Level shifting 5V signal to 3.3V using voltage divider where source has 10k resistor to ground

Question

I made a circuit to read from a dust sensor that operates with a 5V rail. The microcontroller I am using is supplied with a 3.3V rail, and it shares the same ground as the dust sensor.

As the output of the dust sensor is an op-amp with low source impedance, I thought I could shift the level of the sensor output to interface with the microcontroller with simply a 2:1 voltage divider, but this does not work well. I did not realise when I designed the board that the sensor has a 10k resistor to ground on the output of its op-amp (shown as "0-5V signal"):

^{simulate this circuit – Schematic created using CircuitLab}

The dust sensor's signal should smoothly vary between 5V and 0V depending on the air quality. With the above setup, the signal either jumps between around 1.7V and 0.3V with almost no transition in between.

Is there a way to compensate my voltage divider values in the presence of this resistor, such that the 0-5V signal is still translated to within the 0-3.3V range?

The dust sensor is a Shinyei PPD42NS. The circuit diagram can be found here.

Answer 1

The output stage of that sensor is configured as a comparator, so it will only be in either high or low state. That is, no intermediate voltages.

According to the opamp spec, Voh = Vcc-2V. Since you are driving it with 5V Vcc, your voltages should already be in range of your 3.3V logic.

If it is not you could use a suitable divider, or you can always do it this way.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Figure 1. Dust sensor schematic. Source: Taking Space.

As explained in the text of the article:

Sections c and d of the op-amp are set up as comparators to provide output pulses for the P1 and P2 outputs at the connector. Voltage divider R10 and R11 determine a threshold of 1.09V for P1. Similarly, R12 and R13 determine a threshold of 2.5V for P2, P1 and/or P2 will pulse low so long as the input voltage exceeds their respective thresholds. Note that any time the signal is high enough to activate P2, P1 will be active also.

That means that outputs (2) and (3) will be either on or off and not analog as you had hoped.

It seems to me that you could tap off opamp 'b' at point (1) to get the required signal for your ADC. You should probably replicate R7, C4 onto your ADC input to provide the same level of filtering (4 to 8 Hz, if I remember correctly).

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Alternative approach:

Applying an additional resistor or a voltage to the external threshold input could adjust the P2 threshold either up or down.

This means that you could apply a sawtooth ramp voltage to 'thresh' and monitor P2 to see at what voltage it switches. It's a bit more work but avoids hacking the dust sensor board.

Answer 3

Is there a way to compensate my voltage divider values in the presence of this resistor, such that the 0-5V signal is still translated to within the 0-3.3V range?

Since the sensor output is digital, rather than an analog voltage converter, I'd simply use a 3.3 V logic buffer chip with 5-V tolerant inputs. For example, 74LVC1G125. After accounting for assembly costs, using a single logic chip will likely be cheaper than 2 or 3 passive parts needed for a voltage divider solution.