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I have designed a prototype PCB for a product we are working on. It works, but with minor problem the PCB should be able to be supplied from anything from 5V - 12V.

The problem is the 555 timer circuit I chose displays unreliable behavior after about 8V. Turning off without any input after a couple seconds, the time interval in which this happens is diffferent on each try. Another thing I noticed was the enormous 300mA current spike at 12V even when no load is attached at the output.

The schematic of the circuit is below. On the schematic please note that on the PCB R8 is not soldered in as I deemed it unnecessary. It shouldn't change the behaviour. A button is connected to the screw terminal. This is neccesary as the PCB is meant to bring IOT capabilities to an already existing machine model. The button and the PCB need to be placed at different locations inside the device.

enter image description here enter image description here

I had a few theories as to why this my be the case, all proved to be false.

  • I first suspected the RC time constant on the output so I changed the Capacitor values. It did not have an effect.
  • Then I suspected the problem was caused by the DIS input being left to float. I thought spontaneous noise was enough to turn on the BJT and then it would stay on due to the flowing base current, which would explain the high current intake. This was proven false when I grounded the DIS input. Oddly enough this has caused the current cunsumtion to go well above 400mA at 12V which triggered my Power Supplies over current protection.
  • Any influence from the PMOS Source Ciruit(The load is configured as in a source circuit, Attached to 12VA) is out of the question as I ran a test also by desoldering the PMOS and monitoring the output on an ossiloscobe.

Any input on what I may be doing wrong here would be much appriciated. I have tested this circuit on a breadboard and it did work with 12V. I cant make much sense of what is going on here.

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  • \$\begingroup\$ Is Q1 drawn upside down? It makes it hard to read. What does +12Va go to? C8 should be a 100 nF ceramic capacitor, not a 10 uF capacitor. CV should be a 5 nF ceramic capacitor, not a 10 uF capacitor. \$\endgroup\$ Sep 16, 2021 at 20:19
  • \$\begingroup\$ @DavideAndrea yes it is. +12VA goes into a buck converter. But like I said the Source cicruit has no effect on this behaviour. \$\endgroup\$
    – Emre Mutlu
    Sep 16, 2021 at 20:22
  • \$\begingroup\$ Your switch should go to the Reset input instead: that's what it's for. As you have it, when you open the switch, the last state of the output remains, and it could be on or off. \$\endgroup\$ Sep 16, 2021 at 20:24
  • \$\begingroup\$ @DavideAndrea this circuit does work by comparing hald the supply voltage with 1/3rd and 2/3rds the supply voltage. When output is Low, On button press the (-) of Trigger is grounded and set is pulled high, the (+) of Thresh is also lower than 2/3rds. This sets the flip flop. The opposite happens when the output is high. \$\endgroup\$
    – Emre Mutlu
    Sep 16, 2021 at 20:38
  • \$\begingroup\$ @DavideAndrea the problem is not this circuit not working at all, the problem is that it doesnt work above 8V for some reason. \$\endgroup\$
    – Emre Mutlu
    Sep 16, 2021 at 20:38

1 Answer 1

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I've had issues with the NE555 in the past. The NE variant is TTL style. There is a better chip that is pin compatible, the LMC555. It is CMOS.

From the datasheet:

The LMC555 offers the same capability of generating accurate time delays and frequencies as the LM555 but with much lower power dissipation and supply current spikes

When I switched over, all my woes went away, and my battery lasted longer.

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