I followed this schematic almost exactly. I only replaced the BC547's with 2N2222's and some 10K's with 15K's and 1K with 1.5K.

here is the URL to the schematic and info:

http://www.geocities.com/dinceraydin/8051/

I did extreme tests to this circuit and noticed that the 2n2222's aren't working as inverters. In fact, it seems that not enough current is entering the transistor. so why 10K then?

I personally think these resistors should have a lower value so that enough current can get to the base of the NPN's.

I'm gonna use 220 ohms since my LED's dont blow up with 220 ohms in series with them under 5V.

 

Hello,

When you compare the BC547 with the 2N2222, you can see some differences.
The BC547 has higher HFE values and lower Vsat values.
See the attached datasheets.

Greetings,
Bertus

 

Hi Mike,
I suggest that you don't reduce the 10k base resistors below 2.7k. If you do, you place your computer's printer port at grave risk of being destroyed due to overcurrent.

The drivers for your printer port are very sensitive to ESD. It is all to easy to "zap" your printer port. If it's integrated in your motherboard and you zap it, your only repair options are to either use an add-on card, or replace the motherboard.

It's better to start off practicing with an add-on printer port card. They're super cheap nowadays. If you zap that, who cares? Throw it away and plug in another.

 

If your computer is of fairly recent vintage, then you will likely have a spare USB port. If so then you are in luck since you can purchase a USB to Parallel port adaptor such as this one. This way you can do as sgtwookie recommends and use the adaptor as a sacrificial port in case you accidentally damage the port.

I hasten to add that I am not trying to push this particular adaptor. I think WalMart sells one that is probably cheaper.

hgmjr