timed led.

[troy310589]

HI, i want circuit that has an led that stays on even after the powersource has been disconnected, it doesnt have to be long, like 10 seconds or something, im guessing i will need a couple of capacitors, a transistor and some resistors.
(i did try a search for this, but im not sure what this type of circuit is called.)

Thanks

 

[troy310589]

ok, i have figured out that this setup will give me a fading led, (one that fades out) however i would like it to stay at a constant brightness untill it goes out (it doesn't have to be full intensity, but close) so could someone please tell me how i gould do this? im guessing transistor, but i have no idea where.
**broken link removed**

 

[Roff]

What is the supply voltage? How much current do you want for the LED? Will the supply literally be disconnected (with a switch, or by unplugging)?

 

[troy310589]

well, its for my Model Trains, i want a way so that when the train isstopped the LED (headlight) stays on for a little while longer, so the voltage is 12v, however this varies, to vary the speed of the train. oh, if possible could you tell me if there was a way to make the led gain full intensity, without the supply voltage being rite up? there is a resistor on the LED, so it doesnt blow.

 

[Hero999]

Use a constant current sourse like a FET with the gate connected to the source. You might want to consider using a battery on board your train if the voltage is likely to change and you want it to remain powered when the power is disconnected or you could also consider using a super capcitor to power your LED with.

By the way I doubt the voltage varies, I bet the speed controler uses PWM.

 

[troy310589]

What is PWM? either way, the light intensity does change when you turn the controller up or down. i would put a battery onboard, however there is little space, what is the smallest battery/ies i can use to power an LED? (maybe 2 in some cases)

 

[Hero999]

http://en.wikipedia.org/wiki/Pulse-width_modulation
http://www.solorb.com/elect/solarcirc/pwm1/

I was wrong earlier, constant current source probably won't work if it's a PWM controler.

You can use small 3V button cell to power the LEDs from - you know the sort use in those little LED key rings.

 

[troy310589]

aha, yes i know the ones, now igot ideas for a setup, but i dont knoe much bout transistors, a transistor is a switch rite? if i put into my setup somehow, does the switch wire have to be from within the same circuit? so the LED is powered from the battery, but the control/switch wire thingo in the transistor is connected to the train motor somehow, so when the train starts to move, it closes(?) the switch, passing current and the light works? and are transistors only on/off? not variale depending on the control wire?
Thanks

 

[Hero999]

You don't know what a transistor does?

Looks like you've got some reading to do then.

http://www.iguanalabs.com/1stled.htm
http://www.101science.com/transistor.htm
http://www.kpsec.freeuk.com/trancirc.htm
**broken link removed**

 

[troy310589]

ok that has sorta helped, i think my confusion is from the ones i got im testing with, i have a 9v battery, going to a 1.5k resistor, through an LED, then to the Collector, then from the Emittor, i got it goin back to the other pole of the battery, and the LED is on! only dull, but it is on, and the base is disconnected, what have i done here?

 

[audioguru]

i have a 9v battery, going to a 1.5k resistor, through an LED, then to the Collector, then from the Emittor, i got it goin back to the other pole of the battery, and the LED is on! only dull, but it is on, and the base is disconnected, what have i done here?

The collector and emitter of the transistor might be connected backwards, causing it to conduct with avalanch breakdown.
Sketch the circuit and attach it. Show the battery and LED polarities and how the transistor is connected. Show the part number of the transistor so we can see if it is an NPN or a PNP.

 

[troy310589]

**broken link removed**
from what i can read on this transistor:
C945
Q13ZA
i have nothing on the base, and the led is on!

 

[audioguru]

The base of a transistor should always have a resistor to its emitter to make sure it turns off. Without a resistor then heat, mains radiation and radio waves can turn it on. The resistor in series with the base can be the resistance to the emitter if the signal input to the base resistor goes to 0V.
The 2SC945 transistor has an unusual pinout for a small transistor:

 

[troy310589]

What size resistor you recommend? and wont that make the led dimmer?

 

[troy310589]

disregard that last comment, i found it works with a 470k resistor, a 100k it still lit ip a little bit

 

[audioguru]

What size resistor you recommend? and wont that make the led dimmer?

The LED will dim if the transistor doesn't have enough base current to turn on fully.
The 2SC945 transistor has a min current gain of 100, its base-emitter voltage is about 0.65V and the LED current in your circuit is only 4.7mA so the min base current in your circuit should be 47uA. A resistor from the base to the emitter which is 100k has a current of only 6.5uA so if the base resistor supplies at least 47uA + 6.5uA= 53.5uA then the transistor will be fully turned on.

 

[troy310589]

The 2SC945 transistor has an unusual pinout for a small transistor:

i most likely wont be using this transistor in my final gesign, its just whay i got to play with. honestly,after readin the above links, transistors still confuse the heck outta me, i thaught they were a simple on iff switch, and the base wws that made current pass from the collector to emittor, it just had to be over .3v or sumthin, but now i find that their smplifiers n stuff, i got more reading to do i hope to figure it out oneday

 

[catcat]

The basic operation of a transistor is that if you apply a current to the base, the transistor will conduct. In an NPN transistor the collector is the positive terminal and emitter the negative. In PNP, collector is negative and emitter is positive. When you apply a base current, (positive for NPN, negative for PNP) the transistor will conduct. The higher the base current, the more it will conduct. when the current reaches a specific limit, it will be 0 ohms (if we assume the transistor is ideal). This is how they can be used as amplifiers, the small input base current can control the larger emitter-collector current. You should use a 1k ohm resistor before the base to protect the transistor. To know which pin is which, place the transistor flat side tward you with the pins facing down; in NPN or PNP the left pin will be the emitter, middle the base, and right the collector.

 

[Roff]

To know which pin is which, place the transistor flat side tward you with the pins facing down; in NPN or PNP the left pin will be the emitter, middle the base, and right the collector.

There are many exceptions to this.

 

[catcat]

Thats for to-92 case pnp and npn transistors ONLY.