Electronic – 555 soft latch triggers when voltage applied and turned off


I have built a LM555 based soft latch (toggle) with single push button according to the schematic, and while on protoboard everything seemed to work fine, the pcb version is triggering when voltage is first applied.

So when I first connect 220VAC to the pcb, the trigger latches on. After that the circuit behaves normally, every press of pushbutton toggles the relay on or off. If I disconnect 220VAC and the latch was off, it toggles on for a split second before shutting off.

There is a 9V transformer followed by a rectifier bridge and 100uF which provides around 15V when latch is off, and 12,5V when it is on. I measured the middle point between two 10k resistors and it's 7.5V (off) and 6.2V (on).

I already tried a few things:

  1. Adding a startup delay with 2,2k (or 10k) + 10uF connected to pin 4 and GND – doesn't help on its own
  2. Suspected the 100uF cap is staying charged so added a bleed resistor (1k and 2,2k) – doesn't help on its own
  3. Removing 100uF cap renders the circuit unusable (obviously the ripple from he rect bridge is too high)
  4. Increasing the C1 to 4.7uF and 10uF – doesn't help
  5. Adding a 0.01uF from pin 5 to GND – no change

All of the above doesn't fix the problem, although 1. and 2. together seem to make it appear less frequent.
I want the circuit to stay off until I press the button, so what am I missing here? I'm suspecting some kind of circuit instability but can't figure it out.

Any help appreciated.

LM555 soft latch circuit

Best Answer

You have TRIG and THRES set to 1/2 Vcc, so the 555 could power up in either state. To ensure that it is reset on power up you should pull RST low until the supply voltage has stabilized.

Adding a startup delay with 2,2k (or 10k) + 10uF connected to pin 4 and GND - doesn't help on its own

Reset is only active when RST is below ~0.7 V (minimum 0.3 V), so with 10k and 10 uF the reset time will be much shorter than you might expect from the RC time constant. Try a much larger resistance (eg. 100k) to increase the reset time. Also put a diode across the resistor to discharge the capacitor faster on power down.