[TriEmbed] "Latching" device

Shane Trent shanedtrent at gmail.com
Thu Apr 10 12:35:46 CDT 2014

Pololu has a device that lets you turn on an output (FET) and hold the
output on until you turn it off with a digial I/O line. It is available
off-the-shelf, small and $6. I have a vague recollection that Pololu
received a patent on this product.

Pololu Pushbutton Power Switch LV, 2.5V - 7.0V

Instructable on using this item with Arduino.

On Thu, Apr 10, 2014 at 11:38 AM, Pete Soper <pete at soper.us> wrote:

>  In my browser the vertical bar from the cap is further to the right than
> you intended, but I think we get it.
> So the capacitor will be discharged by the mcu pin's input resistance in
> series with the resistor. I agree this should work wonderfully as long as
> the cap is made large enough to keep the voltage high "long enough." If the
> latching action is only needed for a minute or two then a conventional cap
> may work well. For a longer time a supercap might be required.  The MCU (or
> expander port chip) data sheet and the resistor value could be used to
> calculate the time constant. But that's for a perfect cap: the super caps
> will approach this, but regular electrolytics will discharge faster where
> the other resistance involved is very large.
> A key issue is whether the resistor helps or hurts. If the latch needs to
> stay valid for a couple days, for instance, then when the pin is set to
> output and driven to ground it could take a very long time to discharge the
> capacitor. The software also has to keep track of that discharge time,
> re-testing the pin to confirm it's discharged or ignoring the pin "long
> enough" to avoid being tricked by another button press happening too soon
> after the earlier one.
> So the size of the capacitor in relation to how long the "latching" must
> last as well as how fast the discharge has to go would allow judging
> whether this circuit would meet Brian's requirements.  But it surely wins
> the simplicity award!
> If two pins could be used a classic SR latch should work. This would be a
> pair of gates (such as two CMOS nor gates) where the pushbutton applies a
> potential to the "set" input and the mcu pin senses Q or Q bar and applies
> a potential to the "reset" input. Here's a description<http://en.wikipedia.org/wiki/Latch_%28electronics%29#SR_NOR_latch>
> .
> -Pete
> On 04/10/2014 09:31 AM, Bill Farrow wrote:
> I was wondering if there is a "tricky" way you could do this using
> only a single pin, a capacitor, and a resistor.
> When the push button connects it charges the capacitor. The MCU pin,
> configured as an input, can read the state of the capacitor. When the
> MCU wants to reset the latch it can change the pin to an output and
> set it to 0 (low) which discharges the capacitor to ground via the
> resistor.
> Let's see how bad my ascii art is:
> mcu pin --- resistor --- switch --- 3.3v
>                              |
>                        capacitor
>                              |
>                            GND
> Bill
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