I want to use a 555 timer to generate a ramp to my soft starter. The problem is, I want it to be continuous after reaching it's value. Unlike this shown below,

enter image description here

Instead of discharging I want it to remain on. How can I adjust this basic 555 ramp generator circuit to do this?

  • \$\begingroup\$ Why is using a 555 a design requirement? \$\endgroup\$ – jippie Dec 16 '12 at 18:19
  • \$\begingroup\$ It's not necessarily a requirement, I figured it would be the easiest to work with. However I'm open to suggestions. \$\endgroup\$ – Maraldz Dec 16 '12 at 18:21
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    \$\begingroup\$ Did you think about a non inverting integrator? You will need a rail to rail single supply op amp, a couple of resistors and a capacitor. Is the ramp duration time-critical? What are the initial and final voltages? How much current do you need? \$\endgroup\$ – Vladimir Cravero Dec 16 '12 at 18:49
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    \$\begingroup\$ Tell us what you really want to accomplish, not how you think it should be done. A 555 timer for this seems both inappropriate and silly in the first place, but without knowing what you actually want, it is not possible to suggest a alternative. \$\endgroup\$ – Olin Lathrop Dec 16 '12 at 19:32
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    \$\begingroup\$ Then a non inverting integrator should do the trick. You can adjust the timing with a potentiometer too. \$\endgroup\$ – Vladimir Cravero Dec 16 '12 at 19:36

Disconnect pin 6 (threshold) from the output and pull it to ground with a resistor. You can still pull it to supply voltage to manually reset the output.

Note that the output will go to the full supply voltage instead of 2/3 of it as in the given circuit.

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  • \$\begingroup\$ It answers your question, but an OpAmp as integrator is probably better. Maybe even a simple RC suffices. \$\endgroup\$ – starblue Dec 17 '12 at 11:43
  • \$\begingroup\$ Would you be able to elaborate on that please? I am having trouble understanding. \$\endgroup\$ – Maraldz Dec 18 '12 at 1:09
  • \$\begingroup\$ RC = resistor capacitor. \$\endgroup\$ – starblue Dec 18 '12 at 7:13

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