I have made the following circuit:

Viusalisation (not working): https://i.stack.imgur.com/tDA85.png

Circuit (working):


simulate this circuit – Schematic created using CircuitLab

Left to right, the pins on the chip are: vcc out2 in-2 in+2 and ground at the bottom

It always powers the LED with 3.3 V no matter what the inputs are, so the LED is always on. The circuit simply doesn't change, no matter which line or none at all are plugged into the inputs.

It is powered by the 3.3 V and 5 V connections of a Raspberry Pi. The resistor is 10K, but I've tried it with 1K and 100R as well and the circuit still doesn't work.

  • \$\begingroup\$ where is the input!? \$\endgroup\$ – fhlb Dec 29 '18 at 11:25
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    \$\begingroup\$ If you're not plugging anything into the inputs, they're probably floating. Floating inputs == random noise == random outputs == bad. \$\endgroup\$ – The6P4C Dec 29 '18 at 11:45
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    \$\begingroup\$ Max. Input voltage is limited at vcc + 0.3V. You should not apply 5V to a input when vcc is only 3.3V and expect the device to work correctly. \$\endgroup\$ – Unimportant Dec 29 '18 at 12:58
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    \$\begingroup\$ No, you should not leave any inputs floating. Connect vcc to 5V, apply 3.3V to the positive input and connect the negative input to ground. The led should be off. Reverse the inputs and the led should be on. \$\endgroup\$ – Unimportant Dec 29 '18 at 13:07
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    \$\begingroup\$ Could you draw your circuit using the built in circuit designer? Schematic diagrams make it much easier to discuss circuits. The LM393 is so well known I expect half the people here know the pin assignnents for it by heart, but it is still better to use schematics instead of wiring diagrams. It also helps make clear to you what your circuit is supposed to be doing. \$\endgroup\$ – JRE Dec 29 '18 at 13:18

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