I don't understand. Help me! - Page 1
Forum section: electronics / beginners

author: Dat Tran
author_rank: Contributor
timestamp: August 07, 2015, 04:25:17 pm
content: I don't understand why the LED blink when i assemble circuit below:https://youtu.be/KToYHIfOVcEI use my body to conduct (any thing is conductive)any idea help meThank in advance!

author: 8086
author_rank: Super Contributor
timestamp: August 07, 2015, 04:27:02 pm
content: Your image doesn't work.

author: Dat Tran
author_rank: Contributor
timestamp: August 07, 2015, 04:52:51 pm
content: Yes. I fixed. Please explain me!

author: continuo
author_rank: Frequent Contributor
timestamp: August 07, 2015, 05:34:50 pm
content: Was this video taken with your cellphone or do you happen to have any other wireless devices nearby?

author: homebrew
author_rank: Frequent Contributor
timestamp: August 07, 2015, 07:11:21 pm
content: Obviously the transistor starts to conduct. Hence there is some current flowing through your body.The 2N222 has an hfe of around 50-100, thus the current through your body would only need to be around 0.1mA (or even less depending on the type of LED) to light up the LED.Where does the circuit power come from?Maybe an ungrounded SMPS? (Like a plugpack mobile charger?)

author: Dat Tran
author_rank: Contributor
timestamp: August 07, 2015, 07:45:30 pm
content: I used a mobile charger 5V. it's nokia mobile charger

author: retrolefty
author_rank: Super Contributor
timestamp: August 07, 2015, 07:49:56 pm
content: There is no resistor in series with the LED that would limit the led current to it's safe rated value. You are in danger of burning out either the transistor or the led if left on. Although I can't understand how the led could even light up with the base grounded. I suspect you have damaged parts or you didn't draw exactly what you built.

author: Gixy
author_rank: Regular Contributor
timestamp: August 07, 2015, 08:24:30 pm
content: You are 'earthed', not ´grounded'. Depending on your shoes, clothes, you potential is sufficient to polarize the transistor base. So it conducts but then, as retrolefty says, as there is no current limiting device, I suspect that your Nokia charger has too much current to source and maybe it's it which cuts power. And then the system recycles...

author: kezat
author_rank: Contributor
timestamp: August 07, 2015, 10:17:32 pm
content: Could the power supply be going in to over current protection or if its a weak battery powered deal it could be osculating seeing no resistor is added.

author: Dat Tran
author_rank: Contributor
timestamp: August 08, 2015, 10:12:58 am
content: Hi,retrolefty. I don't understand why LED blink. But I draw exactly as I assemble. I sure

author: Dat Tran
author_rank: Contributor
timestamp: August 08, 2015, 10:18:48 am
content: Hey, Gixy. I don't wear shoes or any clothes on foot. My foot on floor without clothes. I don't sure about Nokia charger but it still work

author: Psi
author_rank: Super Contributor
timestamp: August 08, 2015, 11:06:40 am
content: Im going to go with the simple answer.You used a 12V flashing LED, rather than a normal LEDThis explains why its flashing and why it didn't explode due to no current limit resistor.Flashing LEDs have a resistor and flashing circuit built inside.One way you might have come across a flashing LED is from an old car alarm.You can normally see a black blob inside flashing leds if you look close enough.

author: Dat Tran
author_rank: Contributor
timestamp: August 08, 2015, 01:25:07 pm
content: No, no, Psi. I used a normal LED. It has one anode and one cathode, not resistor.

author: onlooker
author_rank: Frequent Contributor
timestamp: August 08, 2015, 02:22:41 pm
content: You should try a few things to get the mimimum setup for reproducing the flash and report back.1. serial a resistor to the led and power them with batteries. Do you still see flash?2. Short the emitter to GND and keep hands off (and using batteries). Do you still see flash? (You said any conductive?)3. Use you finger touch only the emitter.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 08, 2015, 04:55:19 pm
content: I think the lack of an LED series resistor is the key. You've built an oscillator:- current flows into transistor base- transistor switches on- LED lights- power supply sees unexpected large current- power supply protection circuit shuts off the supply- LED goes out- supply is restored- goto 1Try putting a 1K resistor in series with the LED and see what happens then.

author: Gixy
author_rank: Regular Contributor
timestamp: August 09, 2015, 08:25:31 am
content: Hey Andy, that's what I suggested and the proposal of onlooker will allow to fix it.

author: Dat Tran
author_rank: Contributor
timestamp: August 09, 2015, 07:16:39 pm
content: Hey, onlooker1. I serial a resistor to led. When i touch the base-resistor, LED flashes once and then stabilize, not flashes again.2. When i touch emitter, LED no flashesThank you so much

author: Dat Tran
author_rank: Contributor
timestamp: August 09, 2015, 07:23:27 pm
content: Hey, AndyC_772You are right. i learned about Nokia charger, it is true that it will shutdown when excessive current, after it auto restart.

author: Dat Tran
author_rank: Contributor
timestamp: August 09, 2015, 07:26:16 pm
content: Thank all of you. i'm very happy because of explanation... One a gain, thank you so much