitter producing a light beam and said photo-sensor providing an analog signal voltage proportional to light reflected out of said beam by smoke particles;
• means for amplifying said analog signal voltage and for holding a sample of said amplified analog signal voltage;
• an analog-to-digital converter converting said analog sample to a digital representation of said sample;
• and,
• a control comparing said digital representation to said digitally stored alarm threshold;
• said analog-to-digital converter comprising an integrating amplifier operated by said control a) to charge a capacitor for a predetermined period at a first linear rate directly proportional to said analog sample and b) then to discharge said capacitor at a second predetermined linear rate to a predetermined threshold;
• said control counting digitally during said discharging of said capacitor to establish said digital representation of said sample;
• wherein said controller determines a calibrated digital-to-analog conversion factor by a) operating said integrating amplifier to charge said capacitor at a predetermined linear rate, from a first predetermined voltage to a second predetermined voltage, thereby establishing a charging time period, b) counting digitally during said charging time period to establish a digital count representing said charging time period and c) using said digital count and said first and second predetermined voltages to provide said conversion factor in volts per count;
• and,
• wherein said controller operates said integrating amplifier to charge said capacitor from said first predetermined voltage at said predetermined rate for a time period based on said calibrated conversion factor to produce an analog voltage representing detector sensitivity.