Try connecting the anode of the led to +12v through a resistor, then connect the cathode of the led to the emmiter of the pnp tranny, then connect the collector to ground, a logic zero on the base of the tranny through a 10k resistor should light up the led, you need also to connect a diode like a 1n4148 in series with the base of the tranny, cathode to micro and anode to tranny (beacuse when the micro goes logic high - 5v there will still be 12v - 5v = 7v on the base still turning it on).
On 19 Jan, 04:39, David Forbes <dfor...@dakotacom.net> wrote: > On 1/18/12 9:29 PM, Thomas K. wrote: > > > > > > > I have been trying to get this circuit to work for a few days and it > > is driving me nuts! > > > I'm trying to make an array of tricolor LEDs (RBG with common > > cathode). The goal is to use 5v logic to turn on MPSA92 and light up > > the array.I have 4 LEDs with three anode resistos (one for each > > color). I'm working with just the red now, as the others will be > > clones of the same circuit. > > > When I connect the emitter to to +13v, base to ground or +5 through > > any resistor and collector to the array, it lights up. Only when I > > disconnected the resistor (open), does the array turn off. > > > I have tried base resistors from 200 ohm to 200k ohm, and the same > > deal. When I use +5v on the emitter, however, the circuit works as it > > should. I feel like I am missing a fundamental property of PNP > > transistors. Any suggestions? > > Connect the emitter to the same voltage as the base circuit, and it will > work. The base-emitter junction is a forward-biased diode, so any more > than 0.3 volts will cause the transistor to turn on. > > A better design would use common-anode RGB LEDs and an NPN transistor > pulling each cathode of the LED to ground with a 0 to 5V signal on the > base through a series resistor of >10K value. > -- > David Forbes, Tucson AZ- Hide quoted text - > > - Show quoted text - -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to neonixie-l@googlegroups.com. To unsubscribe from this group, send email to neonixie-l+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
