Help with 555 timer not switching off - Page 1
Forum section: electronics / beginners

author: dystopia
author_rank: Newbie
timestamp: May 12, 2022, 04:32:20 am
content: I want to build a timed switch to which turns on a closet light for a fixed amount of time when the doors open. This is for sliding doors, so going to use reed switches as the triggers and use a 555 as a monostable multivibrator for the timing part. The light itself is a generic 2835 based LED light strip.Here is the circuit I have hooked up on a breadboard:The problem I have is this: if I provide the circuit the full amount of current (~1.13A), the LED strips never switches off. If I limit the current to 0.5A, the LED strip does turn off.Here are the traces I see on the scope:[ Specified attachment is not available ]These traces trigger at the time when the 555 output should be going low (~11s) and repeat every ~20ms. My guess was this I am seeing inductive spikes because of the breadboard and perhaps the long/poor LED strip wiring. That's why I added the flyback diode.But nada!I can't also explain why is the voltage at Pin 3 at 7.5V while the VCC is 12V?Any help will be greatly appreciated!

author: pqass
author_rank: Super Contributor
timestamp: May 12, 2022, 06:48:06 am
content: Quote from: dystopia on May 12, 2022, 04:32:20 amI'm not sure what you mean by "...I provide the full amount of current..." It's the load that decides how much current it needs. The 'scope output doesn't show that your PS is current limiting.Why don't you first try to put a simple LED (with resistor) on pin 3 without MOSFET or LED-strip. This should confirm that the timing is working right (and for the duration and voltages expected). Then add the MOSFET and load. Use a separate wire from the PS positive terminal to your LED-strip positive (rather than from your breadboard rail). This will make sure you aren't starving your 555 circuit when the higher LED current kicks on.Non-CMOS 555 output (pin 3) typically won't go as high as Vcc but should be a volt or two below it. At 7.5V, that may not be enough to fully turn-on the MOSFET; it should be provided 10V to be sure it's fully-on. Maybe a dud MOSFET? Divide and conquer.BTW, your circuit seems to work. See attachment. Click below to play with it in the simulator.http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0lwrOGAmOyDs6DMkAcyA2ATiPRHknPMvgFMBaMMAKDAMWXgPHW61R7cwAFijMATiEZCiyKWxDJcuKEjiRmAZ3nclK6SCzw5lCABdxAV1rMA5jsPGHWAisoaAxg70hhKn6YMZEzQJGG46ETqcGCIMDESvv7KSY4maurMZqk+THIB4PJgoWFEuPB+ULDRYKilRASRwlhRKWBwIAAmtABmAIaWADZmzADu4LWGkKJ5aWL2s0Zyi66qGvaVS6kubmKSs2CyE-no6e2ZksIKIqJXuniq55njBofLk29i4wdHbKZHnl8ClQKjuU1E7mq6ggtSh0XhwkgEHiSLsQL403RhgE7jGx0Up0UXAJJkSf3AvwIlE43FM8MB5Jp4Cpc0hNSq0MOJGUnPCtOqLAWLK25J2azxjOJTKZGnG0qlUoesqJugVGIhzC8-EoWBc2J1uAh4CCIHQJXqFtIHNR9m1hkN+t8HVxtqcur4TkR7i+WJujr9yqwDpBWJDyvJnwjwg13xZfrtAeYACVmaZo6mQA01k7wOlIfA8UHRMhEb4syXcZdy6WuNTS3S4KwCHJa4oa8LMXJuv0hmZ6INaJ1Ct74oLyB3RK27biAPbMkAEI2IsqKNnqUjsZtxRTzrDkeDwAA6mgACgBJAByx+QzDnWAgi9Uy-8kORO-vhn3R9Pl+PWFv4AqI+lDPkgVRvssKh7u0ACqABix4AMIAIInswQA

author: Zero999
author_rank: Super Contributor
timestamp: May 12, 2022, 10:43:37 am
content: It appears to be a power supply problem. Disable the current limit should fix it. Increasing the supply decoupling capacitor to 10µF will also help.The circuit attached to the original post pulls the base voltage below the emitter by over 5V, which won't do it any good. Fortunately the BJT isn't required, if the switched can be moved to the positive supply.LEDs aren't inductive, so there's no need for the reverse parallel diode across the LED strip.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 12, 2022, 11:18:35 am
content: If it "works" on your breadboard, but not with the full current, then my first suspicion is some voltage spike caused by the switching that triggers the 555 again.

author: Ian.M
author_rank: Super Contributor
timestamp: May 12, 2022, 11:50:32 am
content: https://www.edn.com/the-ne555-current-spike/Anyone building stuff with bipolar 555 timers is going to have a frustrating time if they don't know the 555 needs *LOTS* of decoupling. 100uF in parallel with 100nF directly across the Vcc and Gnd pins is suitable for most applications.

author: Zero999
author_rank: Super Contributor
timestamp: May 12, 2022, 03:15:02 pm
content: The 555 timer is not a beginner friendly IC. The CMOS variants such as the TS555, TLC555 and 7555 are much easier to work with, because they don't draw huge power supply surges.I'd even consider going discrete. The classic two transistor monostable will work, with one device being a BJT and the other a MOSFET.