Multiplexer CD74HC4051, always giving a true out

arduinomultiplexer

I'm testing a multiplexer from Texas Instruments (CD74HC4051). Both +5V and Ground rails are connected to the Arduino Duemilanove board.

I've got a +5V signal in the channel A7 (pin4) and A0 (pin 13), moving my address selectors to different addresses than A0 and A7 still keeps the output active.

When dashing through all the 8 addresses, the output is basically always +5V.

How come?

The images below are: the CD74HC4051 pinout, my setup connections, and the 4051 truth table. The photos at the end of the post show three different address configurations.

CD74HC4051 Pinout:

CD74HC4051 Pinout

Setup connections:

Setup connections

enter image description here

Photo 1 – Address False, False, False:
Setup with address False, False, False

Photo 2 – Address True, False, False:
Setup with address True, False, False

Photo 3 – Address True, True, True:
Setup with address True, True, True

Below is the schematics with the pull down resistors. This solves the impedance issue of the IC pin internal resistance and possible noise and float signal to the pin.

Schematics with the pull down resistors

Best Answer

Being CMOS, you need to make sure that all of the address inputs are either connected to Vcc or GND since once you charge up the gate capacitances of the decoder's input MOSFETs they'll stay charged up for a long time when the address dip switches go open-circuit.

I don't see any pull-downs on the chip side of your address dip switch, so you need to connect about 10k from each of them to GND.

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As rzrgenesys187 said:

If pin C0 floats HIGH and C1 is connected to AREF which is also HIGH, then there shouldn't be any change. Did you try connecting C1 to ground to get zero when you run select_wire(1)?

Yea, so it actually was switching the entire time.. however, there is a lot more distortion, so the wire connected to ground floated somewhere in the 10s. And AREF isn't high, so I don't understand how that works. AREF for me gave about the same values as ground when using

analogReference(DEFAULT);

so not sure why, but AREF is giving me near-ground values... very strange, but the MUX does infact switch, so this question is answered.. and I actually never had any problems.. meh.

EDIT:

Actually, hooking straight into AREF does make it the input go high to 1023, so I have no idea why the input won't go that high with the MUX. maybe some kinda voltage limit, idk. I have to read up on it..

EDIT2:

Ok, finally think I'm figuring it out. Apparently breadboards leak a lot of current, so that if I was connected to C0 and it was reading C1, then a lot of voltage would leak to C1. Anyway though, connecting to AREF to C3 makes both inputs go high, so now I'm just even more confused.

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