Electronics > Beginners

Daves soft latch Video

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rscott:
So i have been messing around with this circuit for the last 3 years off and on.

I want to actually use it now. However, i noticed in his video, when power is applied, the circuit immediately comes on.

Why is this? What do we do to fix it?

Obviously we dont want the circuit to come on right away
Thanks

rcbuck:
I built the circuit on a breadboard to use in testing a circuit I was building a couple of months ago. It more or less worked. The on-off latch seemed somewhat touchy depending on how long you held the button down. It also required a certain amount of time between button presses to function properly. For my use it did work and I learned how to get around the shortcomings.

If I needed to build a soft latch circuit that had to be 100% reliable I would use a 50 cent PIC chip and software.

TomS_:
Maybe even one of those el cheapo Padauk 3 cent micros. I bet that thing costs less than all of the other components added together.

Of course you do have a one off cost of the programmer... But then you can have soft latching circuits or button debouncers for years. :)

Peabody:
If a microcontroller is involved, then the circuit becomes simpler since you can use a GPIO pin to maintain power-on after the button is released, and then turn off power after the button has been pressed (and released) again.  Attached are two versions of such a circuit.  One uses two GPIO pins - one output to maintain power, and one input to sense the button press.  The second circuit uses a single GPIO pin in INPUT_PULLUP mode for both functions.

Jwillis:
In Daves circuit the gate of the mosfet is charged from the 100k resistor upon power up. Won't it be better to some way have the switch at the gate of the mosfet to isolate the gate until the capacitor charges . So when the switch is pressed the capacitor charges the gate there by turning on the mosfet which turns on the transistor to keeep the gate charged but prevents the capacitor from charging from the power source.So when the switch is pressed again the discharged capacitor shunts the gate charge to ground turning off the mosfet. which turns off the transistor which allows the capacitor to recharge for next turn on.

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