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Need circuit explanation

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without the internal circuit of the chip used it is hard to explain the use.... but as far as i know the comparator might be to trigger the buzzer circuitry based on a threshold and the ntc to sense temperature and trigger alarm based on it....

Hi Alex,

The diode is reverse-biased, and the reverse bias voltage (diode drop) is a function of the illumination that the diode is receiving. In other words the diode is in a photovoltaic topology. OpAmp amplifies the diode voltage to match the dynamic rage of the A/D converter. Here’s an app note about photodiode monitoring, if you want to read further **broken link removed**

By the way, I’d like to invite your question to a specialized group dedicated to sensors **broken link removed** . The group is fairly active and there is a good number of sensor experts.

- Nick

I would say that the the circuit is a fire detector . used to detect smoke through the IRopamp circuit and the heat from the thermister.
The IC must be used to produce an output when output from the 2 sencers trigger the chip.

barrybear said:
heat from [from fire] the thermister
Another possible use (if not the main purpose) for NTC is the temperature compensation of the smoke detection cell. LEDs and photodiodes are temperature dependent. Light output of the LEDs falls with temperature. Forward bias voltage of the photodiode is also dependent (linearly) on the temperature.

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