555 monostable always on - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: December 19, 2015, 12:08:20 pm
content: Hello all,So I am trying to get the following circuit to work in monostable single shot configuration.The idea being that when you press the momentary switch, it should fire one pulse for the duration specified by the RC timing constant (R2 + R3 + C2). Whether I just have a 1K resistor for R2 and whether the potentiometer is there or not, pin 3 is always high. It won't turn off.Am I doing something wrong?Regards,

author: Aodhan145
author_rank: Frequent Contributor
timestamp: December 19, 2015, 12:47:12 pm
content: Your switch is on the wrong pin. Replace C2 with your switch then it should work.I just glanced at the circuit.

author: made2hack
author_rank: Frequent Contributor
timestamp: December 19, 2015, 01:39:48 pm
content: Ok I will try that however according to555 Timer Circuitsit should be on the pin 2 - trigger.

author: Brumby
author_rank: Unknown
timestamp: December 19, 2015, 01:47:33 pm
content: Yes, I was thinking pin 2 for the trigger is right - but I wouldn't have C1 in there. There's no discharge path, so eventually it would charge up to your supply rail and you would cease to get trigger pulses reaching pin 2.

author: mij59
author_rank: Frequent Contributor
timestamp: December 19, 2015, 02:02:03 pm
content: The values of C1 and C2 are swapped, C1 = 10nF, C2 = 100uF.Also connect a pull up resistor to the switch e.g. 10k.

author: made2hack
author_rank: Frequent Contributor
timestamp: December 19, 2015, 02:08:29 pm
content: Quote from: mij59 on December 19, 2015, 02:02:03 pmyeah i noticed the schematic is wrong. I'll try also with a pull up to the switch.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 19, 2015, 02:18:10 pm
content: Yes, monostables are usually triggered by pulling Pin 2 low. Try eliminating C1 and changing R1 to 10k. C2 should be the 100 uF.

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2015, 02:53:37 pm
content: You also forgot the supply decoupling.

author: made2hack
author_rank: Frequent Contributor
timestamp: December 19, 2015, 04:50:22 pm
content: This is what I currently have. Tried with / without decoupling cap.Switched 555 IC, switched the BJT, moved things around the breadboard. At times, the LED would flicker when touching R4. But at no times did it work.I will try to solder it tomorrow, maybe the breadboard is the issue.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 19, 2015, 06:44:01 pm
content: Have you tried turning the LED around? (I know this sounds silly... but I have a batch of Red LEDs that have the "Cup" structure as Anode rather than Cathode, and at first I was using one of these and it flummoxed me until I turned it around.)I breadboarded the circuit exactly as above, using a BC337-25 transistor and no decoupling cap. It works fine, but with R2 as 1k and C1 as 100 uF the on pulse is quite short. I increased the resistor to 10k and now it is behaving reasonably. A quick contact of the switch produces about 1 second On time of the LED. With a 470 uF as C1 I get about 5 seconds On.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: December 19, 2015, 07:15:22 pm
content: The other thing you could try is to remove the transistor and sink the LED directly into the 555. The 555 can easily sink a couple hundred milliamps so ~5mA from the LED and 1k resistor will be fine.If that works, check your pinouts on the 2n2222. Presuming it's in a TO-92 package, double check the datasheet. Some have pins pictured from above and some from below. Also different manufacturers have different pinouts. For example. Fairchild's PN2222A is EBC while OnSemi's P2N2222A is CBE.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 19, 2015, 07:26:08 pm
content: Quote from: Paul Moir on December 19, 2015, 07:15:22 pmDoing that will invert the behaviour of the LED though. That is, the LED will be ON by default and pressing the switch momentarily will turn it OFF for the monostable pulse time.Quote

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2015, 09:48:44 pm
content: Quote from: alsetalokin4017 on December 19, 2015, 07:26:08 pmNot if the LED & resistor is connected between pin 3 and 0V.

author: made2hack
author_rank: Frequent Contributor
timestamp: December 20, 2015, 09:22:26 am
content: OMG!!!Finally got this POS to work. It turns out not only the first 555 timer I had yesterday was bad, so was the second one with which I had replaced it.I felt like suck a tool today, and not the good kind. So, after replacing it with yet another fresh 555, it worked like a charm, both with the LED connected to pin 3 and with it going through the 2n2222.Now, all that remains is to connected this to some optos and see if I can control a Triac.Remember kids, when taking components from old project boards, make sure you use different project boards. Don't take them from the same project board and, if the first fails, assume the second IC will work.

author: made2hack
author_rank: Frequent Contributor
timestamp: December 20, 2015, 09:48:23 am
content: Here's the final schematic if anyone cares. Any idea how I can implement a 1 sec. delay before I can fire the next shot?The reason I ask is that, given a small time constant (11ms -> 121ms) like I have here, it might take longer to press / depress the momentary button, thus allowing longer pulse time (it re-triggers essentially).

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 20, 2015, 09:55:45 am
content: Quote from: Hero999 on December 19, 2015, 09:48:44 pmOf course, but that is different than just "removing the transistor and sinking directly to the 555."

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 20, 2015, 10:20:17 am
content: Quote from: trs80 on December 20, 2015, 09:48:23 amYou could try using two 555 monostables in series (or a 556), or you could try a somewhat different architecture, the Edge-Triggered Monostable 555.Or maybe try a different chip like 74LS121.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: December 20, 2015, 08:28:54 pm
content: C4's purpose is to prevent retriggering while holding the button down. It's supposed to provide a pulse low when the switch is depressed. It really ought to have some high value resistance shunting it to discharge it rather than rely on self-discharge.C4 charges up to about Vcc when you press the button. When you release it and quickly press the button again, the trigger input sees a continuous Vcc since C4 hasn't discharged much. So increasing C4 should provide an increased retriggering delay.I breadboarded your circuit with a LM555 (they're all little different) and it looks like a 1uF with a 1M shunt gets you about that 1 second retriggering delay. That's not surprising since t=R x C, so t=(one million x one millionth) = 1 second.PS - to the other posters, the LED thing in my previous post was meant solely as as quick diagnostic. Easy to try without ripping up too much of the output stage. I was guessing the OP was intentionally going for increased output current for some yet unrevealed purpose, but covering my guess by hinting at the 555's drive capabilities.PPS - bypassing the 555 isn't optional if reliability is important. It takes a good chunk of current when it switches. I noticed a few days ago that someone did a nice video on it:

author: made2hack
author_rank: Frequent Contributor
timestamp: December 24, 2015, 09:29:48 am
content: Hey paul,How did you connect the shunt resistor?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 24, 2015, 11:59:44 am
content: The shunt is simply connected across (in parallel with) the C4 capacitor to allow it to discharge.See my post here, where I used a 10k resistor across the cap:https://www.eevblog.com/forum/beginners/spectacular-failure-of-a-triac/msg826451/#msg826451(the capacitor is now numbered C1)In that scheme, repeated button presses do not cause re-triggering, until the first-triggered monostable pulse is completed.(Scopeshot below is with a 470 uF and 10k timing cap and resistor to give a long output pulse)