In a previous comment, I had remarked on the difficulty I was having finding cheap loudspeakers. I did a little more searching today, and found a good source: Parts Express. I’ve bought stuff from them before, and they seem to be a reliable supplier. They currently have a couple of small speakers for $2 each:
Even better, we could get a pair of 3″ 4Ω speakers in cabinets for 99¢, which could be used as is or cannibalized for parts. All these deals are special offers, and so may not be available when we need to run the class, and probably can’t be purchased with University purchase orders. I certainly wouldn’t want to be the one having to deal with the University purchasing system to get them.
But if we wanted audio output for any of the labs, we could afford to use loudspeakers.
When I complained about my function generator having died, Mylène suggested a kit generator. I looked at the Elenco FG-500K Function Generator Kit she suggested, and read the PDF manual for the kit. The manual reminds me a lot of the HeathKit manuals that I grew up with, though with a bit less explanation of how things work. I might get one just for the nostalgia value of building a kit like that again.
The function generator itself seems almost as functional as the much larger bench generator that died on me. The only feature that seems to be lacking is a frequency-modulation FM input, which I don’t expect to use much anyway. It uses the Exar XR-2206 chip, which does have FM capability (with a current input), but the circuit doesn’t use that capability. It looks possible to add FM input, if needed, since the FSK input and the second timing resistor input are currently unused and hooking them up for FM would not be difficult.
The XR-2206 is supposed to run off 12V (10V to 26V, according to the data sheet), but is being run off a 9V battery. I wonder how much this under-voltage affects the behavior of the function generator. The function generator can be run off an external power supply, and the electrolytic capacitors are 16V capacitors so running off a 12V supply should work, if there are problems with a 9v supply. I would want to change the jack they use for power, though, from a earphone jack to a barrel that matches one of my wall warts (probably the same size as is used for the Arduino, a 5.5mm/2.1mm center-positive barrel). I wish that power barrel connectors were more standardized—every wall wart I have seems to use a different connector, and there is no relationship between the voltages and the connectors. (Actually, 5.5mm/2.1mm seems to be the most common—I have 3 wall warts with that connector, each with a different voltage.)
They also push the high-frequency spec a bit to get 1MHz (which is the typical spec for the upper limit, but the min spec is only 0.5MHz)—their smallest timing capacitor is 820pF, while the spec suggests a minimum of 1000pF. They don’t push the lower end, going down to 1Hz, while the spec claims the chip can go down to 0.01Hz. The Elenco design switches the timing capacitor (100µF, 10µF, 1µF, 0.1µF, 0.01µF, 820pF) and uses a potentiometer to vary the timing resistor from 620Ω to 10620Ω. Since the XR-2206 chip produces an output frequency at 1/(RC), the lowest frequency range should be 0.94Hz to 16.1Hz, and each switch up should provide a factor of 10 increase, except for the last one, which is 115kHz to 1.97MHz. I wonder why they didn’t use a 1000pF capacitor to stay in spec—I might substitute a part there, if I get the kit.