Pin 8 on the Arduino Uno drives the optical solid state relay and when high turns the relay on, adjusting the voltage of the regulator down to 1.4v. ![]() I use an LH1546 optical solid state relay to switch the adjustable voltage of the potentiometer on for the 1.4v heater low voltage. I use a 50k potentiometer to adjust the voltage down to 1.4v for the heater high voltage part of the cycle. The default voltage of the KA278RA05C with Vadj (pin 4) disconnected is 5v which serves for the heater high voltage part of the cycle. ![]() The MQ7 requires a heater voltage that cycles between 5v (60s) and 1.4v (90s), drawing approximately 150mA at 5v which exceeds the power capacity of the Uno, so I use the KA278RA05C adjustable voltage regulator to drive this. This simple project uses the Arduino Uno and the MQ7 Gas Sensor to sense the concentration of CO (Carbon Monoxide) in the air.
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