Source
r/raspberry_pi•
Octetz

Cable between 3.3v and GPIO with on/off switch

I want to have a couple of leds powered by the 3.3v output. I want to have a on/off switch on the cable, and I want to know when the leds are on and off.

I have two problems.

The leds cannot be connected 3.3v - GPIO as the gpio in does not allow enough ampere to let them shine bright. If I connect 3.3v - (GPIO & GND) then I cant identify if the switch is flipped or not.

The second problem - If I ignore that the lights are dim - when having the switch on i get a clear on signal, but when I have the switch off - I guess the cable works as an antenna and rapidly switches between on/off on the rpi.

I would really like a solution where I get bright shining leds and I didn't have to get a switch that grounds the wire when off (currently it is a simple table-lamp-light-switch).

Any Ideas?

Posted Jun 5, 2015, 1:17 PM UTC

1112
richifie•

Not sure I wholly understand the issue, but it sounds like it can be solved by a field effect transistor?

2
OctetzOP•

Sorry, I tried to read the wiki for Field effect transistors - but it didn't help me.

Basically I want this 3.3v --- [switch] -- [LED] -- GPIO, so that when the switch is hit the LED shines and I can read GPIO input to see the state of the switch. My two problems are that, the GPIO cannot drain enough to make the LED shine bright, and, when the switch is off then the input on GPIO changes between on/off because it is not connected to either ground or 3.3v. (I guess the second problem is lesser than the first)

1
richifie•

Why not a SPDT that's connecting a power supply to an LED and connected to the GPIOs?

1
OctetzOP•

I have not been able to find an SPDT that looks like a lamp button, and I only manage to think out a solution that solves the GPIO input flickering problem, but not one that solves the problem with the LED not shining bright.

(Btw, assuming SPDT means http://en.wikipedia.org/wiki/Switch#Contact_terminology )

1
[deleted]•

[deleted]

0
AK_Chrism•

You could do this with a transistor. I don't remember offhand which pin is which (between Base, Collector, Emitter), but you can feed 3.3V to one pin, supply LED with another, and then switch it on and off with GPIO (allowing the 3.3V to the LED or not).

Edit: here is a quick schematic I found online (which is a little different from what I descibed (The LED's Ground connection is allowed/disallowed via the transistor):

http://apps.usd.edu/coglab/psyc770/images/trans5.gif

1
OctetzOP•

I think what I am trying to do can seem confusing. I want to use the GPIO to determine weither the switch is on or off, not use the GPIO as the switching mechanism.

Maybe I simply need to do things differently - with a separate wire pair for the switch and then light the LED using software - but I would really like the led to light instantly and the software to detect the change of the switch... and I need to have a "lamp like" button on the cable (eg http://www.ebay.com/itm/Table-Lamp-Desk-Light-Cord-Cable-Switch-Button-Switch-/331443863229?pt=LH_DefaultDomain_0&var=&hash=item4d2b97febd )

1
AK_Chrism•

Oh gotcha, yea I misunderstood what you were after.

Perhaps the transistor could be inserted for your purpose, still though. Base (the Transistor On/Off control) connected to your lamp switch and 3.3V source), Pull-up resistor between GPIO and another voltage, and GPIO->GND connected to the Collector/Emitter. This should result in a High State on GPIO while the switch is off and a GND State when the switch is turned on.

I would have to play around with the components a bit to know that I'm not leading you astray regarding the connections, but I certainly think that a Transistor is what you want.

1
OctetzOP•

So- Maybe

3.3 --- [Switch] -- [LED] -- GND
                          |
                    [Transistor]
                   /            \
          (Drain) GPIO       3.3V (Source)

Would the GPIO input be low when the switch is off?

1
jmole•

Put the switch on the low side, tie the non-grounded side of the switch to a microcontroller input

1
jmole•

You can also use a pull up resistor on the input pin here.

1
OctetzOP•

Ah, This works! It requires me to add a wire up to the switch, but it works!

Sorry, I didn't understand your comment about the pull up resistor.

1
percykawas•

Check out the Pi EzConnect board I just launched on indiegogo web site. Using that you can easily connect the LED's across each GPIO, drive them from the GPIOs with a current limiting Resistor. If you need additional power - you can use a MOSFET for higher amps - but you may need an external power or an added power source for that.

A blatant plug

http://igg.me/at/piezconnect

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