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Analysis of a 555 circuit design, will it work, maybe there is a better way

colporteur

I'm looking to build an End of Train (EOT) circuit. The model railroad folks I hang out with, just purchased a commercial unit. After gagging at the price, I figured why not build one.

My search found this resource https://techlib.com/area_50/Readers/Lennie/End%20of%20Train-1.pdf complete with PCB. Unfortunately all the links are dead so I had to generate a schematic from the PCB layout.

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Question: Will the circuit work? Are there any reasons for the design?

Instead of bread boarding the design I figured the folks that hang out here are great and offering solutions. Tapping into the experience of the members surely helps. Nothing like experience to find a solution fast.

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I have some concerns with the circuit. I couldn't found a 555 circuit that uses pin 7 as an output. I took a look through Cabe Force Essential 555 IC book (Cabe hangs out here:) and found nothing like this beast.

I'm not sure of the design. It looks like nothing I have found.

I'm thinking it would have been easier to just start from scratch. 

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  • 0 in reply to aswinvenu

    I believe the design I was hoping to use was riddled with errors.   pointed out the chip position error and your drawing suggests a number of other errors. Pins 6 & 7 are connected to together on the PCB and there is no capacitor on Pin 5. your drawing also increases the filter cap.

    I have seen designs were there are caps on pin 5 and some where there are none. What is best practice?

    In your frequency formula there is a constant (0.693). I've seen constants in formulas before. I can't recall if they are the same. Where does this constant value come from.

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  • 0 in reply to colporteur

    The constant is ln(2).

    The 2 comes from the relationship between the switching thresholds. When charging, the lower threshold is twice as far away from VCC as the upper threshold. When discharging, the upper threshold is twice as far away from 0V as the lower one. The natural log is involved because the charging curve is exponential in shape.

    If you want to delve down further, and work it out from first principles, you'll need to read up about RC time constants, and more generally about circuit analysis.

  • 0 in reply to colporteur

    Pin 5 is the Control Voltage (CV). Internally this voltage is generated using a voltage divider. Adding a capacitor will help to produce more a stable control voltage. Good practice is definitely to use the cap.
    The design I shared use the same Resistor R2 for both charging and discharging the Cap C3. If you search for astable multivibrator using NE555 you can find design using two resistor solutions as well.