74HC14 Circuit Not Working

[ebclr]

To easily check if your IC is good

Put only a 100K resistor between 1 and 2 and the capacitor on 1 and ground, on this way you have a oscillator, Check output with a scope you must have a square wave.

This circuit simple work, if didn't work you have any misconnection or bad component

[alsetalokin4017]

The circuit using 74AC14 works just fine on my breadboard too. Also tried 74C14 which does not quite give the total duty cycle range as the AC version. I don't have the HC version to try but I'm sure it will also work very much the same.

Bad breadboard, perhaps? Bad jumper leads?

(Let's emphasize the need for all unused inputs to be grounded, and the decoupling 0.1uF cap right at the chip is needed to quell oscillations at minimum duty cycle setting.)

[RyanG]

I don't know what the problem could be with this thing. I tried 4 different 74hc14 and I tried another breadboard. I checked the diodes, pot, and caps. All are good. I built it on the breadboard the same way as pictured by the other guys. I'm thinking all my 74hc14 are bad, but I used one from the same tube on another project, and that one works fine. There has to be something wrong with them because I am getting a constant ~2.3v on the output pins, which shouldn't happen. I put a 4069 in place of the 74hc14 and I get something more appropriate on the scope, but adjusting the pot makes no difference in the output.

[stj]

74AC series is a bit special, it can sink AND source about 30ma

other 74 series generally only sink OR source, the oposing bias is simply done with an internal pullup/down resistor.

[BrianHG]

Lemon potentiometer? 

[alsetalokin4017]

I don't know what the problem could be with this thing. I tried 4 different 74hc14 and I tried another breadboard. I checked the diodes, pot, and caps. All are good. I built it on the breadboard the same way as pictured by the other guys. I'm thinking all my 74hc14 are bad, but I used one from the same tube on another project, and that one works fine. There has to be something wrong with them because I am getting a constant ~2.3v on the output pins, which shouldn't happen. I put a 4069 in place of the 74hc14 and I get something more appropriate on the scope, but adjusting the pot makes no difference in the output.

Have you grounded all the unused inputs of the chip? Because in your photo they aren't. Leaving them floating can cause the chip to oscillate. The 0.1 uF decoupling cap needs to be as close as possible to the pins 14 and 7, with short leads, as in my breadboard photo. Oscillations can make the chip appear to be not working, although you should be able to see the oscillations on the scope.

Also there is  no need to disconnect the LED + resistor when scoping the circuit. Simply connect the scope probe to the Pin 4 output and the probe reference clip to the common negative rail (ground), in parallel with the LED and resistor.

[RyanG]

I removed the extra wires to ground just for the picture so the rest of the circuit could be easily seen. I have used the scope both with and without the LED and resistor. It made no difference. I tried 3 or 4 pots. I really don't think the 74AC vs 74HC matters here because I am not getting the outputs I should be with or without a load, so even when I am not sinking or sourcing anything the IC doesn't work correctly.

[alsetalokin4017]

Have you tried ebclr's test above? Making a simple, single gate oscillator using only a resistor and capacitor? Then testing the other gates in the same way...

There is something wrong somewhere, that's for sure, and I doubt if it is your whole tube of chips, but try testing the individual gates this way anyhow.

Do you have a function generator? You could even test the gates by putting a pulse train input from your FG into the input and scoping the output to see if it is properly inverting. But when you tested the Schmitt trigger threshold values that should have taken care of that. SO I'm stumped. Gremlins?

[RyanG]

I tested one of my 74HC14 with the single gate oscillator and it would work as long as I unplugged the power supply in between testing each gate. I don't know if that is normal but it is what I had to do.

[Brumby]

Starting to wonder if there might be something happening with the breadboard.

I'd like to suggest something....

 - Remove all your components
 - Pick another area on the breadboard (or another breadboard, if you have one.)
 - Mount the 74HC14, fit the power supply and ground wires
 - add components and interconnects one by one

See if that makes any difference.

[edpalmer42]

Don't test your chips with an oscillator circuit.  Test them with just the pot by connecting the pot between +5 and gnd.  Connect the wiper to one of the inputs.  Adjust the pot from one end to the other.  Monitor the output and as soon as it changes state, measure the input voltage.  Adjust the pot both ways, from +5 to gnd and from gnd to +5.  Compare the results to the datasheet.  For example, Fig. 8 in this datasheet:

http://assets.nexperia.com/documents/data-sheet/74HC_HCT14.pdf

(just chosen at random)

Ed

[grumpydoc]

Don't test your chips with an oscillator circuit.  Test them with just the pot by connecting the pot between +5 and gnd.  Connect the wiper to one of the inputs.  Adjust the pot from one end to the other.  Monitor the output and as soon as it changes state, measure the input voltage.  Adjust the pot both ways, from +5 to gnd and from gnd to +5.  Compare the results to the datasheet. 

+1 but I already suggested that and, apparently, the Schmitt thresholds are at 3.6 and 1.1V which is in line with typical values.

74AC series is a bit special, it can sink AND source about 30ma

other 74 series generally only sink OR source, the oposing bias is simply done with an internal pullup/down resistor.

74AC does appear to be able to drive higher currents but are you sure other 74 series can't both source and sink current - pulling a few 74HC14 datasheets at random suggests  ± 20mA is typical.

