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Simple gas smoke alarm using 555 7805

Published on Apr 11 2011 // Gas sensor circuits

Simple gas smoke alarm using 555 7805.

Shown for the simple gas smoke alarm circuit. The alarm by the buck rectifier and voltage regulator, gas sensing element and trigger the alarm sound circuit. Rectifier and regulator circuit consists of step-down transformer B, the bridge’s circuit, integrated power supply 7805 and so on. Triggered, the alarm sound by the controlled multivibrator circuit (555, R2, W2, C4) and speaker Y and so on. Semiconductor gas sensors using QM-25 type or MQ211 type, it applies to gas, natural gas, gasoline, alcohols, ethers and a variety of smoke alarms, which requires heating voltage stability, we adopted the 7805 pairs of filament voltage regulator, and 3 min warm-boot requirements.
When the QM gas sensors exposed to flammable gases or smoke, the A ~ B resistance between the lower pole, so that the resistance increased with the partial pressure of W1, corresponding to 555 foot potential increase in the potential when feet up to lV above, 555 from the vibration, the oscillation frequency f=1.44 / (R2 2Rw2) C4. Parameters shown in the figure corresponds to a frequency of approximately 0.6 ~ 8kHz. Adjustment potentiometer W2, so that it can be a frequency of 1.5kHz. the corresponding output of 555 feet to promote the speaker output signal Y alarm.
Normal circumstances, the gas sensor resistance between AB extremely large, the resistance and W1 of the partial pressure decreases, corresponding to 555 pin is low, 555 to stop vibration, speaker Y does not work.