Thanks, this works well. I needed a simple pulse generator with separate frequency and pulse width controls that don't interact. I put your circuit into SPICE and massaged the values to get the range I needed, then triggered it with a separate NE555 astable oscillator. Perfect!
I'd like to see the actual values you used and how your test circuit is wired up. Schematic EVERYTHING.Give links if you don't want them in the video. Switches generally don't bounce on opening. Wire them up accordingly. Or add RC to swamp the bounce (~100mS is what we did in the days before TVS and when CMOS had no protection diodes. Gone are the days. Thank the MAKER!!. I'm older than transistors. ) This circuit is EXACTLY what I was looking for. TY-V-V-M !!!
For those asking about values - I built this using 47KΩ and .015uF at the input. The timing circuit consists of a 10uF capacitor with a 10K pot. I added a 91Ω fixed resistor in series with the pot to limit minimum pulse width. These values provide an adjustable pulse width of approximately 400uS to 130mS. I recommend use of a 10-turn pot for easier adjustment. All the formulas needed to ascertain values are in the NE555 data sheets, but I found it easier to model this in LTSPICE and adjust values by trial and error for the exact range needed. Hopefully the values I used will give you a starting point.
Absolutely great! There are tons of videos and examples with NE555 as monostable circuit, but Reset-Pin 4 is always at Vcc and never used. I needed exactly the shown behaviour: Waking up an ESP32 from DeepSleep with a short positive Pulse from 555 Output, while the (high) trigger IN stays high very much longer (30secs) then ESP Routine runs before it switches to DeepSleep Mode again.
6:07 This is exactly what I was looking for, thank you very much! I built the circuit with 0.01s as the time constant for the input circuit (pin 2) and 0.2s as the time constant for the second circuit (pins 6 and 7). As expected, the output LED lights up for about 0.2s as I press the push button, however it also briefly (less than 0.2s) lights up when I release the push button! Any idea what might be going wrong? Unfortunately I don't have access to an oscilloscope. Many thanks!
My best guess is that the push button bounces at the release, or at least it doesn't toggle off in a clean way. This causes a new trigger event that only lasts as long as the bounce since the Master Reset quickly goes low again, terminating the cycle sooner than it would normally. The limitation of this circuit is that the ON time can't exceed the trigger high time. (I said that wrong at the end of the video at 5:24) Hope that helped!
@@5VLogic Thank you very much for taking the time to answer. Switch bouncing was also my very first guess, however it was very easy to rule out as I had some SN74HC14N Schmitt triggers which were promptly put to use :) I eventually found the answer myself and I intended to post the answer here for anybody to use the info, however I got caught up with other things and I didn’t take the time to do it. Since you did answer I now felt it was the right thing to write this up :) I tried the same circuit with a few different types of 555s, 556s and CMOS TS555s that I had around and it was exactly the same with all of them, down to the fact that the second time the LED went on (after _releasing_ the button) it stayed on for a shorter time than when it first went on (after _pressing_ the button). This was a clue already that it was probably linked with the lower capacity capacitor on the trigger pin. So I was really confused why it didn’t appear to be happening with your circuit. I can only presume that the issue was/is present in your circuit as well, however I reckon it was masked by a smaller capacitor (you said the RC constant has to be small(er) however you didn’t state the actual R and C values you used for it so I cannot verify this). I guess it was so small though that you just couldn’t observe the LED turning on for such a brief period of time. If you have any interest to test this hypothesis I suppose you could use another 555 there in the monostable or bistable configuration. But it all comes down to this: there is a hidden issue with the explanations from 3:28 to 4:10, and more exactly at 3:44 - the charge in the capacitor connected to the Trigger and Reset pins doesn’t _instantly_ go from High all the way to Low, as you made it appear at 3:45, it takes a wee bit of time to drop, just like it took to charge. During that wee bit of time though it directly supplies a positive voltage to the Reset pin, just enough to make the 555 NOT reset _yet_, like you said at __3:52__. It turns out that while using 5V Vcc (the _minimum_ specified Vcc for 555 is 4.5V) the Reset pin has a lower threshold for a High level than the Trigger pin has (the latter is 1/3 of Vcc). There’s an article somebody published where he tested different 555 Cis and he found that “The threshold voltage is exceptionally low - lower than any true logic device. Specifications indicate a threshold range of 0.4 to 1.0V in bipolar devices, and 0.4 to 1.5V in CMOS device. Actual measurements indicate a range of 0.65 to 0.75V (bipolar) and 1.1V (CMOS)” (you can search for “Quirky 555 Timer Reset Function” if you want to read the entire article, I’m not including the direct URL here because YT has a habit of just deleting posts which include URLs). So, while that capacitor is discharging it will first reach a voltage just below 1/3 Vcc (~1.67V with a 5V Vcc) but which is still higher than the threshold for High on the Reset pin (which is below 1.5V, see the article I mentioned above), at which point the first comparator will set the 555 output to High (not prevented by Reset _yet_), which will briefly turn the LED on, until the voltage on the capacitor drops below the threshold for Low on the Reset pin, at which point the 555 will finally reset and switch off the output. I hope I managed to explain this well. Your circuit was still a very good inspiration though for what I need and with the addition on two NPN BJTs and a few resistors I think I fixed it. I even simulated it and it’s doing the job I wanted. If you want to see/check the simulation use the 2yw89uxm code with tinyurl com (again, I don’t want to include the URL here so YT doesn’t silently delete the entire post). I reckon if you really wish you could play around with the biasing resistors for the 2 BJTs to further fine tune the switching level for the Reset pin. Hope this helps somebody :)
@@5VLogic Your idea for the Reset trick helped me with the issue I was struggling with (generating only a short pulse following the rising front of a very long pulse - I'm just learning the ropes here), so, yeah, thank you, too! :)
