As Per already mentioned, it is an 74HC32.

Directly at port pin Px,y is a 4k7 pull-up. This pull-up and the OR-gate are needed, because the standard port output cannot drive the following BC337 allone. Some other micros could, but the AT89S52 cannot!


It's not only the flashing that is critical, but also the huge current that can be drawn if all LEDs turn un simultaneously. This can make the power supply pump on and off all the time, if the Vcc of micro is derived from the lamps' supply and if the supply isn't stiff enough. I wouldn't recommend to make any changes on the design.

Nevertheless, if you don't want the OR-gate connected to the MAX1232 at all, disconnect it. But do not omit the OR-gate, otherwise, the micro will not be able to turn on the BC337 properly! Connect the unused input of OR-gate to 0V then.

Behind the OR-gate there is a voltage divider of 10k and 4k7. This makes, that the BC337 doesn't need 0,7V for turning on, but about 2V. This is no problem for the OR-gate at all, and helps to suppress the effect of noise on ground due to the switchings.

The BC337 has a 1k collector resistor. Another 1k resistor goes to the gate of MOSFET. These resistors are critical. Do not make them smaller or bigger, because the BC337 cannot withstand much higher currents if you make them smaller and, on the other hand, the MOSFET will not turn on and off fast enough if you make them bigger.

For the MOSFET I have taken a IRF640. Not because this MOSFET is specially useful here, but because you mentioned to have it in your hands.

By the way, do you want to programm the AT89S52 via ISP? Then you should implement the circuitry shown here:

http://www.8052.com/forum/read/109412

Kai Klaas