stan2004 posted:Did you guys actually prototype circuit #2? I think the "OR" circuit which turns off the Green LED when Red OR Yellow is on is skating on thin ice.
The Yellow and Red LEDs are turned on when one or the other 555 chip is ON (at pin 3 output). This ON signal does NOT go to the positive supply voltage because there's an output transistor as shown in the 555 internal schematic above. As the curve shows on the right, the "ON" voltage might be about 1V below the DC+ supply voltage. Then each Yellow/Red output goes thru a diode which further lowers the voltage.
So this transistor + diode voltage drop now tries to turn off the 2N3906. While a diode appears to have been added above the 2N3906, it's not clear to me that either the Yellow or Red output generates a high enough voltage to turn OFF the 2N3906.
I am a little behind Rod in trying these. He has this circuit working, but I am having what I believe is the exact problem you describe here with this circuit and it does not work properly for me. We are both using 556s instead of 2 555s, but I am using an NE556 and I think Rod may be using a TS556CN (not exactly sure on the exact part he has?). I plan to try it with a couple of 555s next. I have 3 different types of those, NE555, LM555 and a TLC555CP. There might even be one other variety around here after I start hunting them down.
As you describe above, the Base voltage on the PNP transistor (circled) is always about 0.6 to 0.8 volts or so below the voltage coming from the DC supply through the diode. Base voltage never quite gets high enough to turn off the green LED when it's supposed to be the the red's or yellow's solo performances. The red and yellow operate, but the green stays on all the time no matter what.
Rod is using S8050 (3) and S8550 (1) transistors. I have tried those but was mainly using BC547 (3) & BC557 (1), I have a bunch of those and only a few of the others. Also tried a 3906 for the PNP. I don't really think this matters much though? I think it's the shortage of voltage output on pins 3 & 9 of the 556 exactly as you suspected.
