These collars work by releasing a mist of citronella spray each time your dog barks. Citronella is perfectly safe but, because dogs dislike the smell, it acts as a deterrent for barking.

I've tried a bunch of bark collars and deterrents with my noisy pup. Static collars give a small zap when the dog barks. Citronella ones spray a burst of scent. There are also ultrasonic devices ...