LED Flasher

[electronic novice]

hello,
I want to add one transistor in this circuit can anyone help me with the adjustments and value to be used thanks.

 

[Vizier87]

Err. I recommend 555 timers to flash LEDs if that's the only thing you need. They're very cheap.

 

[Ian Rogers]

I think this is that "three transistor project"... Change the LED driver to a darlington pair... that will increase the circuit to three transistors AND allow more current for mulitple LED's

 

[electronic novice]

Q1 and Q2 are BC337 45V 800mA NPN Transistors. If i changed Q2 into a darlington pair what value of transistor could i possibly add.

 

[electronic novice]

maybe still BC337

 

[electronic novice]

do i still need to add a resistor on the collector of the added transistor in a darlington pair?

 

[MrAl]

Hi,


This is a cute little circuit really. You dont see this kind of thing too often anymore. Where did you find this circuit?

At first glance it doesnt look like an LED flasher. It looks like an LED modulator, where the input light level modulates the output LED light level (like an fiber optic repeater might work). If there is very little ambient light level present the LED turns on which produces light, but if we apply some source of light to the light dependent resistor, the LED starts to dim maybe turn off altogether at some point. Now that would be the end of the analysis, but something else interesting comes into play. What if we shine the light from the LED onto the LDR? With the light from the LED shining on the LDR, suddenly the LDR resistance decreases, which turns the first transistor on a little harder which decreases the drive to the second transistor, which then dims the LED or even turns it off. The light from the LED now dimmed, causes the LDR resistance to increase again, which then turns the first transistor on a little less than before which means the second transistor turns on a little more, which in turn turns the LED on harder. Thus we have a feedback loop which given the right conditions, could cause an oscillation which of course will make the LED blink on and off repeatedly. Im guessing that the capacitor helps to set the oscillation frequency.

[ADDED LATER] I didnt recognize the basic circuit configuration at first as a rare type of oscillator circuit that can be made to oscillate without the light shining from the LED to the photo transistor.

Would be a bit interesting to analyze a little more. Would also be interesting to think about what could be placed between the LED and LDR to change the frequency when used in that mode.

 

[electronic novice]

actually the title of this circuit is "Dark-activated led or lamp flasher" from the site where i get this
but can i add another transistor from it?

 

[MrAl]

Hi,

You shouldnt really have to add another transistor unless you need really high output current, is that what you need (to drive a higher power LED)?
Also, you probably need more than 3v to power it if you do add another transistor or even not, depending on LED type.
What kind of transistor you use depends on how much output current you need, if you need more than say 20ma.

A simple solution to adding another transistor would be to use the same kind.
Disconnect the base of Q2.
Connect another transistor emitter to the base.
Connect the collector of the new transistor to the collector of Q2.
Connect the base of the new transistor to the diode D1 where Q2 used to connect.
You may then also need a 100k resistor from the Q2 base to ground, but might get away without needing this.

 

[electronic novice]

yes i want to drive a higher power led but i don't know where to connect the other transistor and what possible adjustments to make.

 

[4pyros]

If all you are after is more LED drive current than all you should have to do is use a higher power transister for Q2.
Your drawing has no values so it would be hard to say what would be needed to change.
What is Q2 now?
Can you post the parts list?

 

[audioguru]

What is Q2 now?
Can you post the parts list?

The project is here: https://www.redcircuits.com/Page64.htm

 

[4pyros]

Q1,Q2____________BC337 45V 800mA NPN Transistors

So the BC337 is rated at 800ma how much more power do you need?

Wow sorry I just noticed that I am a little late in this thread.

But the question remains the same, how much more power do you need?

do you have the spec for the LED you would like to use?

 

[electronic novice]

I really wanted to add another transistor because our requirements is to have at least 3 transistors in a circuit.

 

[4pyros]

I really wanted to add another transistor because our requirements is to have at least 3 transistors in a circuit.

OK try this;
https://freecircuitdiagram.com/2009/05/30/15v-led-flasher-using-3-transistors/

 

[audioguru]

This school assignment reminds me of Transistor AM radios in 1958.
Mine had 6 transistors but the radio bought by my friend had big words on it: 12 TRANSISTORS.

Both radios performed the same and used 3 transistors for the radio circuit and 3 more transistors for the audio amplifier. But the radio with 12 transistors had 6 extra transistors connected together in a circle. They were not used in the circuit.

 

[MrAl]

yes i want to drive a higher power led but i don't know where to connect the other transistor and what possible adjustments to make.

Hi,

You didnt understand my post above? I explained where to connect the second transistor in detail. You know the leads of the transistor are base emitter and collector right/

 

[More Coffee]

http://talkingelectronics.com/projects/200TrCcts/200TrCcts.html

Has a some detail on transistors ,,and a bunch of cool circiuts to boot..

 

[Frank88]

Hello, I am looking to flash 36 LED's, actually want to use a switch to flash them with the other state being solid. I've gotten a start from this forum and have a 555 timer. I'm not skilled with this but it appears I need a SPDT switch where one side will send the power through the 555 timer and on to the LED's, and the other side of the switch will bypass the 555 timer all together? (I thought the other side should go thru another 555 timer in monostable mode but this seems to only be for a timed period)

The LED's are 5mm, 3.8v and 30mA. I've figured out the parallel setup and resistors to use.

Next issue is hooking up the 555 timer for astable state - any suggestions on where I can learn how to do this. I've looked at the schematic but need some training.

Thank you,
Frank

 

[More Coffee]

You don't need just a switch, I'm quite certain you'll need a Transistor as well, there's no way I don't think you can sink the current of 18 leds through a 555 by itself..

I'm hesitant to offer suggestions, other than try to locate some 500ma general purpose transistors, 1 amp maybe.. to suit your needs,

Im green to ,but I would really consider a transistor to handle the current for the LED's