I'm trying to interface 74HC123 monostable with 6N137 optocoupler. Signal from optocoupler will turn on MOSFET driver. Monostable outputs correct signal when optocoupler is not connected - 200 ns pulse. When I connect optocoupler with monostable the voltage on monostable pin 4 is pulled down. I thought that it was due to low output current of monostable, but this is ok according to datasheet. Maximal monostable output current is 25 mA, LED in optocoupler will take around 10 mA.

Monostable is activated by pushbutton. I need positive pulse on optocoupler output. IC's are setted according to highlighted fields in truth tables below. Monostable output pulse width is setted byt C3 and R2.

Osciloscope images of button click, and output monostable pulse with not connected optocoupler are below.

Monostable datasheet: http://www.ti.com/lit/ds/symlink/cd74hc123.pdf

Optocouplet datasheet: http://www.mouser.com/ds/2/239/S_110_6N137%2020140411-596065.pdf

Circuit diagram:

enter image description here

C1 and C4 are decoupling capacitors.

C1, C4 = 100 nF, C2 = 82 nF, C3 = 100 pF

R1 = 8k2, R2 = 2k, R3 = 330, R4 =2k2

74HC123 monostable truth table:

enter image description here

6N137 optocoupler truth table:

enter image description here

Pushbutton click:

enter image description here

Monostable outputpulse:

enter image description here

Any idea what may be wrong? Is this correct way for interfacing this two parts ( high output idle state of monostable )?

  • \$\begingroup\$ Don't you want that output on pin 13? The high pulse to turn ON the LED? \$\endgroup\$
    – Trevor_G
    May 4, 2017 at 21:47
  • \$\begingroup\$ When LED is ON, optocoupler output is low. I need positive output pulse from optocoupler. \$\endgroup\$
    – ToKra
    May 4, 2017 at 22:05
  • \$\begingroup\$ What does V1 do when the optocoupler is connected? \$\endgroup\$
    – Trevor_G
    May 4, 2017 at 22:08
  • \$\begingroup\$ Stays same. I'm powering it with BK Precision 9110. I think that owg60 has to correct answer. \$\endgroup\$
    – ToKra
    May 5, 2017 at 7:58

1 Answer 1


The 25mA is what you can run the 123 at without damage. That doesn't mean the output voltage will not be reduced. If you check the HC family characteristics you will find this graph;

enter image description here

It shows the output will be reduced 0.3-0.9V at 10mA and 25C. You should increase the 320 Ohm resistor as much as you can and still have the isolator work. If that will not work, you need a buffer.

  • \$\begingroup\$ Or use a device with stronger outputs, e.g., (SN)74LVC1G123, or an optocoupler with a more sensitive input, e.g., H11L1 or TLP2361. \$\endgroup\$
    – CL.
    May 5, 2017 at 9:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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