Just to add another possibility, how about a DAC based voltage source. Perhaps more of an exercise for a DAC0800, but here it is anyway.
Objective:
+V to -V out, 9 V to 12 volt range easily adjustable
Cheap.
70 V to 100 mV per step
8-bit digital (256 steps)
100 mA or more output drive capability
+15 V, -15 V and 5 V for logic.
Precision is not so much a concern. Yes 1% resistors are typically suggested. Accuracy is not so much a requirement. I adjust the input until I get the desired output voltage. I have no need to accurately represent the input value. 5% resistors were used. Reference voltage also is not so much a requirement. An LM317 was used to get to 10 V (about).
DAC0800 and LM741 were used because it was what I had on hand. Most any other op amp would work and be an improvement.
Reference voltage could easily be adjusted.
Design one used a DIP switch for the digital input. A later design (not built yet) might use an Up / Down counter. Looking at what I had in stock I had 74xx191, 74xx193 or 74xx169 to choose from. I went with 74LS169 just because I had more of them. The counter option has one switch to set up or down. Two other switches set either a single count per pushbutton or run mode at about 10 counts per second.
The 10 V reference gives about 7.5 mV per step (+9.5 V to -9.5 V about). Acceptable for most diode, transistor and analog exercises.
(edited for a brain fart in math ...)
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