0
\$\begingroup\$

I have an SPST momentary tactile switch and want the pulse, whatever length, to be extended to 500ms (unless of course the button is pressed for longer than 500ms but we can ignore that if we hold the pulse form the rising edge). I don't know if there is a single IC to hold a pulse like this or if I can do it with passive components like resistors and capacitors.

\$\endgroup\$
  • 3
    \$\begingroup\$ You want something called an edge-triggered monostable multivibrator. The 74HC126 is such a beast, although it is not highly regarded. To get reliable 500ms pulses, I'd recommend the smallest microcontroller you can find, and just a bit of code. An ATTiny or PIC10 in a SOT-23 package will be several times smaller than the timing capacitor you'd need to go with an analog chip, totally aside from the fact that it'll be smaller than the analog chip itself. \$\endgroup\$ – TimWescott Jan 4 at 20:37
  • 1
    \$\begingroup\$ What's your preference? You can do it with one mosfet, one BJT, a capacitor, a resistor, and a diode or two. Or you can do it with an MCU. How precise must the timing be? Do you want the duration to last half a second longer than the button is pressed? Or a half-second, regardless of how long? \$\endgroup\$ – jonk Jan 4 at 20:41
  • \$\begingroup\$ Half a second regardless of how long however if it is pressed longer, it doesn't matter if the pulse is longer. I don't mind how accurate, but needs to be at least 500ms. Ideally I don't want to use a microcontroller since its a tiny part of an entirely analog circuit and would have to be duplicated which may become expensive. I don't mind whether its a mosfet transistor cap resistor or diode(s) \$\endgroup\$ – 615283 Jan 4 at 21:08
  • \$\begingroup\$ @TimWescott I believe you were thinking of the '121 or '123. '126 is a quad buffer. \$\endgroup\$ – Sunnyskyguy EE75 Jan 4 at 23:03
  • \$\begingroup\$ @TonyEErocketscientist: Whups. Thanks -- I didn't double-check before I spoke. \$\endgroup\$ – TimWescott Jan 4 at 23:42
0
\$\begingroup\$

You can do this with CMOS inverter, a diode , cap and resistor to discharge the cap slowly.

It's called a monostable Multivibrator or 1 SHOT and there are many other ways to accomplish this.

But the typical IC's are state driven, while some are edge driven. They may be re-triggerable or non-retriggerable and the latter is what you wanted.

The LTC®6993-1 is a monostable multivibrator (also known as a “one-shot” pulse generator) with a programmable pulse width of 1µs to 33.6 seconds, with the non-retrig option and rising edge option.

Rev B

Easy 0.6s with 6 in one IC

schematic

simulate this circuit – Schematic created using CircuitLab

SIM LINK

\$\endgroup\$
  • \$\begingroup\$ Tony how could I set the time on the LTC®6993-1? On the data sheet it says just use a resistor between GDN and SET but how do I caculate the values for that? \$\endgroup\$ – 615283 Jan 5 at 10:25
  • \$\begingroup\$ Page 1 says tOUT = NDIV •RSET 50kΩ • 1µs, NDIV = 1, 8, 64,...,221 \$\endgroup\$ – Sunnyskyguy EE75 Jan 5 at 10:30
  • \$\begingroup\$ not gonna lie im not understanding a lot of that... \$\endgroup\$ – 615283 Jan 5 at 14:42
  • \$\begingroup\$ Why aren't you using the HC14's schmitt triggers? It does the same as a 555 here \$\endgroup\$ – Sunnyskyguy EE75 Jan 5 at 15:56
  • \$\begingroup\$ Im not using any yet because I need to know how to use them. \$\endgroup\$ – 615283 Jan 5 at 18:26
0
\$\begingroup\$

As much as it pains me to say this, a CMOS 555 will do exactly what you want. The standard circuit will give a constant output pulse width for any input pulse shorter than the timeout period. If the input still is low, the output will remain high until the input goes high; then the output will go low immediately. Retriggers caused by switch bounce during the timing period are ignored. The Micrel MIC1555 is in a 5-pin SOT-23 package. 1 chip, 1 R, 1 C (plus decoupling).

https://www.microchip.com/wwwproducts/en/MIC1555

\$\endgroup\$
  • \$\begingroup\$ Could you possibly provide a small schematic or more component and pin details on how the 555 could be used? \$\endgroup\$ – 615283 Jan 4 at 21:36
  • \$\begingroup\$ oh no, not the 555. Then you need to show him how to make it non-retriggerable. \$\endgroup\$ – Sunnyskyguy EE75 Jan 4 at 23:02
  • 1
    \$\begingroup\$ @TonyEErocketscientist: the OP mentions in the comments to the question that he doesn't mind if it's longer if the switch is pressed longer -- so, presumably a retriggerable is OK. \$\endgroup\$ – TimWescott Jan 4 at 23:44
  • \$\begingroup\$ Figure 3 in section 8.4.1 in this datasheet. You really can't lay claim to being an analog circuit designer unless you've designed some circuits with the 555. \$\endgroup\$ – TimWescott Jan 4 at 23:47
  • \$\begingroup\$ You can't really be an Analog Designer® .... unless you know how to make 1-shots with a digital Schmitt Trigger equivalent to six (6) 555's for even less cost. ;) (lol) I've never used a 555 in any product design. \$\endgroup\$ – Sunnyskyguy EE75 Jan 5 at 0:09

Your Answer

By clicking "Post Your Answer", you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.