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What transistor do I need?

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Visaggio

New Member
Hey folks,

I'm trying to build this LED flasher circuit that I found online. I've got limited electronics experience, but i'm learning more everyday.

Here's the circuit diagram:

I've got everything i need except the transistors. Looking at the Radio Shack website, I see a stunning array of available transistors and I've no idea what i should be looking for.

Some more info on the application:
I'll be using an 11.4v Lipo battery in an RC airplane. I'll be powering a 3.6v*20mA*1100mcd white LED.

I've determined that the transistor on the left is a NPN and the transistor on the right is a PNP, but i don't know what values i should be looking for.

The resistors are no issue, but would a "1.0µF 35V 20% Dipped Tantalum Capacitor" (electrolytic capacitor) be appropriate?.....(radioshack model # 272-1434)
 

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audioguru

Well-Known Member
Most Helpful Member
The transistor part numbers 2N3904 and 2N3906 are on the schematic and are very common. The author thinks that RadioShack has them.
 

Visaggio

New Member
Heh, now that you've pointed out the obvious, i see the part numbers right on the diagram.

Thanks for your help :)
 

Hero999

Banned
Any capacitor would do, it doesn't matter whether it's tatalum, electrolytic, polypropylene or ceramic.
 

Visaggio

New Member
OK, I've got the parts as shown on my diagram, but my circuit blows the LED. Considering I've negligible electronics experience, would anyone have a suggestion on where i might be going wrong?
 

audioguru

Well-Known Member
Most Helpful Member
OK, I've got the parts as shown on my diagram, but my circuit blows the LED. Considering I've negligible electronics experience, would anyone have a suggestion on where i might be going wrong?
R3 in the circuit is 470 ohms and is in series with the PNP transistor and the LED. It limits the current in the LED to 21mA which will not blow the LED.
 

fingaz

Member
Hi,

You need to check to make sure the circuit is 'exactly' right. What have you built your circuit on? (strip-board/breadboard etc). Make sure there are no short circuits, no solder blobs joining parts of the circuit which shouldn't be joined (it only takes a tinybit of sodler to cause a short, try looking with a magnifier). Also check the value of R3. the purpose of this resistor is to limit the current to the LED to make sure it doen't get damaged. Based on the info you have given for your LED and the battery (11.4v), R3 needs to be AT LEAST 390 ohms. Make sure you have the transistors the right way round (check the pins) look at a datasheet if you need to.

Hope this helps
Neil
 
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Visaggio

New Member
OK, I'm using a breadboard. I tried to take a pic of what I've got, but the pics won't come out well. Either the flash overpowers the pic so you can't see, or (without flash) the pic comes out too fuzzy.

So i made a representation of my breadboard in Excell and took a screenshot of that.

Would you mind taking a look and see if it makes any sense?
 

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crutschow

Well-Known Member
Most Helpful Member
Try taking a picture of the circuit in sunlight.
 

Roff

Well-Known Member
The image still comes out fuzzy, (taken in sunlight) but here it is.
It looks to me like your transistors and your diode are backward. If that's the case, high current will flow through the circuit as shown below.
I have seen this circuit with a resistor (≈470 ohms) between Q1 collector and Q2 base. This will protect you from problems such as this. It will also save you in case you have the wiring correct, but too low a value for R1, which could otherwise cause something to die.
 

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Mikebits

Well-Known Member
OK, I'm using a breadboard. I tried to take a pic of what I've got, but the pics won't come out well. Either the flash overpowers the pic so you can't see, or (without flash) the pic comes out too fuzzy.

So i made a representation of my breadboard in Excell and took a screenshot of that.

Would you mind taking a look and see if it makes any sense?
That must of took a bit of work. Clever way to show a schematic. Pain to read but clever.
 

Visaggio

New Member
OK, i can see where i was reading the transistors incorrectly. On the diode, the dark band at one end indicates the negative, correct?
 

Roff

Well-Known Member
OK, i can see where i was reading the transistors incorrectly. On the diode, the dark band at one end indicates the negative, correct?
It indicates the cathode. In your circuit, the cathode should be connected to the base of Q1, and the anode to circuit ground.
 

Visaggio

New Member
Neat! it works. Now i just need to figure out how to fit it all on a circuitboard and put it in the model.

Thanks for all the patient help guys.
 

Visaggio

New Member
OK guys, i've run into another stumbling block. I can get the flasher circuit to work fine on a breadboard, but not in the actual application. I've got two images here, C1 is the breadboard configuration that works.

C2 is a breadboard configuration in which i try to isolate the LED in an attempt to imitate the LED being at the end of a 24" wire.

When I isolate the LED, it no longer flashes, I just get a solid light. Would anyone be able to explain why this is, and perhaps how i might isolate the LED successfully?
 

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fingaz

Member
Hi,

I don't see any reason why moving the led further away would make a difference to the circuit. My guess would be that when you moved the led, you may have distubed one of the other components. try moving the led back to where it was, and check if that solves the problem.
Next, I would try building the circuit 'from scratch', and once you have it working then try moving the led again.

Neil
 
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