Push On - Push Off latching Circuit using an NE555 - Page 1
Forum section: electronics / beginners

author: dcherro
author_rank: Newbie
timestamp: February 25, 2020, 11:27:23 pm
content: Hello,I made a flip-flop circuit using theNE555to switch on or off some LED bars that I plan to put in my car.Currently I have the LED bars hocked to a SPDT relay and they are setup to come on when the car dome lights come on (closing or opening the doors)On port 87a on the relay I can either put a simple switch to turn them on when I wantor put my flip flop circuit so I can use a spare button (DSC button from a mazda 3) to make it look like factory OEM.Attached is the circuit schematic so you know what I'm talking about.The problem is the following:When I wan to turn the LED bars with the flop flop circuit it doesn't work as desired. They illuminate for a very brief second (fast flash) then go off.What do I need to change on the circuit so that they turn on ?Also, two videos.Video number 1 shows the circuit working with an yellow LED on the board.https://drive.google.com/open?id=1rYAsWHPrzEF-8H4ycrWxiZOjD1c6xuYZVideo number 2 shows the problem.https://drive.google.com/open?id=1Ok5R4Yos-i5ksmU7QHKmCk7PLgdjDi7ZThanks, looking forward to some help

author: Jwillis
author_rank: Super Contributor
timestamp: February 26, 2020, 01:26:17 am
content: Try increasing C1 to 10uF . Switch bounce may be causing a re trigger because the cap is charging to fast.I don't see the relay . Did you check if the relay will activate with out load ? The signal in may require more current then the 555 can output. The LED could be pulling the signal back down . Try without the LED.

author: dcherro
author_rank: Newbie
timestamp: February 26, 2020, 02:17:38 pm
content: Back with an update.Attached is the updated circuit I used for the flip flop.I changed the cap to a 10 μF, added a 0.01 μF from pin 5 to ground and a Diode to the signal output (N 4007).It now works as it should but there is one problem i'm having.When the relay switches off there is a small feedback that is beeing sent back to the flip flop circuit and for a fraction of a second the yellow LED on the board turns on and off.Is there a way to eliminate this interference ?I was thinking of adding a transistor BC 457, will that help ?If so, can someone please help with it's positioning in the schematic ?Thank you.Here is the video showing the problem.https://drive.google.com/open?id=1LMDLHygDmRUet_3lm714Ks1rPBgv7yYCClick 1 is a switch i use to turn on the relay (to simulate the car door opening). Click 2 is where I switch off the relay from the switch, that's when I get the small feedback in the flip flop.Also attached is a picture of the Relay I am using.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 26, 2020, 02:38:48 pm
content: What is D1's purpose?A 555 when driving a heavy load like the automotive relay + Led will have an output voltage which is about 2 volts less than Vcc. And then you add an extra diode drop to it!If you want to protect the 555 from the inductive kickback, place the freewheeling diode in its correct position, which is in reverse parallel with the coil.EDIT: I've not seen a 555 used in this way, thus I cannot comment whether the circuit operation itself is reliable.

author: dcherro
author_rank: Newbie
timestamp: February 27, 2020, 01:33:20 am
content: Quote from: schmitt trigger on February 26, 2020, 02:38:48 pmAll sorted out.The flywheel diode is in the correct position on the coil of the relay (pins 86 and 85) it's doing it's jobThanks to all that helped !