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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):

enter image description here

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?

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  • \$\begingroup\$ Did you try using 470 ohm resistors instead of 220 ohm? \$\endgroup\$ Feb 14 at 17:20

4 Answers 4

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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.

enter image description here

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.

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  • \$\begingroup\$ Thanks for the detailed answer and for the effort you made. Checked using 3 different brad boards and 4 ICs. Never powered up before checking pin-by-pin for correct connectivity. Looks like it is a bad batch of ICs. \$\endgroup\$
    – Charlie
    Feb 15 at 17:15
  • \$\begingroup\$ Wow, bad luck. Send me your address and I'll mail you a known-good chip! \$\endgroup\$
    – td127
    Feb 15 at 17:18
  • \$\begingroup\$ Thank you very much for the offer. But I'm in Sri Lanka and thinking of changing my oscillator for the inverter I need to build. I have to run my Laptop for 8 hours (need about 100W) to continue my work to face the imminent power crisis in the country. Do you kow an oscillator I can use for this? \$\endgroup\$
    – Charlie
    Feb 15 at 17:29
  • \$\begingroup\$ That's probably worthy of a separate question to this site. You'd probably get several recommendations, with the ubiquitous "555 timer" being a top contender. \$\endgroup\$
    – td127
    Feb 15 at 18:00
  • \$\begingroup\$ Sure. That 555 was in my mind too. \$\endgroup\$
    – Charlie
    Feb 16 at 3:14
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You need to connect pin 6 (Trigger-) to Vdd, as shown in the figure page 2 of the datasheet you linked.

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  • \$\begingroup\$ I tried that- but still the same. Any advice? Also, isn't 6th pin matter only in monostable mode? \$\endgroup\$
    – Charlie
    Feb 5 at 15:22
  • \$\begingroup\$ Check all the connections. Pin 6 is the inverted trigger input so connection to Vdd is required. \$\endgroup\$ Feb 5 at 18:50
  • \$\begingroup\$ Thanks. I'm perplexed why the circuit fails. After all it's just 14 pins and they all are correct. I suspect IC failure. \$\endgroup\$
    – Charlie
    Feb 6 at 4:23
  • \$\begingroup\$ I rebuilt the circuit in another bread board and it shows the same results. \$\endgroup\$
    – Charlie
    Feb 6 at 6:30
  • \$\begingroup\$ Doble checked all the connections. Rebuilt 2 times. \$\endgroup\$
    – Charlie
    Feb 14 at 16:17
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If you check this table in the datasheet: Connections table

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.

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  • \$\begingroup\$ I tried that- but still the same. Any advice? Also, isn't 6th pin matter only in monostable mode? \$\endgroup\$
    – Charlie
    Feb 5 at 15:22
  • \$\begingroup\$ I rebuilt the circuit in another brad board and the result is the same. Could this be IC failure ? \$\endgroup\$
    – Charlie
    Feb 6 at 6:30
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Did you used a polarized capacitor? See data sheet

enter image description here

Increment resistor and then you can use non polarized lower value capacitor.

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  • \$\begingroup\$ I used both types. The result is the same. \$\endgroup\$
    – Charlie
    Feb 15 at 17:16

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