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I was working on a simple monostable IC project and I came acroos this problem.

When the switch is closed which triggers the pin 2 and sets pin 2 on low logic OUTPUT goes HIGH.

As per my settings of C and R it should stay HIGH for nearly 10s. I am using C=82uf and R=120K which gives approx 10s. But, instead OUTPUT is not turning off and it stays HIGH till I disconnect the powersupply.

Here is the schematic.

enter image description here

All grounds are common, and pin 8 is given VCC +5V. I can't figure out the reason. In most all of the circuits I have seen, pin 5 sets capacitor of 10nf. I don't have that a 10nF, so I thought 100pf will work. It may cause some timing malfunction, but I work around that by adjusting the R value. If 10nf is a must and won't work with other than 10nf then please let me know.

Edit: Schematic updated. The 13001s transistor has pin 1 base, pin 2 collector and pin 3 emitter in which pin 3 is connected to gnd, 2 is connected to relay (-) and pin 1 is connected to pin 3 of IC. The relay (+) is connected to pin 4.

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  • \$\begingroup\$ The cap on pin 5 is usually 10 nanofarads. \$\endgroup\$
    – ErikR
    May 24, 2021 at 18:59
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    \$\begingroup\$ When you connect pin2 to the gnd you should then directly disconnect it otherwise the internal RS will be always set and the output will stay high. Try to close and directly open the switch and wait. \$\endgroup\$ May 24, 2021 at 19:05
  • \$\begingroup\$ @PaulGhobril What do you mean by directly open and close? I don't get it.. the switch disconnects and connects pin 2. in between pin 2 and 1 there is only 1 switch. Nothing else is there. \$\endgroup\$ May 25, 2021 at 3:14
  • \$\begingroup\$ Yes when you close the switch it is just to set the output then you must open it to wait for the reset. \$\endgroup\$ May 25, 2021 at 3:40
  • \$\begingroup\$ @PaulGhobril I did the same. As soon as LED started to glow, I opened the switch and waited to be turned off. But, even after 30 mins it is not turning off. if circuit diagram is ok then do you think this can happen because of damaged capacitor? I desoldered the capacitor from old light bulb and capacitor is having value , 82uf 500v. Is this a problem? \$\endgroup\$ May 25, 2021 at 5:03

1 Answer 1

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When you connect pin2 to the GND you should then disconnect it otherwise the internal RS will be always set and the output will stay high. Try to close and directly open the switch and wait. In fact, closing the switch is just set the output then you must open it to wait for the reset.

Since you mentioned in comments that you de-soldered the capacitor from old light bulb and capacitor is having value , 82uf 500v, I guess it is an aluminium electrolytic type. This is a polarised capacitor and if you operate the aluminium electrolytic capacitor with wrong polarity it will act as a short circuit.

To test if the problem comes from the capacitor then after closing and opening the switch just disconnect the capacitor and the output shall become zero. If this happens then either the capacitor has wrong polarity or it is defective and you must change it.

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  • \$\begingroup\$ yes... Polarity is ok. connected cpacitor's - to gnd and + to pin 6. And also when I remove the capacitor after closing and opening the switch output becomes zero. That verifies the circuit diagram is ok. So, this must mean I should change capacitor. Right? \$\endgroup\$ May 25, 2021 at 14:42
  • \$\begingroup\$ Yes I think so. \$\endgroup\$ May 25, 2021 at 15:54
  • \$\begingroup\$ Ok. I tried enough. And I am ordering new capacitors. Do I need to change 100pf capacitor too? because As I understood pin 5 should be connected to gnd with small amount of capacitance. So, 100pf should work. will there be a problem with 100pf? \$\endgroup\$ May 25, 2021 at 17:31
  • \$\begingroup\$ Pin5 is a reference voltage resulting of an internal voltage divider. We usually add a capacitor to avoid the noise. So it is not critical for the functionnality but important for the performance. Datasheet usually recommand 10nF but this is not critical. \$\endgroup\$ May 25, 2021 at 17:50
  • \$\begingroup\$ OK. Thanks... I will post my answer as soon as I receive new capacitor. \$\endgroup\$ May 25, 2021 at 17:54

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