Pulse signal conditioning with a LM311

Question

I am trying to design a sensor to read the pulses on my electricity meter. The meter has 1 LED that gives 1000 pulses / KWh. Each pulse is 30ms in width. I am using a phototransistor and a LM311 to reshape the signal into clean 5V pulses.

Challenges:

1. there is a sheet of plexiglass in front of the meter, which forces me to place the sensor some 3cm away from the pulse LED.
2. The meter has another LED that is always on, consequently shifting the output of my sensor some 120mV up.

Here is the Kicad schematics. The phototransistor has been scavenged from another circuit so I do not know the part number.

and some scope traces

Basically I am trying to detect the edges of the input pulse, shift them to 2.5V then apply them to the comparator that was set up with a 400mV hysteresis between 2.7V - 2.3V.

In the datasheet it says that the recommended input voltage Vi is between VCC– + 0.5 and VCC+ – 1.5. Theoretically, S2 could go above 5V and below 0V.

How can I implement a simple circuit to limit the up and down excursions of S2 to be between 2V and 3V?

Are there any other obvious issues with this design?

Answer 1

If you want to attenuate the amplitude of signal arriving at the comparator input, you could simply increase the source's resistance, by adding a series resistor. The potential divider R1 and R2, providing 2.5V bias, has an equivalent impedance of 50kΩ, as is evident from the Thevenin equivalent:

^{simulate this circuit – Schematic created using CircuitLab}

Adding 50kΩ of resistance between the signal source and this 2.5V source will halve the the signal amplitude:

^{simulate this circuit}

Here the input signal, a 5V/0V square is blue, the unnattenuated, differentiated signal at X is in orange:

This is the potential at S2:

Simply choose R5 for an appropriate level of attenuation.

It might be better for you to adjust the bias potential at S2 to be right in the middle of the comparator's acceptable input voltage range. From the datasheet, the range seems to be between 0.5V above the lower supply, and 2V short of the positive supply:

$$ V_{S2(DC)} = \frac{(0V + 0.5V) + (5V - 2V)}{2} = +1.75V $$

Modify R1 and R2 (and of course Rx1 and Ry2 to match) to obtain about +1.75V bias. I've done that here, and also increased R5 for more attenuation:

^{simulate this circuit}

As you can see, the signal never exceeds the permissible range of input potentials, +0.5V to +3.0V, and is also centered within that range.

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Update

To limit excursions by clipping/clamping, rather than attenuating, one way would be to use zener diodes:

^{simulate this circuit}

You still require R5, to allow the diodes to conduct without large currents flowing, but by making it small compared to R1 and R2, attenuation is very low. D1 prevents S2 from falling further than 4.3V from the positive supply, and D2 prevents S2 from rising more than 3.0V above ground. In practice, with such low diode currents, clamping will begin slightly within those bounds.