Lighting up an LED with 8051

[tonyctsiu]

I am using a TI CC2510. A 8051 compatible chip to control a low power infrared LED as a remote control.

I used some common TNT transistor to amplify the current for the LED.
The connection is as follows:
VDD->100ohms resistor->collector of the transistor-> emitor of the transistor-> ground

8051 signal port -> base of the transitor

When I use the scope to see the voltage across the LED cathod and anode, the upward edge seems to be ok, the downward edge seems to drops relatively slowly. It takes almost 0 us to rise while take almost 200 us to drop to zero.
As some of the pulse are just 100 us apart, this may stick the pulses into one pulse and cause error.

Is there any thing I can do for the current amplification circuit and the choice of the LED so that it can have faster switching time?

Regards

Tony

 

[srizbf]

i cant understand your interface.
you said TNT transistor. what is it ?

you are connecting a transistor to drive cc2510. why ?

 

[tonyctsiu]

It should be NPN transistor. sorry for the typo
I used the cc2510 GPIO port + NPN transistor to control the on/ off of the LED.

 

[IanP]

Try to connect stronger pullup resistor on the uC pin ..
This will give good "drive" for the NPN transistor ..
Try something like 1kΩ - 4.7kΩ ..

IanP


https://en.wikipedia.org/wiki/Pull-up_resistor

 

[keith1200rs]

The connection is as follows:
VDD->100ohms resistor->collector of the transistor-> emitor of the transistor-> ground

8051 signal port -> base of the transitor

Tony,

Where is the LED in this? I assume it is between the collector and the resistor?

Anyway, you should have a current limiting resistor between the I/O pin and the base (assuming the micro has a normal digital output). 1k or more, I would suggest.

I wouldn't worry too much about the voltage on the LED - it won't show a very accurate picture of the current through it. The best way to check it is with a photodiode or to monitor the current through the LED.

Keith

 

[millwood]

It takes almost 0 us to rise while take almost 200 us to drop to zero.

you should make sure that the particular 8051 pin you are using are NOT open-collector output.

and depending on the LED you are using, you may not need any amplification to drive the LED.

 

[tonyctsiu]

Yes, Keith, I agree with you. I think I can measure the voltage across the resistor instead in order to monitor the brightness of the LEDs.
Thanks everyone for the replies!