Gas Detector Circuit ATtiny45

Gas sensors are employed in a wide range of applications in the fields of safety, health, instrumentation etc… Common examples are domestic/commercial alarms for explosive or toxic gases or in automotive application, as gas leakage detectors for LPG powered cars and exhausts detectors inside any fuel-powered truck/car. Such sensors, nowadays, are found also in applications involving air quality control systems and pollution monitoring.

A simple project to build a smart gas alarm fully controlled by the Atmel ATtiny45 chip

Automatic control systems, among the vast electronic areas, have always been one of my favorite topics of interest. When I started “playing” with electronics as a student, to introduce a microprocessor to a system was still an expensive and quite complicated thing to do. This was due both to the large number of components needed to build up the hardware (memories, I/O controller, etc…) and to the cost and difficulty to obtain a good compiler or a development environment. At that time microprocessor systems were seen mainly for educational purposes, while in the electronic consumer market, appliances equipped with a microprocessor were very few and expensive.

Nowadays things have completely changed and I feel always very impressed and attracted by the newest generation of microcontrollers, seeing how many types of peripherals and functions they can include inside the chip. Among the entire market microcontroller’s offer I’m particularly fond of the ATtiny series from Atmel AVR family because of their powerful 20MHz core, the set of peripherals they can contain, while wearing the 555 “dress” and keeping a comparable cost with respect to the most famous timer!

Gas sensors are employed in a wide range of applications in the fields of safety, health, instrumentation etc… Common examples are domestic/commercial alarms for explosive or toxic gases or in automotive application, as gas leakage detectors for LPG powered cars and exhausts detectors inside any fuel-powered truck/car. Such sensors, nowadays, are found also in applications involving air quality control systems and pollution monitoring.

Today’s sensors, while featuring an high sensitivity to a wide gases variety, are very compact in size and have significantly reduced their power consumption to better adapt to portable solutions.

Building a system with a gas sensor is not as easy as it could appear. Despite the sensor could be treated basically as a variable resistor (which value depends on gas concentration in air) the practical implementation in a project should be done considering some design rules, especially if the final circuit is a device to be used in a field where reliability is strongly required (e.g. safety). As an example the internal elements of a sensor (heater and gas sensitive resistors) have to be constantly kept under control to avoid failures leading to a wrong alarm indication; furthermore, if the application needs to achieve a good measurement accuracy, factors like environment temperature, sensor life etc… have to be taken into account.

All those features and controls require to introduce in the project a certain amount of external circuitry (including components like comparators, temperature sensor, spare logic etc…) to carry out functions such us temperature compensation, sensor heating control, sensor warm-up time, sensor life time check and so on…

This project aims to show how the AVR ATtiny45, by means of its peripherals such us ADC, timers and internal temperature sensor, fits perfectly this application and it can be employed to replace a lot of external components while adding extra functionalities at a cost comparable to a simple integrated comparator.

Furthermore the use of an AVR microcontroller makes easy the possibility of the project to be modified and adapted to several sensor types and application areas.
The final circuit is also very compact in dimensions and cost effective.

In the prototype that I am going to present the hardware and microprocessor firmware have been optimized to implement a smart LPG gas alarm (LPG stands for Liquefied Petroleum Gas) for domestic or automotive use. Main features implemented here are:

• full sensor parameters control for maximum reliability;
• temperature compensation for best measurement accuracy;
• six steps of gas alarm levels with Led and acoustical indicators;
• store of “maintenance” parameters in a non-volatile memory (ATtiny45 internal Eeprom);
• logging of any alarm and failure events (all stored data can be transferred via RS232 serial interface).

Gas Detector Circuit ATtiny45 project files: