74HC14 Circuit Not Working - Page 1
Forum section: electronics / beginners

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 05:18:23 am
content: The circuit in the attached picture is not working correctly. I get how it is supposed to work, but I don't get a square wave output. When I adjust the pot I just change the voltage. I checked it with a scope.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 25, 2017, 05:29:26 am
content: The input is just a variable voltage? Why are you expecting an adjustable PWM?EDIT: I think I get it - the cap is going above and below the schmidt threshold. Did you put this on a breadboard? Screen shots?

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 05:33:47 am
content: I was looking for an easy adjustable PWM and I found this on the Internet, so I was trying to use it. According to the picture it should put out a PWM signal.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 25, 2017, 05:36:00 am
content: What did you probe with your scope? Pin 4? The 74AC14 will not output a variable voltage.

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 05:43:25 am
content: Yes, pin 4. I have no idea what is going on here to be honest. If it is working, then it's a very small range somewhere around 45%-55%.

author: Brumby
author_rank: Unknown
timestamp: May 25, 2017, 05:49:32 am
content: Those outputs should be either GND, Vcc or switching between the two.Is there anything else connected to pin 4 other than the scope?

author: edpalmer42
author_rank: Super Contributor
timestamp: May 25, 2017, 05:58:51 am
content: What do you see if you probe the top of the capacitor? What changes if you adjust the pot?Ed

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 06:00:12 am
content: The only thing connected to pin 4 is the scope, and I know that the pins are either "on" or "off." But, I don't get why I get such a small range of values. Without the scope connected, I used a LED and the brightness barely changes when I adjust the pot.

author: Brumby
author_rank: Unknown
timestamp: May 25, 2017, 06:41:41 am
content: I'm thinking it might be time for some photos.Can you take a photo of your setup? It should be close-up, well lit and in sharp focus for us to see exactly what's where. Show it with the scope probe attached, so we know where everything is.Also, images of what is on the scope's screen - one for the pot at minimum, middle and maximum positions, if possible.Two other questions -- What are you using for a power source?and- Where is the scope's ground clip placed?

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2017, 07:27:44 am
content: Check your wiring.Check that all unused inputs to the 74HC14 are grounded or tied to VccPost photos

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 07:54:33 am
content: I have checked the wiring several times. All of my unused inputs are tied to GND. Power source is 5v 7A. The ground clip of the scope is on the - rail of my breadboard. The difference between the maximum and minimum of the voltage reading on the scope from pot minimum to maximum is ~0.05v, and it is the only thing that I can see change. I can do pictures if absolutely necessary, but I don't have the good breadboard wires at the moment. I just have longer jumper wires, so it is messy looking.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 25, 2017, 08:07:31 am
content: Post photos - messy or not. It is not a beauty contest.What happens when you probe Vcc? Does your scope show 5v?

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 08:21:37 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: May 25, 2017, 08:26:49 am
content: Well, for starters, I don't see pin 7 connected to 0V.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2017, 08:29:59 am
content: Quote from: RyanG on May 25, 2017, 07:54:33 amOK, make sure your Schmitt trigger is workingDitch the caps and diodes, connect one end of the 10k pot to Vcc, the other to ground and connect the wiper to pins 1 & 3.Start with the input voltage at 0V, verify that the output of both gates is close to 5V. Gradually raise the input voltage and check that, at some point, the output flips to 0V (that point should be about 3.5V with a 5V supply). It will be worth noting the input voltage at the trigger point - it won't be the same for each gate but will be close. If you want you can verify that as you reduce the input voltage from the first trigger point you need to reach a lower threshold (about 1V) before the output flips high again.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2017, 08:31:19 am
content: Quote from: Brumby on May 25, 2017, 08:26:49 am+1

author: rx8pilot
author_rank: Super Contributor
timestamp: May 25, 2017, 08:35:15 am
content: Put a bypass cap on Vcc, 1uf or so. It should be right at the Vcc pin.What signal do you see on the inputs?Sent from my horrible mobile....

author: rx8pilot
author_rank: Super Contributor
timestamp: May 25, 2017, 08:37:00 am
content: Yeah.... pin 7 is rather critical.Sent from my horrible mobile....

author: stj
author_rank: Super Contributor
timestamp: May 25, 2017, 02:14:37 pm
content: the schematic shows a 74AC series, not a HC series - the outputs are different.besides, using a cap/resister tap circuit often used to generate a reset pulse to create a self-oscillating stage is very poor design to be extremely polite.use a 555 like everybody else.if you need pulse-shaping, use a 555 to drive your HC schmit-triggered inverter.

