Hi again. I have a doubt concerning the parallel port. I need to power a 555 chip & one LED diode at time. Can I use the parallel port as a power supply or external supply needs to be added? Schematic in attachment, thank you!

 

1. About that three transistors LED driver, you should series a resistor to each B of transistors.

2. You can control NE555 through Pin 8 or Pin 1.
a) Pin 8 - Disconnect Pin 8, using one stage of PNP transistor, c connects to Pin 8.E connects to +V, B series a resistor connects to printer port, you also should be add a resistor named Rbe to b,e of transistor.
b) Pin 1 - Disconnect Pin 1, using one stage of NPN transistor, similar as LED driver in your circuit, B series a resistor connects to printer port, E connects to GND, C connects to pin 1.

 

Nice! Thank you.

 

I don't want to open a new topic for this little question, so here it is.

I built a circuit with 4017 counter. Everything works fine, but only if I hold/touch the GND or Vcc wire with my hand. Otherwise the whole circuit works randomly. I connected a 47uF electrolytic cap across the supply wires, but that doesn't help.

Here is the circuit. http://www.kpsec.freeuk.com/projects/trafficlight.htm

Might be worth to mention, the problem disappears if I disconnect the reset pin from the GND line. But then the circuit doesn't work because the reset pin goes high, thus preventing normal operation.

Any suggestions appreciated.