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NE555: NE555 Ripple Detector Simulation in LTspice

Part Number: NE555
Other Parts Discussed in Thread: TINA-TI, OPA192, TLC555

Tool/software:

Dear Team,

I am working on a ripple detector and timer circuit using two NE555 ICs. My goal is to activate a relay when the ripple lasts for more than 2 seconds. Below is a description of the circuit and the issue I'm facing.

In the circuit, I use one NE555 to detect ripples and another NE555 as the timer. A pulse voltage source simulates the ripple in LTspice. However, I am encountering an unexpected behavior: at the beginning of the simulation, the output of the second NE555 (Vout2) remains low for 120 ms. I believe this delay is caused by the filter I placed between the output of the first NE555 and the trigger of the second NE555 to reduce noise.

I would like Vout2 to remain low only when the ripple lasts longer than 2 seconds. Could anyone review my simulation and suggest how I can achieve this behavior?

Thank you in advance for your assistance.

Best regards,
Farzaneh

  • Hi Farzaneh,

    We'd be happy to help with your ripple detection circuit. Given that we don't officially support LTspice, I'll start by recreating your circuit in TINA-TI and see if I can replicate this behavior.

    I'll need some additional time to work on this, but I should get back to you with an update before EOD tomorrow.

    Kind Regards,

    Alex Curtis

  • Morning Alex,

    that would be great as I'm almost ran out of time!

    Regards

    Farzaneh

  • Hello Farzaneh,

    I created the circuit in TINA and was able to replicate the delay you are seeing, but it's much shorter (50ms instead of 120ms). This variance may be due to the different inverters we're using (TINA does not have an inverter built in, so I made one from scratch using the OPA192). Inverters and other logic devices in the signal chain can introduce delays. 

    The model of the NE555 is also a much less accurate representation of the NE555 compared to the TLC555 model, which is our most accurate 555 timer model, which may also explain some of the simulation differences. The RC filter between the two timers will also introduce some delay.

    In the transient response plot below, "VTrig1" is the trigger input of the first timer, and "OutB" is the output of the second timer.

    555RippleDetector.TSC

    So in summary, it's difficult to say what the exact cause of the 120ms delay is, and it is likely a combination of different factors.

    If you have any other questions, please let me know.

    Best Regards,

    Alex Curtis