74HC14 Circuit Not Working

[rx8pilot]

In this project, the most valuable learning opportunity is troubleshooting skill. There is a small problem hidden somewhere. Over time, you build intuition that guides your decisions and the little gotchas are easier to see.

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[RyanG]

I'm back. I tried powering the whole thing with my arduino and it does the same. The output even changes from square wave to the flat ~2.4v if I just touch the output wire going to the scope.

[rx8pilot]

Do another shot of the breadboard so we can see the changes you have made. Attach your power directly to the Vcc strip, make sure your decoupling cap is right next to the Vcc pin. Get rid of the power rails on the side. Attach your gnd directly to pin 7 and pull your other gnd connections from that strip. Not likely the smoking gun, but having antennas on your project will not help.

[ebclr]

Basic troubleshot

Connect only the ic and power nothing else

1) Measure with a multimeter voltage between pin 7 and 14

2) Connect pin's 1, 3 , 5 ,9 ,11 and 13 to ground and measure voltage on pin 2,4,6,8,10 ,12 ,Voltage in all must be +Vcc

3) Connect pin's 1, 3 , 5 ,9 ,11 and 13 to Vcc and measure voltage on pin 2,4,6,8,10 ,12 ,Voltage in all must be 0

4) disconnect all pins except the power supply, and connect a 100K resistor between pin 1 and 2. With a scope check if you have a very high-frequency signal on pin 2 ( you made an oscilator with the resistor and spurious capacitance.

5) Maintain the resistor and add a capacitor from Pin1 to ground, now the frequency must be smaller.

After you reply on this I will give you extra instructions

[rx8pilot]

Basic troubleshot

Connect only the ic and power nothing else

1) Measure with a multimeter voltage between pin 7 and 14

2) Connect pin's 1, 3 , 5 ,9 ,11 and 13 to ground and measure voltage on pin 2,4,6,8,10 ,12 ,Voltage in all must be +Vcc

3) Connect pin's 1, 3 , 5 ,9 ,11 and 13 to Vcc and measure voltage on pin 2,4,6,8,10 ,12 ,Voltage in all must be 0

4) disconnect all pins except the power supply, and connect a 100K resistor between pin 1 and 2. With a scope check if you have a very high-frequency signal on pin 2 ( you made an osiclator with the resistor and spurious capacitance.

5) Maintain the resistor and add a capacitor from Pin1 to ground, now the frequency must be smaller.

After you reply on this I will give you extra instructions

That ^

[grumpydoc]

Did you get some known good HC14's?

[rx8pilot]

Did you get some known good HC14's?

With @ebclr shake down guide - we are about to find out. At this point, I would also start asking about scope setup and probing too. OP - how much scope knowledge do you have?

[StillTrying]

Using my breadbroaded 74AC14, I could sort of reproduce the 14's output stuck at 2.3V by turning the 10k pot to a very low value, and then turning it back up quite a bit leaving the 74AC14's ouput just stuck at 2.3/2.4V.

But, while looking at the 14's input and output stuck at 2.4V the scope's trigger counter flicking between 188MHz and 260MHz gave me a clue that something else was happening! Difficult to see on a 100MHz scope, but the 74AC14 starts oscillating at around 250MHz and it sticks at that frequency until I turn the pot right down into the 100kHz range. Not surprisingly it's something to do with the length of wires to the pot and timing cap.

I don't know what frequency the OP's HC version would HF self-oscillate at, but I'd bet that it was!

[Zero999]

Using my breadbroaded 74AC14, I could sort of reproduce the 14's output stuck at 2.3V by turning the 10k pot to a very low value, and then turning it back up quite a bit leaving the 74AC14's ouput just stuck at 2.3/2.4V.

But, while looking at the 14's input and output stuck at 2.4V the scope's trigger counter flicking between 188MHz and 260MHz gave me a clue that something else was happening! Difficult to see on a 100MHz scope, but the 74AC14 starts oscillating at around 250MHz and it sticks at that frequency until I turn the pot right down into the 100kHz range. Not surprisingly it's something to do with the length of wires to the pot and timing cap.

I don't know what frequency the OP's HC version would HF self-oscillate at, but I'd bet that it was!

It will also oscillate if the capacitor is removed and the input is connected to the output. The frequency will be in the VHF to UHF range and extremely unpredictable. Connecting your 'scope probe will reduce the frequency, by introducing more capacitance into the circuit.

[RyanG]

I did all of the things ebclr said.

1. The voltage was 5v
2. Voltage on all pins was 5v
3. Voltage on all pins was 0v
4. Didn't see anything on the scope except for the flat line at ~2.4v
5. Saw something on the scope for a few seconds then it flat lines at ~2.4v

My first gate wouldn't work at all for the oscillator part, I had to use the 2nd gate

[ebclr]

What is the model and brand of your scope ?

Do your scope have a 1 Khz test signal? try to put the probe there and check the scope / probe

[ebclr]

Can you please post a picture of your probe, specially the X1 GND X10 switch on probe please

[RyanG]

I don't think the scope matters because the circuit doesn't work correctly with a LED in it. I don't have a good scope. I currently have a stupid, worthless DSO138. I realize that the higher frequency oscillation with just the resistor isn't seen probably because of that. I have tried different values on the resistor and cap to change the frequency and no matter what I get to show up it disappears after a few seconds and then just puts out ~2.4v. That is with or without the decoupling cap. Some gates work better than others. I don't really get it. I know a better scope would help but at the moment I don't have one and don't have access to one. I get ~100Hz with the 100k resistor and 0.1uf cap. I get 1KHz with a 10k in place of the 100k. But, either way it goes to the ~2.4v after a few seconds.

[rx8pilot]

I don't think the scope matters because the circuit doesn't work correctly with a LED in it.

I promise.....it matters.

[StillTrying]

The solution on my 74AC14 and 470pF version was to connect the timing cap. directly between the gate's input and GND pin7 of the IC, - do not go down to the breadboard's GND.

The 10k pot gives me 190kHz to ~26MHz with no sign of it shooting off to 245MHz. The 74AC14's hysteresis levels of 1.7V and 2.8V @ 5V, give quite a symmetrical square wave, plus quite a bit of ringing!

[ebclr]

Since you don't have a scope, you need to slow down to visible frequency, Do you have a 10uF electrolyte capacitor?

[RyanG]

Yes I do have 10uf caps

[stj]

use one inplace of the ceramic

[Nusa]

The DSO138 is the cheapest of the cheap scope kits, but not entirely useless. You just need to be acutely aware that it's only a 200KHz scope. It does have a 1 KHz test point.

[ebclr]

Use the 10Mf as your timing capacitor, on this way the Led will blink , and your eyes will be your scope

[RyanG]

It does the same thing. It runs for a few seconds and then flattens out at ~2.4v

[ebclr]

What are you using to measure?

Don't use your scope, The scope can be the real problem

Did the Led blink ?

Did you put a 330R is series with the Led ?

Do you have Skype and camera?

[RyanG]

It is currently working fine.

All unused inputs grounded, with decoupling cap and with 10uf cap.

As soon as I clip the scope on the circuit fails.

[rx8pilot]

Now you know..... it matters. :-)

Sent from my horrible mobile....

[RyanG]

I tried putting the circuit back together the way the picture from my original post and it still doesn't work correctly. That is the point I was trying to make. I get how the crappy scope could screw with the one gate oscillator circuit and therefore the original circuit, but the scope isn't what makes the original circuit not work.