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I am trying to design an op-amp circuit to generate a digital pulse when an LDR is quickly obscured from ambient light (like when you pass your hand over it). This is the circuit I built - a differentiator:

op-amp differentiator

And here is the output when I pass my hand over the LDR:

osc output

Is this the right approach? What should I do to get a cleaner digital pulse output? Should I use a comparator at the output stage?

NOTE: R below LDR is 10k.

Update:

Just wanted to mention that Microchip has a "Tips 'n Tricks Comparator PIC MCU - Microchip" document with a section on the "Data Slicer" circuit mentioned by @Andy.

ww1.microchip.com/downloads/en/DeviceDoc/41215c.pdf‎

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  • \$\begingroup\$ You want a response only when the light source is quickly obscured, but not if the light gradually fades? \$\endgroup\$
    – The Photon
    Commented Jul 10, 2013 at 16:18
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    \$\begingroup\$ A comparator will give you an output pulse with clean digital logic levels. If you also want to have a consistent output pulse width, add a one-shot on the output of the comparator. \$\endgroup\$
    – The Photon
    Commented Jul 10, 2013 at 16:24
  • \$\begingroup\$ Thanks. So I am basically on the right track on the use of a differentiator? \$\endgroup\$
    – M-V
    Commented Jul 10, 2013 at 16:26
  • \$\begingroup\$ Basically yes... \$\endgroup\$
    – DrFriedParts
    Commented Jul 10, 2013 at 17:00

2 Answers 2

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I think one of the main disadvantages of your circuit is that the Vin+ is connected to ground aka negative supply rail and this will be pulling your output into significant saturation because Vin- is several volts difference from it. I'm also concerned a bit that as the device isn't rail to rail you might get some oddball situations where the output inverts because the common-mode range on the inputs is exceeded.

I'm thinking along the lines of what is called as a data slicer. This circuit will tend to keep Vin+ and Vin- closer to each other. Here's a half idea of a circuit which I'll make a few points about underneath: -

enter image description here

Assuming the light input to the LDR is constant, the voltages on Vin+ and Vin- will, apart from noise be very close. In effect the output will be a noise amplifier but in the presense of a small data signal, the output will amplify and reproduce the data as CMOS level signals - this is its main use.

Because you are not using data and you also don't want a big noisy output, you can put a resistor across the capacitor so that the input to Vin+ is slightly less than Vin-. Maybe try 1Mohm. This will seperate the inputs sufficiently to stop noise breaking through and also give you a control value to optimize the response you need. I'd try a 1Mohm pot (in series with 100k) across the cap and do some testing.

The output will be a decent pulse because the op-amp is being used as a comparator. NB I know this device works quite well like this.

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  • \$\begingroup\$ Thanks. Still trying to understand how it works. So the inputs to the comparator are Signal (-) and Low Pass Filtered Signal (+)? A sudden change in data then causes a difference between these signals. Clever! \$\endgroup\$
    – M-V
    Commented Jul 10, 2013 at 17:19
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I would definitely make sure you have enough bypass capacitance on the power line of your IC. You may have it already but I don't see it drawn in there. Any cap in the low uF range will do (preferably two or more with different values but probably don't need to go haywire for this).

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