(as a few had noticed, I had an error in the schematic shown. It’s been updated, thanks!)
A recent question from a friend who made a really cool BlinkM hoodie was: How can you turn a momentary button press into an on/off toggle?
There are tons of ways to do this if you like getting into electronics. Most all work off of some flip-flop like principle. And while I could have suggested a true flip-flop chip, I thought it would be cooler if you could use a 555 timer chip (which contains a single flip-flop and a couple of comparators). After scouring my childhood collection of Forrest Mims electronics books and a few 555 timer devoted websites (two of the best I found were: http://www.bowdenshobbycircuits.info/ & http://www.kpsec.freeuk.com/555timer.htm), I cobbled together the following circuit based off a few almost-what-I-wanted examples.
This is what it looks like in use:
The schematic is pretty straightforward, but does use a bit of feedback trickery to get the toggle functionality:
(old incorrect version here)
The parts cost is pretty low. The 555 timer chip can be had for about $0.43, the 2N3904 transistor for ~$0.40, and various resistors & capacitors are essentially free if you have them.
The circuit has 3 external two-pin connections: 5V&Gnd, button input, and the two pins of the thing to switch. In this case, the switched thing is a power supply to a BlinkM.
By changing the transistor to a beefier one, you can switch much larger loads. The little 2N3904 transistor in there now can switch around 200mA, but a bigger NPN or FET transistor and you could switch a few amps.
It can be made pretty small on a tiny breadboard (courtesy of FunGizmos.com) like this:
It’s not the greatest for battery-powered applications. When “off” it draws about 4-6mA, depending on the brand of 555 timer chip you use. When on it draws that plus whatever power the switched device draws. Best to put a proper power switch on the battery pack to eliminate this quiescent drain.
now ive managed to get this working however i cant seem to get this to pulse aswell, is this possible?
Basically i want this so when the tactile switch is pushed the led will flash rather than stay on.
cant seem to get it to work
I was wondering, do you know of a way to make sure the circuit is always defaulting to “Off” when you power it up? I have the circuit working, but when I initially turn on the power supply I get the LED on sometimes and off sometimes, even though I have been making sure the circuit is in the off position when I power it down. I’d like this as a safety feature, but I lack the insight into that problem. Thanks in advance for any information and thanks for posting this. As a newbie I really needed to see the circuit in another form besides the schematic to make heads or tails out of it.
Gumby,
Disconnect pin 4 from +5V. Put a 10K resistor from pin 4 to +5V and a 0.1uF capacitor from pin 4 to GND. That will give the circuit a short reset pulse when power is applied.
Just curious if there is a way to also keep the timer function, as in – press once, circuit is on for 1 hour then turns off or turns off immediately if pressed again while on.
Hi,
Is there a 555 timer circuit which can toggle between three different states. If yes can you please post the links. It will be really helpful for me.
thanks
moralinverter
i have found that using a 3906 transistor as opposed to a 3904 transistor works exactly as in the video. i had no success getting the 3904 to turn the led off. i popped a 3906 in and it worked just fine.
This is exactly what I’m looking for!
Thank you! :)
(iMac G5 20″-hack into a standalone monitor – view my webpage).
THANKS!!!
I’m a beginner, and doing an ardiuno project. for the project I needed 12v (for the arduino) 5V for a relay and 3.3 for a lcd screen backlight. so I ended up using a ATX power supply.
So I took your sketch and used the 5Vs.b. to “short” the 3.3V sp-on ….
So now I can switch the ATX on by the momentary push button … HAPPY :)
Thanks again, great post
perfect circuit! I was wondering if it’s possible to power it with a 12V power supply, the datasheet states that the 555 can operate at 15V…, does anyone know??
Never mind, I found this website:
http://www.bowdenshobbycircuits.info/page9.htm (chapter: “Relay Toggle Circuit Using a 555 Timer”)
Thanks anyway!
This is exactly what I’m looking for but is it possible add one more push button for reset??? I mean- if circuit is switched on reset push button will all the time reset circuit and circuit stay off
hi :) great little circuit.
ive been trying to replace the button with a transitor or an opto isolator. So the latch can be triggered from a logic 1.
i wonder,has anyone any ideas what part could replace the manual switch?
: P