I want to implement a pulse extender with a retriggerable multivibrator IC called CD74HC123EE4. Here is the datasheet.

The input will be a 32Hz pulse train with a 5 µsec duty cycle; and at the output the duty cycle will be stretched up to 1.5ms. It is okay to keep the duty cycle between %30 to %60.

Based on some examples I came up with the following circuit:

enter image description here

Even though it is extremely slow to simulate this, it seems fine in simulation.

There is also this document and it has a section called "Operating Rules". And somewhere else I read that the input better not exceed 4V(Thats why I use R4 and R2 at the input to lower 5V to around 4V input). I tried to follow "Operating Rules", but before I implement this I wanted to be whether there is some fundamental error or mistake. Is this circuit fine enough for the purpose?

  • 1
    \$\begingroup\$ pulse train with a 5 µsec duty cycle I understand what you mean but that's not how we normally use duty cycle. A 32 Hz signal (31.25 ms) of 5 us pulses has a 5us/31.25ms * 100% = 0.016% duty cycle. I would write: every 31.25 ms (that's 32 Hz) there's a 5 us pulse then it is 100% clear. \$\endgroup\$ Jun 20, 2018 at 10:34

1 Answer 1


The circuit looks fine to me.

You should mention where the I read that the input better not exceed 4V comes from. Indeed it is not advised to make any input voltage higher than the supply voltage. So if you supply the IC from 5 V you are allowed to take the inputs (all inputs!) up to 5 V. I do not understand why 4 V should be the limit for that.

The reason you don't want to exceed the supply voltage at the inputs is due to the ESD protection diodes as these start conducting when Vin > Vcc + 0.5 V.

By itself that is not an issue (it will not damage anything) as long as the current (into an input and through the ESD diode) is limited. You took plenty care of that with R4. So as far as I am concerned you can remove R2 and D1 as R4 by itself protects the input more than enough already. But R2 and D1 also do no harm so it is your choice to remove them or not.

  • \$\begingroup\$ R1 also makes D2 unnecessary. \$\endgroup\$ Jun 20, 2018 at 18:40
  • \$\begingroup\$ @WhatRoughBeast I didn't mention D2 as I expected it to help discharge C1 after power off. But there's already an ESD diode between Rx/Cx and Vcc and it's in parallel with D2 so indeed, D2 serves little purpose. \$\endgroup\$ Jun 20, 2018 at 18:46

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