Hey there, hard to get op-amps so had a go with some transistors with mixed results.
For a start, I tried measuring a DC voltage from a transformer but with a transistor in the way. I got good some results with a PNP transistor (BC640):
Connecting the Emitter to the voltage to be measured (ranging from 3.9V to 17.8V), the Collector to the voltmeter positive terminal and the Base to GND via a 12k resistor, I got a perfect match between measured voltage and voltage at the transformer terminals. A bit suspiciously perfect in fact, not much evidence of a voltage drop. Connecting the Base to the positive transformer terminal completely cut off the voltage.
Now, I tried substituting the transistor Base input with a digital pin from the Arduino and got some weirdness. At the lowest voltages the transformer provides, 3.9V, the pin succesfully switches the transistor on and off (on when it's LOW and vice versa). At the next higher voltage, 5.7V, the voltmetre correctly reads 5.7 when the pin is LOW but when the pin is HIGH, instead of turning it off, the voltmetre reads 5.92V :o
This behaviour is repeated for all higher voltages, with the measured voltage jumping roughly 0.2V when the pin goes HIGH. I have tried R1 values from 12K down to 1K with no apparent change.
I 'm obviously doing something very wrong but I can't figure out what. Help?
P.S. How do I upload an image? I made an award winning diagram but I can't for the life of me figure out how to upload it.