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I am using Schmitt Trigger CD40106BE to make an PWM oscillator with controllable duty cycles. The schematic is shown below: enter image description here enter image description here

However, the LED that should be blinking is constantly on. After some trials and errors I found that even when nothing other than Vdd(9V battery) and Vss(ground) are connected to the CD40106, input pins of the chip with nothing connected shows a 5.4V potential difference relative to the ground (battery negative) and the output pins show a 9V (typical behavior).

Why do I have a 5.4V on my input pins with nothing connected?

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    \$\begingroup\$ Most likely it was oscillating faster than your meter could read with no cap connected , mid scale in the MHz or kHz range \$\endgroup\$
    – D.A.S.
    Commented Oct 13, 2021 at 5:42

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Reset to default
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Check that pin 7 actually measures 0V right at the chip pin - making sure that there is not a loose connection on the ground due to the sometimes unreliable solderless breadboard.

It's also possible the chip is damaged or it is not a CD40106, but it's unlikely that damage would affect all the inputs at once.

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  • \$\begingroup\$ The chip is damaged and the measurement of all pins are ~4.5V. Thanks! \$\endgroup\$
    – Power_tile
    Commented Oct 19, 2021 at 22:34

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