555 driven "Flash then on," circuit needs a switch for the LED...but which one? - Page 1
Forum section: electronics / beginners

author: Nohippychicks
author_rank: Contributor
timestamp: July 15, 2018, 08:53:57 pm
content: I'd like to make a series of LEDs flash, then go on steady; I'd also like to use 555(s) to do it, because I think it'd be fun.This is nothing more than trying to figure things out, but since I'm most familiar with auto and aircraft systems, I've been using 12V to power this:http://downloads.deusm.com/designnews/352-gadget102504.pdf. I also have a 12V wallwart that works easily with my breadboards, so that plays into the situation as well.I learned that what I want is called a "Brake flasher," or some variation of that, even though it's not going into a vehicle.I've tried several of the circuits that I came up with in Google, and the above linked one is the only one I was able to make work...but the ONLY way I was able to make it work was by removing the jumper from U2, pin 8 to +12v.I played around with C3 and C2 to get a series of flashes that I liked with just a single LED and resistor off of U2, pin 3.But that's where I ran into a wall.I figured that if I could get a single LED to blink then be stable on at the press of a button (listed as "Vehicle brake pedal switch," in the link) that it would be a matter of sliding a transistor in place of the LED I had from U2, pin 3 to +12, to drive a larger LED lamp array. When I put anything but an LED/resistor on U2, pin 3, however, all I get is a steady on at button press.I don't have the exact item called out for in the link for Q2 (called out is "IRF5210"...seems to be a P-chan FET, but an N-channel seems like it should be what's there...) but I've tried a 2222 (NPN), 30N06L(N-chan) and an IRF4905 (P-chan) and the best I can get is for the LED to turn on dim for a few seconds, then bright as long as the button is held.I don't have an O-scope, so I can't tell what's really going on at U2, pin 3, but my DMM reads 9-ish volts out there after the button is pressed and held, and if I have an LED hooked up between there and +12V, I get a very pleasing flash, then steady at every button press.It really seems like at that voltage output on U2, pin 3, I should be able to put almost anything on pin 3 to control ... FET, relay or whatever. I still get nothing more than an on/off with anything other than an LED-resistor load.All that being said, can someone point me in the right direction to drive a higher current load, have it flash then steady on at a button press, and maybe even break it down Barney style why your solution is good and mine failed?

author: romons
author_rank: Contributor
timestamp: July 15, 2018, 11:05:54 pm
content: How powerful is your 12V wall wart? 555s are pretty sensitive to a droopy power rail.Also, look at the 555 datasheet, and you'll notice that, while it isn't required to connect the rst pin, it is probably a good idea. Also, the CV inputs should be grounded through a small cap.The circuit you posted is for cars, which have a basically unlimited 12V rail, so it would probably work fine in a car. Just protect the power rail, either with a big cap, or power your LEDs from a different supply/battery (with the grounds connected, obviously)Good luck. I like playing with 555s. They have lots of uses, and are simple to program.

author: Nohippychicks
author_rank: Contributor
timestamp: July 16, 2018, 02:37:29 am
content: Quote from: romons on July 15, 2018, 11:05:54 pmI thought about this, but I'm not familiar enough with 555s to know how sensitive they are; the supply I'm using CLAIMS to be 2A. I think that should be plenty because I've seen lots of online 555 circuits running at 5V, but I'm not an electrologist, so maybe I'm wrong?Quote from: romons on July 15, 2018, 11:05:54 pmI think I've read this before, but I'm not sure how I should have pulled this out of the data sheet...do you feel like going into any detail as to why I should have seen this? What I linked to has the U1 reset floating and both CV (control voltage, right??) floating as well. I can't quite get to my little breadboard circuit to try adding a cap to those #5 pins for a bit. Do you have a suggestion for what value cap to put there? Would it have any effect on either the number of flashes or the speed of them?Quote from: romons on July 15, 2018, 11:05:54 pmI don't intend to run this in a vehicle, but as a learning point, would that "Big cap," you mentioned be run between +12V and -12V before the circuit?Quote from: romons on July 15, 2018, 11:05:54 pmThanks! I've tried to learn about 555s. When I put them in circuit on a breadboard, I seem to get some mixed results. I've been going off of some old books from RadioShack and what I've found online...honestly, a lot of the pages I've found online seem like English may not have been what they were originally written in, so trial and error has lead me to where I am now.Would having a power supply restricted to 2A cause the transistors I've tried to not modulate the LEDs? The LED I have on U2, pin 3, reliably does a flash then steady as long as I don't have some sort of transistor there. With a transistor is there, the LED switches on dim, then bright when the button ("vehicle brake pedal switch" in the linked diagram) is pressed.

author: romons
author_rank: Contributor
timestamp: July 19, 2018, 08:12:21 pm
content: I'm guessing that the one-shot that is controlling the RST pin on the second 555 is going off prematurely. That can be caused by droop, or by noise, or by the RST pin not being held up to V+.I'm not saying you should have known about anything. However, leaving the reset pin on the 555 floating is bound to cause problems. Just hook it up to the same point as the V+ input on the leftmost 555.The CV input is actually there to enable you to modify the frequency by changing one of the internal voltage points for the timing circuit. If noise gets on it, it'll cause inconsistent results, so the datasheet always shows a small cap from that input to the GND rail.Regarding the cap to prevent droop, put it between the V+ and GND rail, as close to the leftmost 555s as possible. Use a big one, like 1000uF or something, just to see if it helps.. When the flasher turns on your LEDs, it'll suck all the current out of the system, and possibly droop the power rail. Again, that can cause the one-shot 555 on the left to turn off prematurely, which will prevent the flasher from flashing.I simulated the circuit, but I haven't built it, so take what I say with a grain of salt.Regards