NE555 Timer - Monostable Operation - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: April 30, 2023, 07:11:10 am
content: I am trying to build a monostable circuit from a NE555 Timer, but find it difficult to understandhow to choose suitable component values. I can not find any article or video explaining the choices made while building, or referencing back to the datasheet.From the circuit configuration I've ripped from the Texas Instruments datasheet, I have the following questions: RC NetworkApart from the RC Network being used to control the timing delay of the monostable operation, is RAalso used for the purpose of limiting current/voltage to either Pin6-THRES and Pin7-DISCH? If so, how do we calculate a suitable range of resistors to use?The output pulse duration is governed by the equation tw=1.1RAC. However, if five time constants is approximately the time it takes for a capacitor to charge to full capacity, wouldn't the pulse duration then be 2/3 of the five time constants instead? So the equation would be tw=3.3RAC? Decoupling capacitorIt is mentioned that this capacitor connected to Pin5-CONT is optional and only required to improve the operation of the timer by cleaning the input signal to the comparators. Texas Instruments recommended a value of CL=0.01uF. Are there any other possible values this can take? How would we determine appropriate valued capacitors to use? Pull-up resistorI've seen this pull-up resistor mentioned in some articles, but videos on YouTube have a different configuration of this circuit and usually omit it. I'm wondering how to calculate the value of this RLbecause when I built my own circuit using a 10K resistor, the LED at Pin3-OUT just stayed on and my button presses weren't doing anything.And just an aside, when reading datasheets what does Part 7.4 Electrical Characteristics tell us? If I'm looking at the typical threshold current for an NE555 Timer, the datasheet tells me this Pin operates at 30nA. If I'm using a 9V source, then that means my RAshould be 300MOhms. But this seems like an absurd value...

author: iMo
author_rank: Super Contributor
timestamp: April 30, 2023, 08:12:24 am
content: The 555 is one of the most discussed chips ever.. For last 50 years already..The chip includes 2 comparators and compares the timing capacitor voltages at two levels, 1/3 Vcc and 2/3 Vcc.The capacitor never charge itself to "full capacity" as you wrote, but to 2/3 of the Vcc only.That is the trick with the 555. Thus the capacitor gets charged/discharged between those two levels, or up to 0V (based on the configuration). There are tons of on-line calculators and simulators for the 555 where you may get the idea behind.The suitable range of resistors depends on whether you use a CMOS version or a bipolar version of the 555 - see the datasheets for the ranges.Decoupling capacitor - 10-100nF is ok.Pull-up resistor - it is rather the "load", not a pull-up. The value of the resistor depends on the current you want sink/source, approximately the current through the RL is I=Vcc/RL.https://en.wikipedia.org/wiki/555_timer_IC

author: jonovid
author_rank: Super Contributor
timestamp: April 30, 2023, 08:24:17 am
content: best place to visit for 555 timer circuits isTalking Electronicshttps://www.talkingelectronics.com/https://www.talkingelectronics.com/te_interactive_index.htmllots of tips here. also easy to understood projects for beginners to electronics.

author: sy
author_rank: Regular Contributor
timestamp: April 30, 2023, 11:06:08 am
content: Okay so I've tried building the circuit. When I connect the circuit to the battery, nothing happens -- as expected -- however, when the button is pushed, the LED seemingly stays on forever until I disconnect it from the battery. I suspect something is wrong with the capacitor not charging over 2/3 VCC to trigger the threshold comparator reset.I have a 9V battery, however I measured it to contain only 8.1V.The decoupling capacitor was 10pF as suggested on the Texas Instruments datasheet:https://www.ti.com/lit/ds/symlink/ne555.pdfI used a 10KOhm resistor for the output to protect my LED.As for the output pulse, I wanted to have a 5 second delay so I chose 100KOhm and 47uF for my RC network.tW=1.1*100K*47u=5.17secsI have attached an image of the build without pressing the button.RED: power from my 8.1V batteryBLACK: groundYELLOW: holds my trigger pin high until button is pressedGREEN: pin 6 and 7 inputs from the RC network

author: gamalot
author_rank: Super Contributor
timestamp: April 30, 2023, 12:15:10 pm
content: Quote from: sy on April 30, 2023, 11:06:08 amI thought maybe that cap should be 10nF, a 10pF cap wouldn't make sense here.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2023, 12:23:20 pm
content: Quote from: sy on April 30, 2023, 11:06:08 amWhere the heck does the data sheet recommend a decoupling capacitor of 10pF? The only circuit I saw in that data sheet, which has a decoupling capacitor shows 0.01µF, or 10nF, which is 1000s times bigger than 10pF. I would opt for a much larger value, say 10µF.Quote8.1V shouldn't be a problem.There doesn't appear to be any supply decoupling. That 10pF capacitor is on pin 5, not across the supply. Connect 10µF across the supply pins, as close to the 555 as possible. Although 10nF is suggested on pin 5, I've found it often requires more than that to be reliable and would recommend swapping that 10pF capacitor to 100nF.Contrary to popular belief,the 555 timer is not a beginner friendly IC. A decent layout and proper supply decoupling are essential, for reliable operation, especially in monostable mode.

author: gamalot
author_rank: Super Contributor
timestamp: April 30, 2023, 01:12:19 pm
content: I tried this and it works perfectly, even without the capacitor connected to pin 5.https://www.electronics-tutorials.ws/waveforms/555_timer.html

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 30, 2023, 01:21:17 pm
content: Datasheet clearly states typical 0.1uF bypass cap.Other smaller valyes are for timing circuits only.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2023, 01:44:31 pm
content: Quote from: gamalot on April 30, 2023, 01:12:19 pmIt clearly has a supply decoupling capacitor though.How does it respond when powered up? Does it sometimes trigger, during power on? If so, that's what the capacitor on pin 5 helps to minimise.

