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I've been working on designing a positive rising edge trigger monostable circuit using a 555 IC, aiming for a pulse duration of around 5 seconds. However, my simulation attempts have been unsuccessful so far. I'm wondering what might be going wrong and if there are any adjustments I should make to the circuit diagram.

enter image description here

**Edited: ** As advised by @Antonio51, I modified my circuit like so, but it seems to work for small values of output pulse durations. I need the output pulse duration to be in the range 5-7 seconds. How to go about? enter image description here

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  • \$\begingroup\$ Do you want edge-triggered or level triggered? Because the trigger input to the 555 is level triggered. Also, do you need it to be exclusively positive triggered? \$\endgroup\$
    – MOSFET
    Commented May 13 at 3:31
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    \$\begingroup\$ Does your simulation require a GND connection? I can see 600+ V on your 555's output. Where is that voltage referenced to? You may want to put a GND and tie it to battery's negative. \$\endgroup\$
    – Rohat Kılıç
    Commented May 13 at 11:37
  • \$\begingroup\$ GND connection solved the 600V issue on the simulator. Thanks \$\endgroup\$
    – ObliteratedJillo
    Commented May 13 at 15:06
  • \$\begingroup\$ You have changed your R1 value ... and supply voltage ... \$\endgroup\$
    – Antonio51
    Commented May 13 at 15:59
  • \$\begingroup\$ @Antonio51: Sorry for the confusion. The supply voltage and trigger pulse should be 5V. I have tried with and with R1. Ckt works if R2 is kept below 1K, otherwise C2 can't discharge quickly enough. \$\endgroup\$
    – ObliteratedJillo
    Commented May 13 at 16:12

2 Answers 2

Reset to default
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Seems working well. Made wit microcap v12.
You could just "verify" if the wires are all well "wired".

enter image description here

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  • \$\begingroup\$ +1 for the details. But I can't get it right with your circuit on my simulator. What is the purpose of the diode? I need to trigger with a step pulse of longer duration. \$\endgroup\$
    – ObliteratedJillo
    Commented May 13 at 15:07
  • \$\begingroup\$ The above ckt works if the output pulse duration is kept small, but I need to keep around 5-7 secs \$\endgroup\$
    – ObliteratedJillo
    Commented May 13 at 15:24
  • \$\begingroup\$ The diode suppresses the positive "part" of the "derivating" circuit made with R3-C3. You see it in the Vin waveform. This pulse amplitude is reduced to ~ 0.7 V above the supply voltage. Pulse output is already 5 s. Increment the "green" 10k if needed. \$\endgroup\$
    – Antonio51
    Commented May 13 at 15:54
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You are asking the 555 to do something it was built from the ground up not to do.

In order for a hysteretic oscillator to work, there must be something in the circuit that responds to each transition direction. In a CMOS CD4093 gate or an opamp circuit, there is one comparator with a reference voltage that changes based on the output state. In the 555, Hans separated this function into two comparators, each with a different fixed reference voltage, but the function is the same. Indeed, for the 555 astable circuit the two inputs are connected directly together for this reason.

Separating the standard arrangement into two inputs, each sensitive to only one signal direction, gives the 555 its unique flexibility. The Threshold input responds to a positive-going voltage, and the Trigger input responds to a negative-going voltage, and there is not much you can do about it when driving the Trigger input. The 555 internal schematic is not super complex, and you can track the effects of voltage directions through it. Notice that the threshold input stage is all NPN transistors, while the Trigger input is all PNP transistors. This alone implies which signal direction causes action, and is confirmed by the Simplified Schematic on page one of the TI datasheet.

TI NE555 Datasheet

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