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I'm trying to build a simple astable multivibrator using a CD4047. I try to demonstrate using 3 LEDs on the OSC, Q, and Q` pins. Following is the circuit I built (copied diagram):
The only difference I have is that I have 220 Ω as the LED driver resistor and Pin 6 is connected to Vdd.
I can only see the OSC LED blinking at about 5 Hz. The Q` is always on, Q is always off after just one blink at the startup..
I tried with two CD4047s (purchased from the same store).
What could be the problem?
Analyzing the schematic for the 4047, you can see that if ASTABLE is high then the quad NOR will be low no matter what is happening on pins 4,6, 8 so those are actual don’t care pins, as they don’t affect anything but other inputs to the NOR. This is why changing pin 6 had no effect.
Your OSC is oscillating as expected so the internal CL and CL_BAR clocks should be working.
Your Q is stuck low and Q’ high. With Q low that should cause the 2-input NAND gate feeding the final flip flop’s D input to go high and Q should flip on the next clock, yet it remains stuck low.
The only thing (other than a bad chip) that would cause this symptom is if the EXT RESET pin 9 was high: that would keep the final flip flop into the state you describe – Q low, Q’ high.
So the only explanations I see are:
-you’ve wired pin 9 high by mistake, or
-you’ve wired pin 9 low but there’s a bad connection to the actual pin and it is in fact high, or
-the chip is bad
**** EDIT, a few hour later ****
Upon further reflection I realized the "pin 9 stuck high" theory did not jibe with the brief blink of the Q LED on power up: if pin 9 were high then that LED would never light.
That left "bad chip" as the only answer but this is a very unsatisfying answer.
So I went through my ancient stash of ICs and found a CD4047B and quickly wired up your circuit on an equally ancient superstrip breadboard.
And it didn't work - not exactly your symptom but something clearly not right. I checked the wiring, it looked fine. Then I beeped out with a multimeter the legs of the resistor, cap, diodes, etc to the actual 4047 pins just to make sure and lo, one pin wasn't making a connection. I removed the 4047, polished up the pins with DeoxIT, put it back, connection was now good, and voila, the circuit works fine. OSC blinks and Q and Q_BAR blink at half rate.
So your circuit is fine, you likely just have a bad connection or a wiring mistake, simple as that. I think this is more likely than a "bad chip".
Don't trust you breadboard connections - do a continuity check from each of the 14 pins to where you think they should be going.
You need to connect pin 6 (Trigger-) to Vdd, as shown in the figure page 2 of the datasheet you linked.
If you check this table in the datasheet:
You can see that for "Free running Astable Multivibrator" you should pull pin 4,5,6,14 high. But you are pulling pin 6 (-Trigger) Low.
Pull pin 6 to +5V instead and it should work.
Did you used a polarized capacitor? See data sheet
Increment resistor and then you can use non polarized lower value capacitor.
