Gas-smoke-detector
In the presence of smoke or gas, the AC output voltage increases and becomes rectified, filtered, and Zener-diode coupled (D2 for thresholding) to sensitivity control R3. Under no gas condition, the output is approximately 0 V (high). When gas is present, the output becomes a negative value (low) sufficient to overcome the threshold of MeMOS gate 2 and D2. The circuit utilizes a TGS 308 sensor, a general-purpose gas detector that is not sensitive to smoke or carbon monoxide. If smoke is the primary element to be detected, the TGS 202 sensor should be used. The two sensors are fundamentally identical, with the main differences being the heater voltage and the required warm-up time delay. The TGS 308 requires a 1.2 V heater and a 2-minute delay, whereas the TGS 202 requires 1.5 V and 5 minutes, respectively. The system employs a MeMOS gated oscillator directly interfacing with a triac-controlled AC horn. Using the MC14572 HEX functional gate, four inverters, one two-input NAND gate, and one two-input NOR gate, the circuit provides complete gas/smoke detector logic functions, including time delay, gated astable multivibrator control, and buffer operations. The 24-V AC horn produces an 85/90 dB sound level output at a distance of 10 ft. Controlled by the astable multivibrator, the horn generates a pulsating alarm signal, which may be advantageous over a continuous one in certain noise environments.
The circuit operates by monitoring the output from the selected gas sensor, either the TGS 308 or the TGS 202, based on the specific application requirements. The gas sensor detects the presence of specific gases or smoke, and its output voltage varies accordingly. The rectification and filtering process ensures that the AC signal is converted into a stable DC voltage suitable for further processing. The Zener diode (D2) is employed to establish a precise threshold voltage, which is critical for the sensitivity control resistor (R3). This configuration allows for fine-tuning the sensor's response to gas concentrations, ensuring reliable detection.
When gas is detected, the output voltage drops to a negative level, which is interpreted by the MeMOS gate. The MeMOS gated oscillator plays a vital role in controlling the operation of the triac, which in turn activates the AC horn. The use of the MC14572 HEX functional gate enables a compact design with multiple logic functions integrated into a single package. The combination of inverters, NAND, and NOR gates allows for the implementation of complex logic operations, including time delay mechanisms that prevent false alarms by requiring the sensor to detect gas consistently over a predetermined duration.
The output from the system is designed to drive a 24-V AC horn capable of producing a significant sound level, ensuring that the alarm is audible even in noisy environments. The pulsating nature of the alarm signal generated by the astable multivibrator is particularly effective in drawing attention without becoming monotonous, which can lead to desensitization in continuous alarm scenarios. This design is suitable for various applications, including residential and commercial environments, where gas or smoke detection is critical for safety.In the presence of smoke or gas, the ac output voltage increases and becomes rectified, filtered and zener-diode coupled (D2 for thresholding) to sensitivity control R3. Under no gas condition, the output equals approximately 0 V (high). When gas is present, the output will be a negative value (low) sufficient to overcome the threshold of MeMOS gate 2 and D2.
The circuit shown uses a TGS 308 sensor, a generalpurpose gas detector that is not sensitive to smoke or carbon monoxide. If smoke is the primary element to be detected, use the TGS 202 sensor. The two sensors are basically identical; the main differences lie in the heater voltage and the required warm up time delay. The TGS requires a 1.2 V heater and a 2 minute delay, whereas the TGS 202 requires 1.5 V and 5 minutes, respectively.
The system uses a MeMOS gated oscillator directly interfacing with a triac-controlled ac hom. Using the MC14572 HEX functional gate, four inverters, one two-input NAND gate and one two-input NOR gate, the circuit provides the complete gas/smoke detector logic functions time delay, gated astable multivibrator control and buffers operation. The 24-Vac hom produces an 85/90-dB sound level output at a distance oflO ft. Controlled by the astable multivibrator, the hom generates a pulsating alarm-a signal that may be advantageous over a continuous one in some noise environments.
🔗 External reference
The circuit operates by monitoring the output from the selected gas sensor, either the TGS 308 or the TGS 202, based on the specific application requirements. The gas sensor detects the presence of specific gases or smoke, and its output voltage varies accordingly. The rectification and filtering process ensures that the AC signal is converted into a stable DC voltage suitable for further processing. The Zener diode (D2) is employed to establish a precise threshold voltage, which is critical for the sensitivity control resistor (R3). This configuration allows for fine-tuning the sensor's response to gas concentrations, ensuring reliable detection.
When gas is detected, the output voltage drops to a negative level, which is interpreted by the MeMOS gate. The MeMOS gated oscillator plays a vital role in controlling the operation of the triac, which in turn activates the AC horn. The use of the MC14572 HEX functional gate enables a compact design with multiple logic functions integrated into a single package. The combination of inverters, NAND, and NOR gates allows for the implementation of complex logic operations, including time delay mechanisms that prevent false alarms by requiring the sensor to detect gas consistently over a predetermined duration.
The output from the system is designed to drive a 24-V AC horn capable of producing a significant sound level, ensuring that the alarm is audible even in noisy environments. The pulsating nature of the alarm signal generated by the astable multivibrator is particularly effective in drawing attention without becoming monotonous, which can lead to desensitization in continuous alarm scenarios. This design is suitable for various applications, including residential and commercial environments, where gas or smoke detection is critical for safety.In the presence of smoke or gas, the ac output voltage increases and becomes rectified, filtered and zener-diode coupled (D2 for thresholding) to sensitivity control R3. Under no gas condition, the output equals approximately 0 V (high). When gas is present, the output will be a negative value (low) sufficient to overcome the threshold of MeMOS gate 2 and D2.
The circuit shown uses a TGS 308 sensor, a generalpurpose gas detector that is not sensitive to smoke or carbon monoxide. If smoke is the primary element to be detected, use the TGS 202 sensor. The two sensors are basically identical; the main differences lie in the heater voltage and the required warm up time delay. The TGS requires a 1.2 V heater and a 2 minute delay, whereas the TGS 202 requires 1.5 V and 5 minutes, respectively.
The system uses a MeMOS gated oscillator directly interfacing with a triac-controlled ac hom. Using the MC14572 HEX functional gate, four inverters, one two-input NAND gate and one two-input NOR gate, the circuit provides the complete gas/smoke detector logic functions time delay, gated astable multivibrator control and buffers operation. The 24-Vac hom produces an 85/90-dB sound level output at a distance oflO ft. Controlled by the astable multivibrator, the hom generates a pulsating alarm-a signal that may be advantageous over a continuous one in some noise environments.
🔗 External reference
Related Circuits
The sensor operates based on the selective absorption of hydrocarbons by an n-type metal-oxide surface. The device features a heater designed to eliminate hydrocarbons once smoke or gas is no longer detected in the immediate vicinity, allowing for reuse....