momentary button as onoff toggle using 555

Posted on Feb 4, 2014

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

momentary button as onoff toggle using 555
Click here to download the full size of the above Circuit.

Forrest Mims electronics books and a few 555 timer devoted websites (two of the best I found were: & ), I cobbled together the following circuit based off a few almost-what-I-wanted examples. 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`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. 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...

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