Another reason I often ask posters about their motives is to simply understand their needs relative to something tangible. In other words, what are they looking for that the commercial product doesn't offer?

The satisfaction of building something to work from the scratch with zero knowledge is priceless. True, there are a lot of commercial products came with numerous kind of function. But i can't find any of them fit to my requirement.

 

The BC108 transistor is obsolete and was replaced by the BC548 many years ago. The max allowed current for a BC108 was 200mA and the max allowed current for a BC548 is only 100mA.

With 15 strings of two LEDs the total current is 300mA so a stronger transistor is needed.

thanks, i never knew about that...but now i would like to reduce the number of led to 4 units,so should i just stick with the transistor or it would be better to use the 555 like bill mention before.

 

A transistorized 555 timer can source or sink up to 200mA from the pin 3 output.

I suggest using it to sink current, as pin 3 can go almost down to ground, but won't go higher than about Vcc-1.3v.

 

The satisfaction of building something to work from the scratch with zero knowledge is priceless. True, there are a lot of commercial products came with numerous kind of function. But i can't find any of them fit to my requirement.

Those are 2 key requirements, IMHO, for launching into a project. Well put.

If you really want blinking (wasn't in your first post?), the 555 strategy is best to pursue. It takes some learning, but you'll enjoy how powerful and versatile that one IC can be.

If you don't need blinking, just on or off, you still need a "control" that is either on or off and not in between. The 555 was suggested as a solution for this but there are others (eg. comparator LM339 with hysteresis or equivalent op-amp circuits).

 

Are you breadboarding or simulating these circuits?

Here is a thought I've never done, but I believe would work well.

It is based off this schematic...

I would have put more LEDs, but you get the idea. If you draw 20ma per leg, and use 30 LEDs, you will pull about 1/3 amps. Higher power supply voltage on the LED section can compensate. It you need to see how this would work let me know.

 

Thanks Bill for your help....though i can't fully understand the operation of 555 hysteretic oscillator.
As your diagram show:
How does it control through the reset pin?
What is the function of C3?
If i want to use a CMOS555, what kind of changes should be done?

 

I'm still not sure what your power supply voltage is. It matters a lot.

To use this variation with a CMOS, you would need a completely different set of transistor drivers. CMOS 555 has very low drive, and I'm depending on the 555 to provide 30ma to the base of the transistor alone! If you use 300ma for the LEDs it is needed.

I have a separate project for a Hysteresis Oscillator, complete with theory of operation, here...

555 Hysteretic Oscillator

 

C3 is a supply bypass capacitor that keeps the supply voltage steady so it does not jump up and down as the load current changes. It also helps prevent the circuit from oscillating at a high frequency. All electronic circuits need a supply bypass capacitor.

 

My objective now is to prolonged the battery life. I would like to get a small warning light rather than keep replacing up the battery.Therefore i want to reduce the numbers of led to 4 units. And the supply voltage i use now is 6v. So,does cmos better to use for this function?

 

My objective now is to prolonged the battery life. I would like to get a small warning light rather than keep replacing up the battery.Therefore i want to reduce the numbers of led to 4 units. And the supply voltage i use now is 6v. So,does cmos better to use for this function?

I built some 3V Chasers with high-speed Cmos ICs (their minimum supply is 2.0V) and red 1.8V red LEDs. Two AA alkaline cells last for about 4 months.
The average current is very low because my circuit blinks 10 LEDs in sequence then a pause before chasing around and around again. Each blink lasts for 30ms so our vision sees it brightly.

I built 6V Chasers with ordinary and high-speed Cmos ICs. The bright green and blue LEDs use 3.5V and four AA alkaline cells drop to 4V when they are dead. Again the LEDs blink briefly in a sequence then a pause and the battery cells last for about 3 months.

 

If you look at my 555 projects I have several projects that last a good long time using something like a CMOS 555 and 3VDC for a power supply. Most of them lasted 3 months continuous using 2X AAA batteries. The reason I had so many examples is different LEDs drop different voltages. My suggestion, pick one and use pin 4 to turn it on/off with the LDR circuit I drew for you.

CMOS 555 Long Duration Minimum Parts LED Flasher
CMOS 555 Long Duration Red LED Flasher
CMOS 555 Long Duration Blue LED Flasher
CMOS 555 Long Duration LED Flyback Flasher

The flyback flasher will handle 4 LEDs easily. It is important that the LEDs need much more voltage than the power supply, and therefore in series with each other.

 

Thanks again Bill. Its perfect for my project...One thing though,if i would like to increase the supply voltage like 6v. Do i have to change the resistor and capacitor value?

 

No, but be vary careful to have more LEDs (by a wide margin) than the power supply can light. If you have a 6V power supply 4 white LEDs in series are perfect. It will be pretty bright too.

 

I breadboard the circuit,but the result doesn't go the way i want to....i'm not sure what i did wrong. By the oscillating circuit alone it work fine...but once i connect the reset input to be controlled by ldr circuit,the flash goes weird. Sometimes the flash goes off by itself. I would show the schematic later...but i wonder if,is it due to the connection is not stable enough which little vibration would disturb the circuit. Another thing,if i use a not-gate inverter to control the reset input,does it take the current which decrease the battery life.

 

Are you using the schematic on post #25?

 

yes,i did....i use a 9v battery instead of 6v to put 4 units of red led in series. I only change the resistor in lower the frequency.

 

Sounds like we need to condition the signal a bit more. Something like the 555 in this circuit in addition to what you have.

Let me get a good night sleep and I'll modify the schematic.

 

thanks bill....btw,i did try the schmitt trigger to control the input and i think it gave better result than the transistor. But,its still not gave a precise one. When i block the light it did stop the flash and it continue to flash again after a short time in dark.But in long duration of dark,the flash didn't start after i exposed it to the light. well, at least it improved much better.

 

It will flash in the dark, but the pot sets the level of light this happens.

A 555 has something called hysteresis. The level of light it stops flashing is going to be different than the level is starts. This prevents the flickering you were getting with the transistor, which didn't have hysteresis.

http://en.wikipedia.org/wiki/Hysteresis

It is possible to make a Schmitt Trigger with less hysteresis. Put a pot of pin 5 of the 555 connected to the LDR.

Betcha didn't know you were getting a lesson in electronics when you started!

 

Betcha didn't know you were getting a lesson in electronics when you started!

you got that right...lol

anyway,i would like to ask...what is the different between a comparator and the schmitt trigger in circuit?