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I've made a circuit that's connected to a 12V+5V power source (attached a gallery. last photo is the schematic) enter image description here

it is simply a 555 timer that goes off whenever one of the two input pins goes high (that's what the 4070N is for). I've attached some readings under 5 different conditions: - potentiometer at 50kOhms (one input pin LOW and the other NC)

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  • potentiometer at 1kOhms (one input pin LOW and the other NC)

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  • input pins HIGH and LOW

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  • input pins both LOW

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I'm a bit stumped with this one.. could it be that the 555 is not working properly? i can read the 4070 with the multimeter and the only weird thing is that it outputs 2Volts when both inputs are LOW. and what's weirder is that the frequency doesn't seem to change with the potentiometer..

If any kind soul has time to take a look, take a shot. I think i'll replace both chips in the meantime just in case, because i'm not sure anything else is wrong :/

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    \$\begingroup\$ Sorry, cannot read that schematic. Can you reformat and repost? Also, it's better to include the graphics here, rather than to link to an external provider. \$\endgroup\$
    – mike65535
    Jun 21, 2018 at 22:42
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    \$\begingroup\$ With CMOS logic, all unused inputs (including those on unused gates) MUST be connected to either Ground or the positive supply, either directly or through resistors of 10K or so - this includes the inputs of the one gate that you do use. I can't read any signal names on the A4988 Carrier block, so don't know what happens there. What is a "12V+5V power source"? \$\endgroup\$
    – Peter Bennett
    Jun 21, 2018 at 22:54
  • \$\begingroup\$ pasteboard.co/HrfO4KM.png Here is the schematic reuploaded. I didn't connect the unused pins of the 4070, i'll give that a try before changing the chips. I'll come back with results probably today. Thanks a lot!!! \$\endgroup\$
    – Agustin
    Jun 23, 2018 at 17:32
  • \$\begingroup\$ Update: I've grounded all inputs of the 4070, and now i get full 5Volts in the output. However, the 555 timer is still working like it was before. When the Reset pin is high it goes full high on the outpu. only when the output is about 2.5V does it start to oscillate, and even then, i can't seem to be able to change the frequency with the R1 potentiometer. \$\endgroup\$
    – Agustin
    Jun 23, 2018 at 17:53
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    \$\begingroup\$ What does "goes off" mean? Output a 1-shot pulse? Output a square wave for a while? .... \$\endgroup\$
    – Scott Seidman
    Jun 25, 2018 at 18:09

1 Answer 1

Reset to default
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The problem lies in the duty cycle and the discharge times. In the circuit i've labeled R1 and R2 in the wrong order, so in the usual formulaes that are found i should've changed those up. In any case, i didn't pay attention to it. The duty cycle is more than 80% for almost all of the potentiometer range, as you can see in this plot (y axis is the duty cycle and x axis is the potentiometer value in kOhms)

plot duty cycle vs pot value

My guess the 555 is just getting stuck at 100% duty cycle or something very near that.

Sorry for the mess, hope this helps someone in the future.

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