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 03:18:26 pm
content: The pin 7 wire came out when I pulled the other ground wires for the picture by accident. I'm not that good at this but also not that dumb.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2017, 05:38:05 pm
content: Quote from: stj on May 25, 2017, 02:14:37 pmIt should work for the 74HC series too.What's wrong with this design? The only problem I can see is poor frequency stability and accuracy, which is a non-issue for a simple PWM circuit.Quote from: grumpydoc on May 25, 2017, 08:29:59 amYes, this is good advice. Check the Schmitt trigger circuit is working before doing anything else.

author: RyanG
author_rank: Regular Contributor
timestamp: May 25, 2017, 09:18:49 pm
content: I'm getting about 3.6v and 1.1v for the Schmitt trigger points

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2017, 09:51:43 pm
content: The next steps are to test the diodes and the potentiometer.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2017, 10:18:28 pm
content: Quote from: RyanG on May 25, 2017, 09:18:49 pmThat looks about rightJust to confirm that the circuit should work here is my breadboarded versionSlight differences from your original circuit, I could only quickly put my hands on a 5k pot so I increased the cap to 220nF. The resistor on the LED is 200ohms rather than 220, finally there's a 220nF decoupling cap on the supply (badly placed, BTW as it should be close to the chip but I just stuck it where it was easy). You can also see the logic analyser leads. None of those changes should make much difference to be honest.The 74HC14 is a pull I had lying around. Provenance unknown but that looks like a 1993 date code!And changing the duty cycle works as well although one limitation of this circuit does show up which is that the frequency is not independent of duty cycle varying from approx 1.2 to 1.5kHz

author: rdl
author_rank: Super Contributor
timestamp: May 25, 2017, 10:50:45 pm
content: I also put one together and it worked pretty well other than the frequency stability. I used a 10pf capacitor and a CD74HC14E. The square wave looked fine for a breadboard circuit. At midrange on the adjust pot, frequency was about 6 MHz and varied from around 5 to almost 9 MHz as the duty cycle was adjusted. I didn't see the frequency stability get much better when the cap was changed to get down in the single digit KHz range. This kind square wave generator is not something I ever really looked at. Maybe there are better circuits.

author: ebclr
author_rank: Super Contributor
timestamp: May 25, 2017, 11:21:22 pm
content: To easily check if your IC is goodPut 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

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 26, 2017, 12:17:04 am
content: 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.)

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2017, 01:50:22 am
content: 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.

author: stj
author_rank: Super Contributor
timestamp: May 26, 2017, 02:13:28 am
content: 74AC series is a bit special, it can sink AND source about 30maother 74 series generally only sink OR source, the oposing bias is simply done with an internal pullup/down resistor.

author: BrianHG
author_rank: Super Contributor
timestamp: May 26, 2017, 02:42:49 am
content: Lemon potentiometer?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 26, 2017, 02:53:39 am
content: Quote from: RyanG on May 26, 2017, 01:50:22 amHave 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.

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2017, 02:58:57 am
content: 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.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 26, 2017, 03:11:01 am
content: 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?

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2017, 04:21:46 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: May 26, 2017, 06:02:41 am
content: 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 oneSee if that makes any difference.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 26, 2017, 06:20:58 am
content: 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

author: grumpydoc
author_rank: Super Contributor
timestamp: May 26, 2017, 07:22:06 am
content: Quote from: edpalmer42 on May 26, 2017, 06:20:58 am+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.Quote from: stj on May 26, 2017, 02:13:28 am74AC 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.

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2017, 07:29:40 am
content: 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

author: grumpydoc
author_rank: Super Contributor
timestamp: May 26, 2017, 07:49:15 am
content: Quote from: RyanG on May 26, 2017, 07:29:40 amProbably best to grab some more from somewhere, I'd avoid ebay if possible.

author: newbrain
author_rank: Super Contributor
timestamp: May 26, 2017, 09:16:06 am
content: Quote from: grumpydoc on May 26, 2017, 07:22:06 amJust 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.

author: mikerj
author_rank: Super Contributor
timestamp: May 26, 2017, 09:24:55 am
content: Have you added a decoupling cap across the IC as suggested previously? Having the supply impedance too high makes logic devices do strange things.

author: ludzinc
author_rank: Unknown
timestamp: May 26, 2017, 10:59:03 am
content: Quote from: RyanG on May 25, 2017, 08:21:37 amHow 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 fromhttp://www.robotroom.com/PWM.html) to build a RGB colour mixer for the kids and it works a treat.

