555 is a good idea, but opamps may be better. You have the option of building oscillators, amplfiers, active filters, and even analog computing circuits. If your 9th graders are proficient with algebra, they can even predict the output of most of these circuits. This can provide a window into the practical application of mathematics and might even intice the more motivated students to learn how and why the circuit models and math allow predictions to be made. A decent performance opamp (TL074, quad FET input) can be had for less than a dollar at Digikey. Resistors and capacitors are inexpensive if bought in reasonable bulk (1% resistors are $8.12 for 200, at Digikey, less at surplus places like All Electronics, though selection will be spotty). The big cost will be in the prototyping board for wiring all of this together, although they are very reusable (about $8.00 at Digikey, $4.00 at All Electronics). $5.00 per student is doable for the consumables, but figure about $15.00 to get started.
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555 is a good idea, but opamps may be better. You have the option of building oscillators, amplfiers, active filters, and even analog computing circuits. If your 9th graders are proficient with algebra, they can even predict the output of most of these circuits. This can provide a window into the practical application of mathematics and might even intice the more motivated students to learn how and why the circuit models and math allow predictions to be made. A decent performance opamp (TL074, quad FET input) can be had for less than a dollar at Digikey. Resistors and capacitors are inexpensive if bought in reasonable bulk (1% resistors are $8.12 for 200, at Digikey, less at surplus places like All Electronics, though selection will be spotty). The big cost will be in the prototyping board for wiring all of this together, although they are very reusable (about $8.00 at Digikey, $4.00 at All Electronics). $5.00 per student is doable for the consumables, but figure about $15.00 to get started.