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I am designing a photodiode sensor that will sit on a computer monitor and will output a +5V TTL signal if the screen is displaying and image and a 0V signal if the screen is not. The prototype circuit that I built works almost as expected (schematic below).

Prototype Photodiode TTL Circuit

Original Photodiode Trigger Circuit

The one problem is that the signal when the screen is turned on transitions (drops to 0V and then back up to 5V) every ~4 milliseconds (I am only recording the TTL signal at 2kHz). I tracked down the cause of this and the culprit seems to be the PWM for the LCD monitor back light. If I turn the brightness of my screen all the way up the unwanted transitions go away and the signal is a pristine 5V for the duration of time that the image is on the screen. Unfortunately looking at a monitor that is set at 100% brightness is unpleasant to say the least. Since I only need accurate TTL onset I was thinking that modifying the circuit above to include an RC-debounce circuit should fix my problem. I am wondering if the revised schematic (below) would work as drawn. The reason for the different opamp is that I am laying out a board using SMT components and ordering from Mouser instead of using parts that I could find at Fry's (that have been on the shelf since 1970). I am also aware that I have no bypass caps on my IC's and that my pinouts are a little wonky (figured I'd use actual circuit symbols instead of the rectangles that my eagle library has for the chips that I'm using).

Revised Photodiode TTL Circuit enter image description here

The thought with this circuit is to replace the inverter with an OR gate and use an RC circuit to the other input of the OR gate. The idea being when the photodiode is on that the OR gate's output will be high and the inverter's output will be low turning on the MOSFET and allowing the RC circuit to charge. Then when the photdiode sees a monitor PWM event the inverters output goes high shutting off the MOSFET and allowing the RC to begin discharging. This event also causes the OR gate input connected to the opamp to go low but the other OR gate input is high because it is tied to the discharging RC circuit keeping the output of the or gate high during the the monitors PWM events. The output would only go low when the photodiode sees a sees the screen off for more then the RC time constant of my "debounce" circuit.

Am I missing anything? Will the circuit work as drawn?

EDIT: The leading edge of the logic needs to be accurate (not delayed) but the falling edge can be delayed without affecting the use of the circuit. The output of this circuit needs to go from low to high (when screen goes from black to white) in less than 0.5ms and needs to transition from high to low (white to black) in 10ms or less. This diagram hopefully shows what I'm trying to achieve Transition Diagram Actual is what I see, the transition in the middle of this line is what I am trying to get rid of. I think the line marked acceptable is how the revised schematic would work.

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  • \$\begingroup\$ A drop every 4 ms is almost like a ripple in a power supply, albeit at four times the normal frequency of regular AC. Can't you just put a large cap (e.g. 100 µF) from the input to ground to smooth it out? \$\endgroup\$ – tcrosley Jul 5 '15 at 18:43
  • \$\begingroup\$ The input of the opamp? Also the frequency of the drops changes with the brightness of the the monitor so I'm pretty sure that I'm picking up the PWM of the monitor back-light. \$\endgroup\$ – Lucas Evans Jul 5 '15 at 19:05
  • \$\begingroup\$ It's a convention to put +VDC at the top of a diagram, and ground at the bottom; it makes understanding the diagram easier. \$\endgroup\$ – Daniel Griscom Jul 5 '15 at 21:03
  • \$\begingroup\$ I understand about the PWM thing. I was just thinking you could smooth the input out using a fairly large capacitor. Whatever. \$\endgroup\$ – tcrosley Jul 5 '15 at 21:04
  • \$\begingroup\$ Are you sure the backlight, when on, pulses at least every 10ms (and hopefully more often)? \$\endgroup\$ – Daniel Griscom Jul 6 '15 at 10:57
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You need to transition on the screen going white within 0.5ms, transition on the screen going black within 10ms, but ignore periods of black lasting less than 4ms. This is more constraining than simple R/C circuits can provide: you need a timer.

The venerable 555 timer will do what you need. Set it up in monostable mode, with a duration of 6ms (make it adjustable), and drive the reset line from the output of your OPA141. Then, any glow from the screen will reset the timer, but if there's no light for 6ms then the output will go active. (You may have to play with the various polarities.)

An alternative would be using one of the simplest microcontrollers (e.g. an 8-pin PIC12) with an analog input. Less than $2, but incredibly versatile.

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  • \$\begingroup\$ Yes, but what if one day he decides he wants to lower/increase the brightness of the screen than that 4ms interval will change to. For this to work you need to know the maximum inteval, and then go on from there. \$\endgroup\$ – Golaž Jul 6 '15 at 11:52
  • \$\begingroup\$ Depends on whether his screen driver changes frequency as it changes brightness. If it does, then he may be SOL. \$\endgroup\$ – Daniel Griscom Jul 6 '15 at 12:20
  • \$\begingroup\$ I really appreciate the help your idea is excellent !Monostable Circuit. This is what you had in mind isn't it. With the values that I've chosen for the pot and cap should give me a range between 1 and 10ms shouldn't it? \$\endgroup\$ – Lucas Evans Jul 6 '15 at 17:23
  • \$\begingroup\$ I haven't gone through the details, but in general it looks right. \$\endgroup\$ – Daniel Griscom Jul 6 '15 at 17:35
  • \$\begingroup\$ I definitely plan on using the 555, but, just out of curiosity, as long as the inputs to the OR gate that I am using are schmitt triggered would the MOSFET and RC circuit I drew in my revised schematic work similarly to the triggered 555? \$\endgroup\$ – Lucas Evans Jul 6 '15 at 21:05

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