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2017, 12:20:27 pm
content: 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.

author: ebclr
author_rank: Super Contributor
timestamp: May 26, 2017, 08:00:11 pm
content: Do you have a scope to check the frequency that the single oscillator is running ?

author: BrianHG
author_rank: Super Contributor
timestamp: May 26, 2017, 08:19:33 pm
content: Quote from: RyanG on May 26, 2017, 12:20:27 pmThis 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).

author: bson
author_rank: Unknown
timestamp: May 26, 2017, 08:32:05 pm
content: 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.

author: edavid
author_rank: Super Contributor
timestamp: May 26, 2017, 09:04:46 pm
content: Quote from: stj on May 26, 2017, 02:13:28 amJust in case anyone is confused, the above isnotcorrect. 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.

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2017, 09:55:31 pm
content: 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.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 26, 2017, 11:01:47 pm
content: Quote from: BrianHG on May 26, 2017, 08:19:33 pmThat 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.

author: RyanG
author_rank: Regular Contributor
timestamp: May 27, 2017, 03:45:16 am
content: 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.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 27, 2017, 04:45:05 am
content: 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.Sent from my horrible mobile....

author: RyanG
author_rank: Regular Contributor
timestamp: May 29, 2017, 07:55:53 pm
content: 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.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 29, 2017, 08:10:49 pm
content: 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.

author: ebclr
author_rank: Super Contributor
timestamp: May 29, 2017, 08:10:54 pm
content: Basic troubleshotConnect only the ic and power nothing else1) Measure with a multimeter voltage between pin 7 and 142) 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 +Vcc3) 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 04) 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

author: rx8pilot
author_rank: Super Contributor
timestamp: May 29, 2017, 08:12:02 pm
content: Quote from: ebclr on May 29, 2017, 08:10:54 pmThat ^

author: grumpydoc
author_rank: Super Contributor
timestamp: May 29, 2017, 08:16:42 pm
content: Did you get some known good HC14's?

author: rx8pilot
author_rank: Super Contributor
timestamp: May 29, 2017, 08:21:15 pm
content: Quote from: grumpydoc on May 29, 2017, 08:16:42 pmWith @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?

author: StillTrying
author_rank: Super Contributor
timestamp: May 29, 2017, 08:23:38 pm
content: 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!

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2017, 08:55:22 pm
content: Quote from: StillTrying on May 29, 2017, 08:23:38 pmIt 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.

author: RyanG
author_rank: Regular Contributor
timestamp: May 29, 2017, 09:16:18 pm
content: I did all of the things ebclr said.1. The voltage was 5v2. Voltage on all pins was 5v3. Voltage on all pins was 0v4. Didn't see anything on the scope except for the flat line at ~2.4v5. Saw something on the scope for a few seconds then it flat lines at ~2.4vMy first gate wouldn't work at all for the oscillator part, I had to use the 2nd gate

author: ebclr
author_rank: Super Contributor
timestamp: May 29, 2017, 10:25:10 pm
content: 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

author: ebclr
author_rank: Super Contributor
timestamp: May 29, 2017, 10:26:40 pm
content: Can you please post a picture of your probe, specially the X1 GND X10 switch on probe please

author: RyanG
author_rank: Regular Contributor
timestamp: May 29, 2017, 11:24:18 pm
content: 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.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 29, 2017, 11:39:46 pm
content: Quote from: RyanG on May 29, 2017, 11:24:18 pmI promise.....it matters.

author: StillTrying
author_rank: Super Contributor
timestamp: May 30, 2017, 12:21:02 am
content: 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!

author: ebclr
author_rank: Super Contributor
timestamp: May 30, 2017, 12:23:26 am
content: Since you don't have a scope, you need to slow down to visible frequency, Do you have a 10uF electrolyte capacitor?

author: RyanG
author_rank: Regular Contributor
timestamp: May 30, 2017, 12:32:44 am
content: Yes I do have 10uf caps

author: stj
author_rank: Super Contributor
timestamp: May 30, 2017, 12:34:40 am
content: use one inplace of the ceramic

author: Nusa
author_rank: Super Contributor
timestamp: May 30, 2017, 12:35:58 am
content: 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.

author: ebclr
author_rank: Super Contributor
timestamp: May 30, 2017, 12:54:41 am
content: Use the 10Mf as your timing capacitor, on this way the Led will blink , and your eyes will be your scope

author: RyanG
author_rank: Regular Contributor
timestamp: May 30, 2017, 01:19:59 am
content: It does the same thing. It runs for a few seconds and then flattens out at ~2.4v

author: ebclr
author_rank: Super Contributor
timestamp: May 30, 2017, 01:41:05 am
content: What are you using to measure?Don't use your scope, The scope can be the real problemDid the Led blink ?Did you put a 330R is series with the Led ?Do you have Skype and camera?

