For this just add a button the the end of the orange part and press it every single time you want to shoot. Technically you are finished here, and you can use it as a semi-automatic cannon without a clock. Both the repeaters on the green and red wool should not have the ticks set from the default 1 tick. For this, add a dispenser on top of the open block above the fence, as you can see in the picture and then connect up the redstone dust which then leads into a repeater that will power the dispenser. Then we come to the red part of the build. Put another redstone dust behind it on the corner and connect this to the next repeater. First connect the repeaters from the previous picture to the first repeater on the green field. For this place repeaters along the outside of the foundation, connected by a bit of redstone dust on the corner. Next you will add the redstone that leads to the launching TNT. And add a repeater so it faces the block behind the dispensers. As seen in the first picture create a line of redstone accross the dispensers (Crouch so you do not access their inventory when right clicking the dispensers).

This is the redstone aspect of the TNT cannon, not yet the clock. This will be where the dispenser is later placed at and it also holds some redstone. Lastly add blocks in the position where the red wool is on the picture. This is a 2x3 field which will hold aditional redstone later.

After you do this, add blocks in the spot where the green wool is on the third picture. Also this time dont place your repeater next to the launching tnt, which is going to be at the fenct. In this case this is not bad as the TNT is already primed when you put it in. Also you will move your whole row of these back by one from the original design so that the last TNT will touch the sourceblock of water. Each of the has to be between the orange blocks and one block above them as well. This acts as a color code so I can more easily show you which blocks I mean, in which case for you these blocks would be the ones you chose as your solid blocks. For the next step you will be adding some more solid blocks in the positions that you can see the orange wool in.

Minecraft computer aimed tnt launcher