what am I doing wrong...MC4028BE

[strokedmaro]

Well, I come to the electro gods again for help... What seemed so simple (in theory) operates completely unexpectedly. Attached is the datasheet for my problem child...the MC4028BE BCD to decimal decoder.

SETUP: Vdd=12vdc, Vss=ground, input "D"=grounded, Inputs A,B,C are attached to pushbuttons to 12vdc.

With no inputs I get 6vdc on (pin 3)...shouldn't it be closer to 12vdc?
With inputs on A, B, and C (pin 4) reads 12vdc but I also noticed it stays at 6vdc when inputs removed...so It jumps only 6vdc when activated.
All other outputs start at 0vdc and go to 6vdc or so.

I would think they all start at 0vdc and output Vdd when activated but that is not whats happening obviously. This happens with 3 exact chips...I've also set it up several times on the breadboard to make sure I didn't mess the wiring up.

Ive also attached a pic of exactly what I have on the breadboard. Keep in mind that pin 4 always has 6vdc on it so the LED dims immediately. when pin 4 is active the 12vdc is enough to kill the LED completely....when 15 is active the led goes out completely but only outputs 6vdc. I think the 6vdc from pin 4 plus the 6vdc from pin 15 together make this happen because no other output will extinguish the LED including pin 15 if pin 4 is not connected. HELP HELP HELP!!!

Any help appreciated!!!

 

[strokedmaro]

AHH...maybe I just thought of it myself...do I need pull down resistors on A, B, and C when not active? I remember someone posting something like that on another post. If so, can someone draw a quick example? thanks!

EDIT: I guess A, B and C are technically not "0" without being grounded when not high right?

EDIT: I was right...."low" A,B,C inputs have to be grounded when not high or it does not work....now, can I just put a reisitor to ground on each of these inputs??

 

[Roff]

You should indeed use pulldown resistors. Anything in the range 10k - 100k should be fine.
You need to put the resistor values on your schematic.
Also, how are you limiting your LED current? There should be a resistor in series.
What is the purpose of D7? Are you driving a relay, solenoid, or motor?
Maybe you could explain in detail what you are trying to do.

 

[strokedmaro]

You should indeed use pulldown resistors. Anything in the range 10k - 100k should be fine.
You need to put the resistor values on your schematic.
Also, how are you limiting your LED current? There should be a resistor in series.
What is the purpose of D7? Are you driving a relay, solenoid, or motor?
Maybe you could explain in detail what you are trying to do.

Good observations....the circuit does indeed drive a pair of solenoids (the reason for D7) . The pic attached is for one half of the circuit and only shows 1 solenoid (LED-which does have a resistor in series) I also wasnt thinking about the inputs to the decoder when I was testing....normal operation will switch between 12vdc and ground from within a computer this project will attach to. on the breadboard however I only had pushbuttons on going to power with no pull downs. I think I've finally got it right. THANKS!

 

[Roff]

Good observations....the circuit does indeed drive a pair of solenoids (the reason for D7) . The pic attached is for one half of the circuit and only shows 1 solenoid (LED-which does have a resistor in series) I also wasnt thinking about the inputs to the decoder when I was testing....normal operation will switch between 12vdc and ground from within a computer this project will attach to. on the breadboard however I only had pushbuttons on going to power with no pull downs. I think I've finally got it right. THANKS!

Unless you have other connections to the circuit you posted, you could get rid of about half the parts. You wouldn't need any of the diodes except D7, or any of the base-to-ground resistors.