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Feedback on 555 timer Schmidt Trigger design

colporteur

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Can I get members insight into using a 555 timer as a schmidt-trigger design that I pulled from the Internet?

I found this schmidt-trigger design using a 555 timer. I would like to use it to condition the output of a photo-resistor. The 555 timer output will feed the input to a Arduino Mega. I'm in the process of bench testing the setup to finalize component values. While testing I figured I would throw it out to some more enlightened minds than mine for feedback.

I'm using the 555 only because I have a stock of them. I don't have any readily available smidth-trigger IC's so I figured I would go with what I have and build from scratch. Building from scratch especially using Internet designs comes with risk. I was hoping to mitigate the risk by getting community feedback.

If inquiring minds suggest you don't want to do this, that is a good thing.

  • 0

    It will be interesting to see this circuit in operation.

    I wonder what the response curve will be.

  • 0

    Hi Sean,

    There are known working circuits here: https://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html

    (press Ctrl-F and search for "light", since that's a very long page.

    There are light detectors using an LDR, and some with phototransistor.

    Otherwise I think you'd have to either try that for real, or simulate it, since that circuit could have come from anywhere, and it's not one of the two normal topologies seen regularly with a 555. That doesn't mean it won't work, it just means it's perhaps a lot less common. Definitely worth simulating or trying for real especially if it's for the train use-case.

    What's the actual requirement incidentally? Is it for train detection, or some other purpose? 

    image

  • 0 in reply to shabaz

    Detector for KY-008 laser light. I'm using what consumables I have on hand. Photo resistors and 55 timers. This circuit might have some merit. I will checkout the resources. Thanks for the insight.

  • 0

    Thanks for the input. I found the original circuit on a 555 timer schmidt-trigger search. It looked simple enough. I questioned the cap myself but with my limited knowledge who was I to argue.

    It has been close to 20years since I played with 555's so it will be interesting. They are alot cheaper now than then:)

  • 0

    Awesome feedback folks. Sorry for not responding. The regional telco decided to take my money and not provide me internet. Just got it fixed today. I was beginning to twitch and go to the library for a fix.

  • 0 in reply to colporteur

    Hi Sean,

    I see! Is it a break-beam kind of thing with no modulation? If so, then there is an easier way that may work better than the 555 circuit:

    Since the laser is powerful, the LDR resistance will change significantly. You could experiment with connecting just the LDR and a resistor (i.e. potential divider circuit) directly to the analog input of the Arduino. It could even have an automatic calibration on start-up, or if a button is pressed, whereby it reads the analog level when a train is present or when a train is not present, and the code then uses its own threshold. 

    But probably there may be no need for calibration, since the difference with the laser blocked and unblocked may be significant (if it is well aligned).

  • 0 in reply to shabaz

    But really this should be a stopgap solution, since it is worth experimenting with the modulation method mentioned in the earlier discussion, it is cheaper and less need for precise alignment, unlike with an unmodulated laser. With modulation you get great sensitivity in your detection.

  • 0 in reply to shabaz

    I agree there is a more simpler solution. I just don't have it! My ideal was to buy rather than build. That failed miserably with the IR sensors (well I learned a lot). I haven't found a reliable IR solution so I am on to plan B. 

    The IR solution was a disappointment. The noise reduced their performance. I'm rolling around the idea of further bench testing the IR sensor. Discuss their operation. Explore some of the problems and look at some solutions. I discovered (wasn't aware of this) that not all the IR sensors have the same circuit. I found four different schematics all for the same IR sensor image. They all may look the same in the pic but the circuits are different. My schedule doesn't afford me the time to park what I am doing and take the tangent learning opportunity. 

    I've been experimenting with the KY-008 laser and the photo resistors. I have had good success detecting the beam on the bench setup. I haven't incorporated the schmidt-trigger design yet. I got distracted by another project to help an Arduino code developer. I'm hoping to get back at the sensor this week. My thinking is using the simple trigger is a quantitative transition feeding the Mega would improve performance. 

  • 0

    It might help us help you, if you tell us what you are trying to accomplish.

  • 0 in reply to colporteur

    I have a fair amount of experience with 555 timers, IR sensing, break beam circuits, etc. I'd like to help. But I need to know what you are trying to do. What is the ultimate goal?