It is of some relevance though because the output needs to be able to sink the capacitor discharge current as well as source the charge current - which brings up another weakness in this design. At either end of the potentiometer range the cap is effectively connected directly to the output, so the current spikes might be quite large. Probably not much of an issue in practice but it might be better to stick a couple of fixed resistors between the ends of the pot and the diodes.

However this is a "I need a quick PWM on a breadboard" circuit, not something which is "production ready" - I wouldn't get too hung up about the details.

We've had two working with 74HC (mine and rdl's) and one with 74AC (alsetalokin4017's) and with 74C (also alsetalokin4017). RyanG tried with HC but I think we can stop telling him it won't work without a 74AC14.

At this point I think trying a different breadboard would be useful.

[RyanG]

I think there is something wrong with these ICs. If I put any resitive load on the output pins the square wave disappears. It turns into a flat ~2.3v. I put the gate of a 2n7000 on the output and it works fine, but only if I have 5v on the drain of the 2n7000. If I pull the 5v from the 2n7000 it goes to the flat 2.3v and I have to reset the power to the 74hc14 if I want the square wave to return. I've tried this on 2 breadboards and in different places on each of them, and it is on an input I already tested with the pot test.

This datasheet says that it can sink or source 4ma @ 4.5v on the first page.
https://www.diodes.com/assets/Datasheets/74HC14.pdf

[grumpydoc]

I think there is something wrong with these ICs.

Probably best to grab some more from somewhere, I'd avoid ebay if possible.

[newbrain]

We've had two working with 74HC (mine and rdl's) and one with 74AC (alsetalokin4017's) and with 74C (also alsetalokin4017). RyanG tried with HC but I think we can stop telling him it won't work without a 74AC14.

Just to add a datapoint, no problems whatsoever with an AC (scope captures on request), and I also tried an ACT.
As the thresholds in ACT are asymmetrical (TTL compatible input), the frequency is much more dependent on the duty cycle setting, but that's all.

[mikerj]

Have you added a decoupling cap across the IC as suggested previously?  Having the supply impedance too high makes logic devices do strange things.

[ludzinc]

Yes, the scope shows 5v. Picture of breadboard and schematic attached. When I attach the scope it replaces the series resistor and LED. I removed the wires for the other inputs so you can see the rest of the circuit on both the breadboard and the schematic. The diodes are not touching, it is just the angle of the picture. I also tried another 74HC14 just in case the one I was using was bad. Every other component was also tested.

How thick are the pins on the pot?  Not pot I've had would fit easily into the breadboard.

If you've had to force it in, you've introduced an intermittent connection....

I learnt that the hard way many moons ago with a thick diode lead.

What are you using for a power supply?  Is it collapsing under load? 

As for checking your chips - daisy chain outputs to inputs, set the first input high (or low) and check you have expected highs and lows on required gates.

I've built this circuit with 74AC14 (I recognize that pic from http://www.robotroom.com/PWM.html) to build a RGB colour mixer for the kids and it works a treat.

[RyanG]

The potentiometer works fine in the same spot as a voltage divider, so I think that is okay. Using a 5v 7A power supply running the 74hc14 and a 5mm LED. I've run larger things off the same supply so that should be fine. The chip works absolutely fine until I connect the output of one gate to the input of another or if I add a load. I have also added the cap across the IC as previously suggested.

[ebclr]

Do you have a scope to check the frequency that the single oscillator is running ?

[BrianHG]

The potentiometer works fine in the same spot as a voltage divider, so I think that is okay.

This means nothing.  You can have a 1 meg impedance at the center of the pot which would work fine as a voltage divider, as would a 1k pot, a 10k pot, and a 100k pot.  The circuit demands a 10k pot for a degree of drive onto the timing cap.

As for the power supply. have you measured it to be 5v.  With so many amps, especially if it's a switching supply, without a minimum load, it may be overshooting 5v and this would seize up the 74HC14 at somewhere above 6v.

Try powering the circuit with 2x AA batteries (3v) or 3x AA batteries (4.5v).

[bson]

The hysteresis on a '14 is not a well-controlled property, and it needs to oscillate at least this range plus two diode drops.  If the inverter input hysteresis is too close to Vdd or Vss (within one diode drop) it won't be able to oscillate.

[edavid]

74AC series is a bit special, it can sink AND source about 30ma

other 74 series generally only sink OR source, the oposing bias is simply done with an internal pullup/down resistor.

Just in case anyone is confused, the above is not correct.  There is no 74xx series which does not have push-pull outputs.

There are a few parts that are open collector/open drain (e.g. 74xx05), and even fewer parts with built in output pullup resistors (7448, 74248).  But, they are exceptions.

There are no parts with pulldown resistors.

[RyanG]

I've tried several pots and my power supply is 5v for sure. I've run one LED off of it before with no problems. I am not going to be able to test anything for the next few days because I'm away from my stuff, but I will try anything suggested when I get back.

[rx8pilot]

As for the power supply. have you measured it to be 5v.  With so many amps, especially if it's a switching supply, without a minimum load, it may be overshooting 5v and this would seize up the 74HC14 at somewhere above 6v.

Try powering the circuit with 2x AA batteries (3v) or 3x AA batteries (4.5v).

That is a good point, the supply may be pulse skipping or something with only a few mA of load. It would potentially make a mess of the voltage until a reasonable load is applied.

[RyanG]

I'll try powering it off of my Arduino when I get back. I know that I could just use the Arduino for pwm or a 555, but I'd like to learn other ways of doing things than the most common.