Toggle between 1.5V and 5V Supply for MQ7 CO Sensor
VCE saturated on the OnSemi PN2222 is less than 0.1V at 150mA or less 
(datasheet figure 11)

With nearly 150mA through the transistor you'll need 15mA or more into the base to meet the requiremnts of that graph your 100 ohm resistor should work fine for this.