Smartest way to add a 'switch' to this circuit via IR trigger?

Discussion in 'The Projects Forum' started by gnolivos, Jul 14, 2011.

  1. gnolivos

    Thread Starter New Member

    Jul 14, 2011

    I have built a circuit that will allow for 2 LEDs to gradually light up when switched ON, and then gradually fade out when switched OFF.

    All works fine as is, and here is the circuit:

    Now, I want to control the switch via a PIR Motion Sensing Module. The module is this one from FUTURLEC, and I have tested it with my ~ 4.5V source and works fine.

    The issue is that the output trigger is high at just 3.3V, so I cannot use this to feed my main circuit, nor is that the intent. I want to use this as a 'trigger' to actuate my main switch on the circuit linked above.

    I toyed with the idea of using a NPN Transistor, Can anyone help me determine if this is possible? I tried a few things and gave up with no success on my design.

    I also thought about using a relay, but that seems overkill, and I cannot even find a relay that will trigger at 3.3V (only see 5V relays). I am trying to stay away from relay, so if anyone can help me with simpler alternatives, (transistor or otherwise) that's the help I am looking for.

  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    It appears that your LED circuit is using a 4.5V source, which can be the same source you use to power the Motion Sensor. an output of 3.3V High is more than enough to bias the transistor(by the way, the transistor in the circuit IS an NPN). The only issue I see is that if the motion stops, eventually so will the Cap charging and the LED fading. Not sure if this is what you want or not. Also, the datasheet suggest that the output can be High or Low.
  3. gnolivos

    Thread Starter New Member

    Jul 14, 2011
    Thanks for your reply. Indeed, I meant adding an NPN transistor, much like the one already in the circuit... that was my mistake. be aware, the transistor in my circuit serves a different purpose... I want to add a SECOND NPN to help build a switching mechanism to replace the current switch (assisted by motion sensor). Hope that makes sense.

    Motion stopping is not an issue. Already tested that.

    Anyhow, I tried modeling the added transistor with a 3.3V source, and I think this will require my entire circuit to be re-assesed for new R,C values. The current seems to change significantly throughout as a result. But it looks possible... I was mistakenly under the impression that to build a switch, the voltage feeding the collector had to be equal to that of the base... not so?

    I tried modeling the circuit by adding