activate vs let down

activate

verb
  • To render more reactive; excite. 

  • To organize or create a military unit or station. 

  • To bring a player back after an injury. 

  • To put a device, mechanism (alarm etc.) or system into action or motion; to trigger, to actuate, to set off, to enable. 

  • To render a substance radioactive. 

  • To hasten a chemical reaction, especially by heating. 

  • To remove the limitations of demoware by providing a license; to unlock. 

  • To render a molecule reactive, active, or effective in performing its function. 

  • To encourage development or induce increased activity; to stimulate. 

  • To aerate in order to aid decomposition of organic matter. 

let down

verb
  • To soften in tempering. 

  • To disappoint; to betray or fail somebody 

  • To reduce one's level of effort. 

  • Used other than figuratively or idiomatically: see let, down. 

  • To allow to descend. 

  • To lengthen by undoing and resewing a hem. 

How often have the words activate and let down occurred in a corpus of books? (source: Google Ngram Viewer )