Charge pump to a specific potential somewhere else in the circuit
Firstly a multimeter is a simple Wein bridge circuit, however It all depends on what you are measuring, if unknown an opto isolator could be used in the feedback circuit of an op amp, the output being given by the amount of feedback.
You should look at charge pump ccts to gain more insight
https://www.maximintegrated.com/en/app-notes/index.mvp/id/725

If you redraw this cct, you will see you are not doing what you set out to do. You have 30V feeding the collector of a transistor through a resistor R2, which will always pull the collector higher than 12V. The transistor relies on its switch action pulling the collector low, through the R4C1 network.