Normally the microphones that exist in these things are triggers for us to 'knock' right? Well, isn't there something that automatically does the triggering, so that the drums play automatically and i play guitar along?
With this little thing i have to be triggin' it all the time, right?
Simplest solution is a 555 timer, run at a low frequency.
Google for 555 metronome and you are sure to get a million circuits. The click will trigger your drum.
But, once you want to make something that does actual beats......boom cha cha etc ....... it's a lot easier & prob cheaper to gut the percussion out of a discarded home organ!