I've got this 555 circuit that triggers a high output state for 3 minutes when pin 4 of U14 is pulled low from an open circuit state. I've had it working in the past with U14 as a typical SSR however I opted to use an OptoMOS type photocoupled MOSFET as it fitted the design better. The issue I'm having is that when I power the circuit the system incorrectly triggers. If I remove U14 it no longer incorrectly triggers.

The 555trig input on the left is floating so the input to U14 is definitely not enabled at any point. R133 is 560R and R154 is 10k. The other resistors are 100k, C50 is 1u (100u has the same issue. It doesn't matter too much what value this is), and C51 is 1000u. It's powered by a 5v DC-DC converter and initially turned off until I press a button that connects 5v to the circuit through a rather overkill CS147 which is another optically isolated MOSFET device (http://cotorelay.com/wp-content/uploads/2014/10/ct147_cs147_mosfet_relay_datasheet.pdf)

U14 datasheet: http://www.ixysic.com/home/pdfs.nsf/www/CPC1002N.pdf/$file/CPC1002N.pdf

Any ideas on why this might be happening? I tried a multimeter beep test on startup to see if the U14 contact was closed for any prolonged period but I got nothing. Whatever transient is occurring is a bit tricky to pinpoint. Considering that I can remove U14 and it doesn't trigger tells me that it does indeed make the connection however briefly. What do you think I should try in order to make this not trigger on power-on?


3min output high when triggered

  • \$\begingroup\$ can anyone mention the purpose of C50? \$\endgroup\$
    – User323693
    Commented Jan 24, 2017 at 7:46
  • \$\begingroup\$ @Umar It couples the AC signal from U14 to the trigger pin. When U14 is an open circuit, at steady state C50 has no charge across it. When U14 is then activated, the left plate is brought to ground, and because there is no charge accumulated in the capacitor, the right side is also brought to ground. This triggers the 555 timer (U13). The pull-up resistors could be used without C50 for the same effect. \$\endgroup\$
    – Alex
    Commented Jan 24, 2017 at 8:08
  • \$\begingroup\$ @Cameron I'd try pulling 555trig to ground and seeing if the problem still persists. If it does, my guess would be capacitance between pins 3 and 4 of U14 holds pin 4 low for long enough that it triggers the 555 \$\endgroup\$
    – Alex
    Commented Jan 24, 2017 at 8:17
  • \$\begingroup\$ pin 4 should be connected to pin 8 \$\endgroup\$ Commented Jan 24, 2017 at 10:06

1 Answer 1


A few minor mods to the circuit should cure any problems.

enter image description here

R133 (10k) and C52 (47uF) are connected to pin 4 (reset) so that on power up the reset pin in held low preventing any spurious trigger signals.

D1 is added to prevent the positive edge taking the trigger input higher than the supply.

C53 is the 0.1uF capacitor normally added to 'smooth' the internal divider. The circuit will work without it so it may be omitted.

  • \$\begingroup\$ Well I just added the 10k and 47uF for the reset line and it stops triggering, but it actually stops it triggering even when I want it to. I'm measuring the supply voltage at pin 4 so the reset shouldn't be triggering other than when the cap initially starts charging up, right? Were the other component value changes necessary? \$\endgroup\$
    – Cameron
    Commented Jan 30, 2017 at 3:35
  • \$\begingroup\$ pin 4 should be at 5V once the cap is charged after power on reset. If its not triggering then by all means play with the R132,C50,R131 values. If you have a scope you should be able to see pin 2 pulled low (either to ground or below 1.7V). If the output pulse from U14 is not the full 5V you could add another 10k resistor from pin 2 to ground which sets pin 2 at 2.5V. The pulse then only needs to pull it down to below 1.7V to trigger the 555. \$\endgroup\$ Commented Jan 30, 2017 at 12:55

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