LED Flasher Circuit

[nicksydney]

Hi,

I've created a circuit to flash 1 LED using transistor and capacitor. Here is the circuit that I've created, the only thing I saw with the LED is when the first time I connect the battery the LED lit up and it fade away, it does not go through the cycle of lit up and dim continously. Can anybody help and advice me me what is the problem with the circuit ?

The component I'm using

1. 9v battery
2. Transistor NPN MPSA42 (the datasheet is attached with this port)
3. 100uF capacitor
4. 5mm LED (Red)

Thanks for your help.

Cheers

 

[dr peppers]

That circuit looks a little hard on the transistor, its shorting the supply.
I think you need a resistor in series with the supply at the least.
Youd be better with a 2 transistor multivibrator circuit, or better still a 555 chip.

Edit: see the post on this page 'single transistor oscilators'.

 

[audioguru]

You need an oscillator circuit to light and dim an LED over and over.
But your circuit is not an oscillator:
1) The LED, 470 ohm resistor and 47uF capacitor in series with the base-emitter diode of the transistor simply cause the LED to light up as the capacitor charges, then the LED dims as the capacitor approaches a full charge.
2) The base current for the transistor from the 4.7k resistor plus the charging current of the capacitor simply caused the transistor to turn on and short-circuit the supply voltage.
3) If the supply is from a nearly dead battery then the LED might light for a moment then dim as the transistor shorts the battery which causes the battery voltage to drop which causes the transistor to turn off. Then the battery refreshes its voltage and the LED tuns on and off again, over and over.

 

[nicksydney]

Hi,

I found this video that shows how to flash LED with a single transistor

I tried replacing the resistor mentioned in the video with 4.7K and the capacitor with 100uF it did not work ?

Cheers

 

[audioguru]

His drawing of the the transistor looks funny.

 

[ShawnR]

That is kind of interesting. I don't recall seeing BJT used with an open base....seems to work. The more I read, the more I realize how little I know.

 

[audioguru]

The transistor is connected upside down so its collector to emitter has reverse polarity. Then the emitter-base junction has avalanche breakdown (at about 8V) and the transistor conducts with its forward-biased collector base junction in series with the avalanche breakdown conducting emitter-base junction. They discharge the capacitor until its voltage is low enough for the transistor to turn off.