author: RyanG
author_rank: Regular Contributor
timestamp: May 30, 2017, 02:42:24 am
content: 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.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 30, 2017, 04:00:42 am
content: Now you know..... it matters. :-)Sent from my horrible mobile....

author: RyanG
author_rank: Regular Contributor
timestamp: May 30, 2017, 05:46:04 am
content: 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.

author: ludzinc
author_rank: Unknown
timestamp: May 30, 2017, 05:58:17 am
content: Can you provide a pic of your layout?This time don't unplug anything - show us the exact layout you are trying now.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 30, 2017, 07:01:06 am
content: I doubt the scope is to blame, it just needs to be used correctly. All details as are important which is why more than one person has requested very specific information. You seem to be worried you will be judged on the equipment you have and that is blocking the help you are seeking.You should be able to make this work with no test equipment at all - a low end scope is a nice bonus.Sent from my horrible mobile....

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 09:46:19 am
content: There could also be a problem with the 'scope or probe. Try measuring the resistance of the probe with a multimeter. It should be 1M on the x1 setting and 10M on the x10 setting.

author: andybarrett1
author_rank: Regular Contributor
timestamp: May 30, 2017, 10:03:06 am
content: Quote from: rx8pilot on May 30, 2017, 07:01:06 am+1

author: RyanG
author_rank: Regular Contributor
timestamp: May 30, 2017, 06:09:29 pm
content: I believe the DSO138 does only 1X mode, and it is built into the board. The probe that it comes with has 0 ohms across it. I tore everything off the breadboard again today and rebuilt the whole circuit for about the 10th time, and it works.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 07:50:32 pm
content: Quote from: RyanG on May 30, 2017, 06:09:29 pmThen the probe or 'scope input is broken. 0 Ohms would cause a short circuit and it would behave exactly as you've described.QuoteSo is it working with the probe connected now? If it is, then it seems like 'scope or probe has an intermittent short circuit. Try flexing the probe's cable and checking the resistance at the same time. Another possibility is something metallic short circuited the input on the 'scope's PCB.

author: RyanG
author_rank: Regular Contributor
timestamp: May 30, 2017, 08:10:27 pm
content: It was working while controlling a LED. The scope has 1Mohm built into the board with no switch on the probe. I did not check the circuit with the scope.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 08:15:44 pm
content: Quote from: RyanG on May 30, 2017, 08:10:27 pmWell if what you said in the other post above "The probe that it comes with has 0 ohms across it." then you have a problem with the 'scope/probe. I was going to suggest you use another channel but I Googled the DSO138 and it only has one channel so you're SOL.

author: Nusa
author_rank: Super Contributor
timestamp: May 30, 2017, 11:31:03 pm
content: The "probe" that comes with the DSO138 is literally a short BNC cable with two alligator clips on the other end. However, the BNC output is already compensated and measures about 1MOhm. There are two trimmer caps on the board to calibrate it.https://youtu.be/F1oDhaqyIvk

author: RyanG
author_rank: Regular Contributor
timestamp: May 31, 2017, 01:52:21 am
content: That is what I was trying to say. Now that I have this circuit working, somehow, I wanted to know the actual best way to do a PWM signal for the full range of duty cycle from 0%-100% without a micro

author: StillTrying
author_rank: Super Contributor
timestamp: May 31, 2017, 10:36:55 am
content: I'm amazed that people still think the '14 stuck at 2.4V problem was/is something to do with a scope probe, or a bad scope probe, when I was reproducing it for 2 hours in Reply#57.Quote from: RyanG on May 31, 2017, 01:52:21 amhttps://www.eevblog.com/forum/projects/change-pulse-width-of-a-signal/msg1129000/#msg1129000If you're not too bothered about the frequency changing a bit many other methods are available.

author: stj
author_rank: Super Contributor
timestamp: May 31, 2017, 11:52:35 am
content: 2.4v should be impossible, unless it's the average of a very fast squarewave.a frequency counter or similar function in a multimeter would be handy here.

author: ebclr
author_rank: Super Contributor
timestamp: May 31, 2017, 03:15:26 pm
content: Duty Cycle from 0%-100 is impossible.O% and 100% means no oscillation , no cycle no duty

author: Brumby
author_rank: Unknown
timestamp: May 31, 2017, 03:20:14 pm
content: Quote from: ebclr on May 31, 2017, 03:15:26 pmFrom an oscillator output - I agree. However, if you wanted to, you could design a circuit that would switch out the oscillator and connect the output to 0V or Vcc - which would effectively give you the 0%-100% range.You could also program it out of a micro - if you were to include those in your range of chips.

