Make a momentary switch control a toggle

Question

What are the simplest, cheapest, smallest ways to make a momentary switch produce a 2-state toggling output (latching momentary switch)?

In other words, the output is continuously low, and when you momentarily press the button/tact switch, the output changes to continuously high, and then when you press it again, it switches back to low.

Answer 1

Here's another alternative with two inverters and debouncing:

For logic output, only the two inverters, RC, and feedback resistor are needed:

NC7WZ14 is $0.06 with 2 inverters, small 6-pin package, and Schmitt trigger inputs, though I think this circuit doesn't even need Schmitt inputs because of the RC. Also it can drive 24 mA, so you could connect the LED directly to it if you're using an LED for whatever reason.

Also shown here and here as a toggle touch switch.

I think the feedback resistor has to be much smaller than the center resistor to prevent it from oscillating if you hold down the button, because the low-impedance connection to the output will prevent the capacitor from charging to the middle voltage until you let go.

Variants are described at Press ON - Press OFF soft latching circuits:

Answer 2

One possible method:

Use an RC to debounce the switch

and then feed it into a D flip-flop with the output feeding back to the input to implement a T flip flop.

The D flip-flop needs to be positive edge-triggered only, and needs an inverted output.

• The 8-pin NC7SZ74 should work, is very small, and costs $0.12 in quantity.
• The 74HC74 is a dual D flip-flop with 14 pins for $0.05.

Are there any issues with this? The switch would need to be held down for a short period of time for the rise time to trigger the clock input, which is probably good. If you hold down the button, it will only trigger once, which is good. Is there a better way to debounce in the feedback loop or something instead?

Answer 3

Or you could do it with a single tiny 6-pin microcontroller and no other components. It might cost a tiny bit more, but it's simpler and takes up less space. Debouncing a switch in software is simple, then it's just a manner of storing a boolean if the output should be high or low. The smallest version of this mcu comes in a 2mm x 2mm surface mount package. In large quantities it might be $0.50.

Answer 4

This circuit (also described here) is very cheap, but is more complex and takes up some space with all the components. If transistors are like $0.02 in large quantities, it would be maybe $0.10 total?

Answer 5

Make a T flip-Flop by shorting the inputs of a positive edge-triggered j-k flipflop and connect the pushbutton output to the clock input of j-k flip flop.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 6

Could you not just do it in software?

Will the button be an input on a programmable device, or do you need to do the toggle in the hardware realm?

By storing a button state in memory it's quite easy to use a momentary switch as a toggle.

boolean buttonState = false; // store for toggle state, false = off true = on

if(yourButton == HIGH && buttonState == false) // if button is pressed and toggled off
{
     Serial.println("Button Toggled On"); // do something while button is on

     buttonState = true // set button state to on
}

if(yourButton == HIGH && buttonState == true) // if button is pressed and toggled on
{
     Serial.println("Button Toggled Off");  // do something else while button is off

     buttonState = false // set button state to off
}

Sorry if this is off the mark, obviously this answer depends on you using an MCU or programmable chip.

Answer 7

I know it's kinda an "indirect" way of doing it, but you can get a hall effect sensor like sparkfun has: http://www.sparkfun.com/commerce/product_info.php?products_id=9312

and just use a magnet, I found out (the hard way) that this particular hall effect is latching, so it won't work for what I need it to, but looks like it could in theory work for what you're looking for.

I just plugged it into a breadboard, I think I may have used a resistor (It's been probably a month so I'm a little fuzzy on the details), and when I move the magnet close (momentary) it closes the sensor and there you go.

Answer 8

The easiest alternative is to use an alternating pushbutton switch.

If you ignore the fact that it latches at different positions, it's almost exactly like a momentary switch. I understand that this isn't the spirit of the question, but it is a simple and compact solution requiring no software and no external hardware.

Answer 9

Not sure exactly what the end goal is but I thought I'd put this out there:

This is a great little circuit I like to use to drive bistable relays. It's pretty cheap to build as well.

Pressing the momentary switch toggles the state of the relay and the LED. It's not really picky about how long you hold down the button. Since the relay is latching, it won't draw much current most of the time.

In this configuration, the LED will turn on when the relay is in its set condition and off when the relay is reset. You can also connect R7 to the + terminal of C3 to make the LED turn on when the relay is reset instead.

I like to use this in guitar effects so I can bypass or engage the device with a momentary footswitch. Of course a 3PDT switch would do the same thing on its own, but the switching is quieter this way (no big pop) and momentary soft-touch footswitches feel much nicer than the big 3PDT ones.

Hope this helps.