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In the Smoke Detector industrial there is much debate over using a photo sensor or an ion sensor to detect smoke. The debate is being decided country by country and though the reasons for selection may vary consensus is being reached.

Use of an infrared diode and photo sensor is predominant in most of Europe and Asia. Many nations have simply decided that the amount of radioactive material needed to operate the ion chamber is unacceptable and regulated its use. Even places like the U.K. where the use has not been outlawed the requirement for a nuclear license to store more than 30 detectors has made low cost shipping and receiving untenable.

In the U.S the amount of radioactive material required for a smoke detector is considered acceptable and so the debate continues on the merit of system performance. From the standpoint of cost and fast fires the ion chamber has reigned supreme, but photo sensors have been shown to more quickly detect smoldering fires that cause poisoning or suffocation. As a result both photo and ion sensors are used together to provide better coverage while ion chamber devices dominate single sensor smoke detector sales.

Regardless of the type of sensor used Microchip offers detection I.C. ideal for battery operation, horn driving and sensor interfacing as well as the microcontrollers for systems that require the flexibility of programming.
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Smoke Detector Devices and Horn Drivers

In the previous post we talked about the charging and discharging behavior or a Li-ion battery.   On our way to a full fledged fuel gauge we need to first measure battery voltage.   Figure 1 shows the battery voltage measurement circuit using and ADC, in this case a MCP3421 18-bit ADC (U1).

Figure 1 Battery Voltage Measurement

Since the MCP3421 device has an internal reference voltage, the measurable maximum input voltage range is limited to the internal voltage reference voltage of up to 2.048V.  To measure the input voltage higher than the internal reference, a voltage divider is used, which is formed by R1, R2, and R3.  The R3 is optional and is used to calibrate the R1 and R2 component tolerance.  By choosing the series resistance value of the voltage divider to be very high (> 1 MΩ), the current losses due to the voltage divider is negligible.

In the example circuit as shown in Figure 1, the ADC is configured as single ended by connecting the positive input pin (VIN+) to the battery voltage, while the negative input pin (VIN-) to the VSS.   The ADC output is available to the MCU via the I2C bus line.
Figure 2 shows the discharge curve of a 3.7V Li-Polymer battery (3.7V, 170 mAH).  The curve shows that the battery voltage reduces linearly until it reaches about 80% of its full capacity.

Figure 2 Li-Polymer Battery Voltage Discharging Curve
Since the battery discharging characteristics are very linear until the point where the curve falls off sharply, measuring only the battery voltage is an alternative low-cost method to estimate the current status of the battery. In this case, the measured battery voltage can be compared with the fuel values in the lookup table in the MCU firmware.
The circuit shown can be used for measuring the battery voltage of any battery type. When the circuit is used, the voltage divider (R1, R2, R3) must be properly adjusted in order to keep the maximum input voltage (or the voltage at VIN+ pin when the battery is fully charged) to the ADC device is less than the ADC internal reference voltage (2.048V).
Although using the voltage alone is not sufficient to represent the battery fuel status, this method is widely used for simple and cost-sensitive applications because of its straightforward implementation.

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Battery Management

Delta-Sigma Analog-to-Digital Converters

The battery discharging behavior changes with various parameters, such as battery chemical type, load current, temperature, and aging. The figure below shows the general behavior of battery discharging curves of several battery chemical types.  The battery discharging curve of most batteries is almost flat until it reaches about 80% of its full range, and then falls off sharply after that. Intelligent battery chargers, or delta V chargers look for this drop off.
Since the battery’s internal chemical reaction is largely governed by voltage and temperature, the battery discharging behavior is greatly affected by the temperature.  The low temperature limit is determined by the freezing temperature of the electrolyte.  Most batteries do not work well below -40°C.  The battery performs better at higher temperatures because the chemical reaction processing is accelerated.  However, the rate of undesirable chemical reactions increases and results in a decrease of battery life.  At extremely high temperatures, the active chemicals become unstable and can destroy the battery and your laptop.

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Battery Management