author: RyanG
author_rank: Regular Contributor
timestamp: May 31, 2017, 04:05:04 pm
content: I understand that 0% and 100% aren't possible with oscillation. As far as the 2.4v thing it was definitely my cheap scope interfering, in my case. I'm sure it can be replicated in other ways.

author: rdl
author_rank: Super Contributor
timestamp: May 31, 2017, 07:41:42 pm
content: You can get close to 0-100% with a 555 timer as long as you're not too concerned with constant frequency. Dave did this in episode #392 if you haven't seen it.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 10:08:33 pm
content: The easiest way I know of to get 0% to 100% PWM, with jellybean parts and no MCU, is to use the LM393.

author: RyanG
author_rank: Regular Contributor
timestamp: June 01, 2017, 10:21:42 pm
content: This may not be the simplest way of doing it, but does this look like it would work? It works in everycircuit.http://everycircuit.com/circuit/6706632576991232I think it will only show in Chrome

author: MK14
author_rank: Super Contributor
timestamp: June 01, 2017, 10:26:54 pm
content: Quote from: RyanG on June 01, 2017, 10:21:42 pmNo, not apparently showing in Chrome.Ideally, it would be nice if you can post circuits in some kind of standardized format. E.g. image files.Something which needs to go onto a weird website and/or needs downloading executables is not a good idea, for easy viewing on forums.Quote

author: rdl
author_rank: Super Contributor
timestamp: June 01, 2017, 10:33:18 pm
content: I had this in my notebook from a long time back. I didn't have time to build it again to test until today. It works okay and you may already have the parts. With a 10k pot and 0.1 uf cap I got from less than 1% to over 99% duty cycle at about 650 - 925 Hz.

author: RyanG
author_rank: Regular Contributor
timestamp: June 01, 2017, 10:35:44 pm
content: I thought that the website I posted worked in chrome no matter what. I sent it to other people to check. Here is the picture.

author: StillTrying
author_rank: Super Contributor
timestamp: June 01, 2017, 10:44:17 pm
content: That can't be right.Edity: The first inverter and resistor don't make sense.Edity2: Is the value of R1 critical.

author: rdl
author_rank: Super Contributor
timestamp: June 01, 2017, 11:18:50 pm
content: It's not my circuit just one I found. Maybe I drew it wrong, but I ripped it up and built it again and it still works. Maybe someone else could give it a try? It's pretty simple and quick to build. Sorry for the poor quality image.

author: rdl
author_rank: Super Contributor
timestamp: June 02, 2017, 02:39:28 am
content: Quote from: StillTrying on June 01, 2017, 10:44:17 pm1. I drew the circuit close to the way I originally built it on perfboard so it doesn't look the way it's usually shown.2. The original circuit used a 4049 and I have R1 noted as "improves shape of square wave - use about 10x value of R2"Maybe it's the different chip, but R1 seems to make little difference here. As a matter of fact, at higher values and with small values of C1 and R2, it causes the output to become a sine wave at around 10 times higher frequency than expected. The circuit seems to work fine without it and just a wire directly to the input of the first inverter.

author: BrianHG
author_rank: Super Contributor
timestamp: June 02, 2017, 03:15:21 am
content: In that schematic, shouldn't R1 be between the diodes and cap with the cap wired direct to the first inverter input. Otherwise, when your trimpot is wired all the way to one side or the other, that circuit's output will sort of be short driving the cap distorting the second inverter's output when the trimpot is set to 0% or 100%.

author: ebclr
author_rank: Super Contributor
timestamp: June 02, 2017, 03:16:51 am
content: In my opinion, the capacitor must be in GND, for me no sense to connect on outputMy circuit will be like this one

author: BrianHG
author_rank: Super Contributor
timestamp: June 02, 2017, 03:41:18 am
content: Quote from: ebclr on June 02, 2017, 03:16:51 amThe advantage of placing the cap on the second inverter output instead of GND is that it increases hysteresis, making a clear cut guarantee not to be so sensitive to the small schmitt region. It artificially enlarges the size, but, the input may now exceed VCC & GND slightly during the transition.I guess there are different ways of wiring this circuit and will still function.

author: Zero999
author_rank: Super Contributor
timestamp: June 02, 2017, 07:47:42 am
content: That circuit will work with the capacitor going to +V too.There are plenty of variations on this circuit.