my final year project need some urgent help - Electronic Circuits Projects Diagrams Free

jitenderrawal · 12th April 2006, 08:11 PM

I am doing my final year project and i need some serious help i am using a 89c51 microcontroller and ULN2803 darlington transistor and four relays
how to connect the microcontroller with the ULN2803 and relay please helpgive me some idea as soon as possible
how to interconnect microcontroller,uln 2803and relays.

Papabravo · 12th April 2006, 09:09 PM

If there is a problem , it will be the quasi-bi-directional structure of the port pins on the 89C51. They can't source very much current, only about 60 microamps. That may be enough to turn the darlington on, but not very hard.

The data sheet from Allegro lists a typical input current of .9 to 1.35 milliamps.

If I were you I would use a PNP predriver pulled low by a port pin to connect the input of the ULN2803 to Vcc. That should turn the output on smartly! In addition your outputs will be off while RESET is asserted.

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12th April 2006, 08:11 PM	(permalink)
jitenderrawal	my final year project need some urgent help I am doing my final year project and i need some serious help i am using a 89c51 microcontroller and ULN2803 darlington transistor and four relays how to connect the microcontroller with the ULN2803 and relay please helpgive me some idea as soon as possible how to interconnect microcontroller,uln 2803and relays.

12th April 2006, 09:09 PM	(permalink)
Papabravo	If there is a problem , it will be the quasi-bi-directional structure of the port pins on the 89C51. They can't source very much current, only about 60 microamps. That may be enough to turn the darlington on, but not very hard. The data sheet from Allegro lists a typical input current of .9 to 1.35 milliamps. If I were you I would use a PNP predriver pulled low by a port pin to connect the input of the ULN2803 to Vcc. That should turn the output on smartly! In addition your outputs will be off while RESET is asserted.