This is a mouse that was on­ce bro­ken but is now healed. (A me­chan­i­cal switch on the cir­cuit board was bro­ken; upon fix­ing that, the mouse works al­most like-new.) I was ac­tu­al­ly quite sur­prised at the ex­treme sim­plic­i­ty of the ac­tu­al mouse hard­ware - a cou­ple of re­sis­tors, ca­pac­i­tors, an LED for the sen­sor, and the sen­sor chip it­self. It seems to me that the costs for build­ing such a mouse must be rather cheap in­deed. In fact, I won­der now about the op­ti­cal track­balls out on the mar­ket to­day - mine (the Kens­ing­ton Ex­pert Mouse) is to all ap­pear­ances just an up­side-down op­ti­cal mouse with a ball and a cou­ple of ex­tra but­tons; would it be per­haps pos­si­ble to con­struct my own per­fect­ly-func­tion­ing track­ball with on­ly cheap op­ti­cal mouse parts?

Fas­ci­nat­ing as that may seem, there are in­deed more ex­cit­ing pro­jects to at­tend to first.