The line-in voltmeter is a partner to the Audio Relay Switch outlined below. The two modules are intended to allow a vast number of computing devices to interface with and control external hardware modules without the need for complex or hardware-dependent drivers. The 44Khz scope is designed to receive a signal between +5 and 0 volts. It feeds this voltage through a resistor into the base of an NPN transistor. The transistor is attached to a 555 timer that outputs into the collector of the transistor. The effect is that the oscillator output is modulated by the NPN transistor, and the resulting PWM signal is stepped down, but proportional in magnitude of the input voltage on the NPN. This DC-AC conversion is necessary because line inputs are equipped with a high-pass filter to attenuate DC current.
The device worked as intended, boasting 4-channels of input. I did not test at higher frequencies, as the current intended purpose did not call for it, but it was acceptable for measuring knobs, button pushes, and other voltage-dependent sensors.
This design has been dicontinued due to a few key flaws in its design. The common-ground BJT amplifiers become unreliable when targeting devices with high internal
resistance. The voltage-sensing resistor is set to 10k, and since the beta cannot be raised to accomodate a higher base resistor, the current will be too
skewed to accurately represent inputs.
Another key flaw was external to the device. Inputs to computers are not well standardized. The resistance of line inputs may vary between hardware, and many computers omit the line interface at all. It would have improved the design had I reserved two of the op-amps for stabilizing DC-offset, rather than adding an additional two channels to the design. This would have allowed the device to operate with microphone inputs in addition to line inputs, diversifying the available level of host hardware. Most computers do not possess more than one easily-accessible microphone or line input, so this tradeoff is generally favorable.