author: gamalot
author_rank: Super Contributor
timestamp: April 30, 2023, 01:57:13 pm
content: Quote from: Zero999 on April 30, 2023, 01:44:31 pmIt works without any capacitors, except the 47uF one for timing, the LED stays off during power up.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2023, 02:14:48 pm
content: Quote from: gamalot on April 30, 2023, 01:57:13 pmLooking, closely at your photograph, it doesn't appear to be the standard NE555 timer, but the TS555, which is a CMOS version and is more tolerant to poor supply decoupling. The original poster is using the bipolar NE555. Try using that in your circuit.

author: gamalot
author_rank: Super Contributor
timestamp: April 30, 2023, 02:57:11 pm
content: Quote from: Zero999 on April 30, 2023, 02:14:48 pmHere's the clear picture, you can see the mark on the top of the NE555, and of course when a chip costs less than two cents you have every reason to suspect it's not genuine.I measured the resistance between pin1 and pin8 is about 18K, the resistance between pin1 and pin5 is about 12K, and the resistance between pin5 and pin8 is about 6K, so it is more like NE555 not the low power version TS555.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2023, 03:23:11 pm
content: Try repeatedly cycling the power. . .Just because it works for you in this instance, it doesn't mean it's good practice or that it's not causing the OP's problem. I have built plenty of 555 monostables. The most recent one did have proper decoupling, but it did still have the issue of occasional false triggering, during power on.

author: gamalot
author_rank: Super Contributor
timestamp: April 30, 2023, 03:57:21 pm
content: Quote from: Zero999 on April 30, 2023, 03:23:11 pmI'm certainly not saying don't use decoupling caps, I'm just saying that the 555 monostable isn't that difficult and I'd love to see what would happen if the OP copied exactly the circuit I used.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 30, 2023, 05:04:03 pm
content: Your pushbutton is probably wired incorrectly- they can be a pain even after you think you’ve identified the the correct pins to use with an ohmmeter.The trick with any of those square snap action switches is to use any two pins - on opposite corners. Any two pins on opposite (diagonal) corners will always be a normally open contact that closes when pressed.In your circuit, just move the black wire on the switch down to the pin directly below the currently connected pin.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 02, 2023, 12:01:58 am
content: For this sort of situation, "don't overthink it" is probably a fairly good philosophy.The 555 is a pretty rugged chip compared to more modern ones, it is not too easy to destroy unless you do something really silly. Experimenting with different values is probably a good practice for you here. Especially for something like the RC network, have a play around with different choices of R and C so as to get the same product. Look at the range of resistor and capacitor sizes you have to hand to get a feel for the maximum and minimum values you can get for each.A hint about decoupling capacitors, as long as it is fast acting any reasonably large(ish) value will do. 0.01uF can be ok, but the important thing here is to use ceramic capacitors which react very fast, not electrolytics which can be quite slow. I often use a ceramic 1uF to 10uF* for decoupling, because it is ceramic it is just as quick as a lower capacitance ceramic. Above 10u you might start to hit situations where you get big voltage spikes when the circuit is first powered on or where you get weird behaviour for a while during initial power-up if your power supply has a low maximum current. 100n to 10u is typically a good range, or have both sizes of cap in parallel, but make sure they are ceramic.*10uF as a ceramic can be pricey and trickier to find mostly focused on SMD physical format, as you sound new to electronics you are probably using through hole parts on breadboards, so SMD is something you don't need to worry about. There are a very few 10u ceramics made with through hole legs, 100n is easy to find as a through hole ceramic, and 1u reasonably find-able too.

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 02, 2023, 01:50:28 am
content: Quote from: Infraviolet on May 02, 2023, 12:01:58 amFast acting? So what’s a slow capacitor?

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 02, 2023, 02:29:31 am
content: Quote from: Infraviolet on May 02, 2023, 12:01:58 amBy "quick" you mean it has better high frequency decoupling capabilities than an electrolytic?A good cheap compromise is to use two capacitors in parallel rather than a "expensive" 10uF ceramic.ie: The OP could use a 10uF electrolytic in parallel with a 0.1uF ceramic instead.

author: sy
author_rank: Regular Contributor
timestamp: May 02, 2023, 11:22:03 am
content: Quick update. I've checked my circuit over again and realized the trigger portion of the circuit wasn't working as intended -- one of my insulated wires were broken from inside somehow. Also swapped my IC for a new one, and boom it worked. Somehow I fryed the old one, it doesn't work properly.That aside, whenever I'm pressing the button to ground the trigger pin, I'm finding that it doesn't activate the circuit all the time and sometimes I have to press it a couple times for it to activate -- as sometimes I only see the LED flash on for a split second and turn off again. I'm not sure if this could be due to switch debouncing? The other thing is that I've also measured the resistor used for my trigger circuit, but when I'm holding the push button down it reads a 0.6V voltage drop, which means the trigger pin is not being completely grounded.I've built my circuit with the same values as @gamalot however with an 800Ohm resistor instead of 3.9KOhm at the output.Edit: the circuit not activating all the time was because I built the trigger circuit wrong, rebuilt it and it works fine now.My next question is how do I know what values to be putting in? Cos so far I've just copied a circuit. What are some things to look for when designing with this chip? And what values specifically from the datasheet?

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 02, 2023, 04:00:22 pm
content: You can use an online calculator or do the math manually. (You can find these formulas in the datasheets)The problem with most online calculators is they allow ridiculous component values that won't work in real life. So you have to look at the datasheet and understand those limits.ie: Don't use resistor values less than 1K or so...