Safe testing with LEDs

LoopyByteloose

I have a microcontroller (in a radio controlled reciever)·that I want to test the outputs for ground or +5.

Can I use any old LED and a 470ohm resistor to verify the outputs?
My thinking is that the resistor will safely limit the current in the forward direction and that if the LED is backwards, the diode function will prevent and damaging reverse current.

Or, is there a risk that I will hurt the microcontroller?

I don't have a scope.· And, the instructions are in Chinese.
There are no specifications for maximum current outputs, so I am assuming 20ma and trying to use only 10ma.

I am having to·reverse engineering this.

Any· useful tips on how to safely do this kind of thing would be welcome.

UAEmbedded

This should work if the LED is capable of handling 5V reverse bias. Many can. A few cannot. I would probably increase the resistance to 1 kiloohm. < 5 mA should get enough output from an LED to be noticable without endangering much of any output pin.

LoopyByteloose

Thanks, I was thinking I should add more resistance. I may have already fried the unit as it nicely toggled on and off for a while then stopped (there goes $25US).

I had planned to use it as a remote control with the Basic Stamp once I verified the logic.

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G. Herzog in Taiwan

KenM

With 470 ohms in series it is unlikey you fried your device, unless you misread the color code and it is only 47 ohms?

;-) Herzog said...
Thanks, I was thinking I should add more resistance. I may have already fried the unit as it nicely toggled on and off for a while then stopped (there goes $25US).

I had planned to use it as a remote control with the Basic Stamp once I verified the logic.

LoopyByteloose

Thanks, but I have learned to check the resistors on a digital multimeter BEFORE using them.
It is much safer than misreading the value (in fact, I think I should get a capacitance meter for the same reason)

I just got back to the device to try 1K, but I found it is all working fine with the 470ohms.

The real problem is that I am not used to making good, solid connections with the breadboarding.
It seems that the resistor leads are not the idea 22 gage wire.

I spent a lot of time thinking about all this and nothing beats having a scope.
(I had an old Textronics 'boatanchor' that weighed about 75 pounds, but it was really well designed and stable)

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G. Herzog in Taiwan