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Randomplayer
modified 6 years ago

Some help please

3
7
174
02:55:05
I built this circuit on a breadboard with a 2N3906 PNP transistor, a 10V 3300 uf capacitor and tested it. Immediately the LED turns on and stays on. I don't know why this happens as a PNP transistor is only supposed to to turn on when the base is grounded. The function of this circuit is that the LED is on until the PNP transistor sees that the capacitor is below a threshold. Then the transistor should switch to charge the capacitor and quickly turn off. Could I please have some advice on why this happens? Thanks in advance.
published 6 years ago
alexpu
6 years ago
The collector of a pnp only needs to be ~700mV above the base to bias. Follow your voltages from battery to ground. 9V- 150mV voltage drop across the 270 ohm resistor = 8.85V. 8.85V - 730mV to bias the transistor = 8.12V. 8.12V - 5.71V voltage drop across the 10K ohm resistor = 2.41V applied to your LED. Take a look at this circuit. I replaced your 10K ohm resistor with a seperate battery. When the voltage is equal to the voltage at the collector, the transistor won't activate. Now change the voltage from 8.85V down to around 8V and it will begin to let current flow. I hope this helps! http://everycircuit.com/circuit/6573326854782976
Randomplayer
6 years ago
Your circuit helps but it sort of misses the point of what I wanted to create. The circuit U am trying to make is supposed to give a brief pulse to the capacitor so it becomes charged. This cycle continues so the LED turns on and begins to fade away before the transistor recharges the capacitor.
Randomplayer
6 years ago
When I said U I mean I (keyboard typo)
Randomplayer
6 years ago
Is there a way to create what I aimed for?
hurz
6 years ago
transistor capacitor are not needed for this effect. Modern LEDs will glow with very little current. Take a 9V DC a 10kΩ resistor and your LED. Connect them correct in series and watch the effect. Forget the transistor and capacitor, they dont make it brighter.
BillyT
6 years ago
There are many ways to achieve the effect you are taking about. The basic idea is for the base turn on voltage to disappear when you turn the switch on, you can use the transistor itself to blend the voltage away; try http://everycircuit.com/circuit/6091040514375680
Randomplayer
6 years ago
Thank you @hurz @BillyT @alexpu your comments have been very helpful and I will look further into this circuit and effect.
hurz
6 years ago
@Randonplayer, you call a glowing LED "effect"? Yes sure thats an effect, but not a big surprise. http://everycircuit.com/circuit/5121009433444352

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