A square wave generator with tunable frequency and amplitude, to control a subwoofer.
The co-op I call home uses a large subwoofer as a doorbell, driven by a signal from a 30+ year-old function generator. Not to speak of the overall system, the latter alone is quoted at nearly 40W (it has active cooling). The power drawn by the subwoofer-amplifier is more or less unavoidable if we want an instant response, but I figured it's a bit of a waste to have the function generator running continuously, especially when we're paying energy for functionalities we don't need; the output was just a 44Hz sine wave, after all.
My goal was to replace this function generator with a more specialized, lower-power device. I figured a square wave would do just as well, what with the subwoofer effectively being a low-pass filter. To get this, I set up a 555 timer for 50% duty cycle with a potentiometer and a switch between low and high capacitance on the timing RC circuit. This would let me tune the frequency about an order of magnitude with the pot, and an additional order of magnitude with the switch.
Variable frequency square wave in hand, the next order of business was to restrict the output amplitudes. I decided the easiest way to do this was with another potentiometer and resistors, setting up a voltage divider. The whole deal was powered from a cheap LM7805, and from experience we didn't need peak-to-peak voltages over 2V, so the values were set appropriately. To then get a low impedance output from this, I used a simple op-amp buffer, and limited its current with another resistor.
Then, all that was left to do was plug it in! With Boston electrical prices as of 2021, this would save us around $25-30 per year, for an investment of less than $5 and a few hours. I guess it just isn't as cool as the function generator, though.