I am using a "CD74HC4051 High-speed cmos logic analog multiplexer/demultiplexer"

I put 5V to VCC, put my oscilloscope probe to the common out/in, put 3.3V to A1 (channel in/out), and have a wire holding 3.3V ready to plug into S0 (address select) so that I can activate A1 from on to off state easily. I expect the oscilloscope to either give me a 3.3V or 0V output depending on whether or not S0 gets a voltage (according to the truth table, S0 on high activates A1).

However when I power the circuit, the oscilloscope outputs what appears as pulses. The oscilloscope also states "Square wave or pulse detected on CH1". When I put 3.3V to S0, another output appears but it's still a noisey square wave.

Any ideas as to why I'm getting such strange outputs? I'm not exceeding the voltage range so I don't think my device is blown?

I noticed the device has a VEE input, should I be putting a voltage here?

  • 2
    \$\begingroup\$ Are you driving a signal to all of the inputs of the chip? \$\endgroup\$
    – user39382
    Feb 21, 2015 at 5:10

1 Answer 1


Vee should be connected to Ground, or to a negative supply.

As this is a CMOS part, all digital inputs MUST be connected (possibly via a resistor) to either Vcc or Ground at all times - otherwise the inputs may float to a "maybe" state, with unknown consequences.

Connect all S0 - S2 and /E to ground with 10K (or so) resistors. Also connect A0 to ground with a 10K resistor, and A1 to Vcc.

Now, the output should initially be Low. Pulling S0 up to Vcc will select A1, and the output should be High.


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