ADC Machine Monitoring System w/ External Voltage Source
Based on the circuit provided, the machine output looks like this except for the resistor:







simulate this circuit &ndash; Schematic created using CircuitLab

You will see that this is an open circuit if the output transistor is off without the resistor present (the resistor is necessary as we will see).

The circuit functionality is quite simple; if the machine is running, there is no current in the output transistor (or in the opto) but the base of the output transistor has been pulled up by the capacitor - measuring this with some instruments might read the value at Din, but for an ADC we need to develop a voltage so I have added a resistor for that purpose.

Adding this resistor will tend to discharge the base capacitor giving known behaviour.

The operation now is:

If the machine is off, Dout is 0V (or very close to it) and if the machine is on (transistors are conducting) Dout will be approximately Din - 1.4V.

Using a 1k resistor will likely work (and you will not need the series resistor although a perhaps 10 ohm in series might be prudent).

The capacitor across the opto also reduces any noise across the device and acts as a glitch filter for the output transistor.

So if you add a pulldown resistor and make your series resistors 10 ohms, the operation is now very simple.

V(ADC) ~ 0V: machine is off
V(ADC) ~ Din - 1.4V: machine is on

Note that the ADC will not output 00 for 0V in (from the datasheet) so I would look for any value below 100 (decimal) for machine off as the capacitor may not completely discharge through the base emitter junction of the output transistor; the machine on voltage will vary with temperature and may be as much as 1.5V below Din.

For a 5V reference with Din = 5V (to make things simple) then when the machine is on you should read 0x2C0 or higher.