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The point of this circuit is to allow rising edges to send a negative pulse to a 555 timer. C1's purpose is to allow rising edges to trigger a positive pulse which in turn connects output to ground.

I ran a time domain simulation from 0 - 1.2 seconds with .001 interval. I sweep R1 from 1k to 100k in 33k increments. The bottom blue line is the signal from the square wave generator. The circuit passes rising edges as negative pulses as I would expect to "out" but at the startup a negative pulse is initially sent out as C1 charges. Is there a circuit that can prevent the initial firing to the timer?

schematic

simulate this circuit – Schematic created using CircuitLab enter image description here

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  • \$\begingroup\$ This is just a canned comment to let you know that what you're trying to build from discrete analog components (possibly incorporating Opamps and/or NE555) is a digital control problem and thus can easily and with lower parts count be solved with a microcontroller with really minimal firmware to write. \$\endgroup\$
    – Marcus Müller
    Commented Dec 8, 2019 at 18:03
  • \$\begingroup\$ The point of this timing circuit is to prevent users who are writing code on a microcontroller from doing dangerous things. \$\endgroup\$
    – flips
    Commented Dec 8, 2019 at 18:45
  • \$\begingroup\$ you might want to explain in detail the application of all this. To me, this sounds like you need to learn about built-in watchdog circuits that are meant for exactly this! \$\endgroup\$
    – Marcus Müller
    Commented Dec 8, 2019 at 18:53
  • \$\begingroup\$ So the application is literally to send a low pulse to a 555 timer. The rest of the circuit operates exactly as expected. The only thing I want to do here is to make sure that Vout here does not activate the transistor while the cap is charging. One option I have considered is removing this edge detection logic here for logic gates but most of the gates I have won't pull down long enough to cause the delay specifed by the TS555C chip I am using without using way too many of them. \$\endgroup\$
    – flips
    Commented Dec 8, 2019 at 19:02
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    \$\begingroup\$ I got that you just want to send a negative pulse to a 555 :) but I'd like to understand for what purpose that happens; if you've already got a microcontroller in your system, it really sounds like you could use the hardware built into that to do what you want. But: I don't know in which bigger context you're doing this, so I can't help you. \$\endgroup\$
    – Marcus Müller
    Commented Dec 8, 2019 at 19:50

2 Answers 2

Reset to default
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You are asking to disable negative edge pulsing while both input and Vcc power up without a comparator to disable input or output.

Therefore it is not possible with only 1 transistor.

Therefore you need a powerup disable timer or DC OK detector to disable inputs.

As Marcus correctly states this can be solved logically or with more analog parts or a register to indicate State for output enable.

What is missing is the time delay after power up and more analog parts.

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  • \$\begingroup\$ Agreed. The solution I ended up adopting was electronics.stackexchange.com/questions/305150/… . Where a transistor pulls reset low for an amount of time until the capacitors are at their "steady state". I don't know what the right word for this is. \$\endgroup\$
    – flips
    Commented Dec 9, 2019 at 14:21
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555 Timer Monostable Circuit Triggered When Circuit is Powerd

Solves This

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  • \$\begingroup\$ In my LTspice simulations adding C2 shows the opposite: the first negative pulse (caused by charging C1) is still there, the others negative pulses are (almost) completely gone. Stepping decades, I tried with rise and fall times of 10n to 10m. \$\endgroup\$
    – Huisman
    Commented Dec 8, 2019 at 20:40

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