A comparison compares two voltages, can comes out with a digital high or low depending on which input is higher.

A Schmitt Trigger fires high on one low voltage and low on another voltage. A 555 Schmitt Trigger goes low on the 2/3 power supply level, and goes high on the 1/3 power supply level.

You can make a Schmitt Trigger with a comparator and 2 resistors.

 

here the schematic that i have done....please comment any unnecessary component need to be remove or any improvement should i take. And what is your opinion of the battery(Energizer) for this circuit can hold,like few days or month?
*noted: LED that i used is 6 units RED led(2.5 v,20mA)

 

When the 9V battery is new and the LEDs are turned on, the current in each LED is 32mA which might burn them out. Change the 120 ohm resistors to 180 ohms for a current of 21mA. The total current will be 63mA.

For each blink, the LEDs light for 1.5 times longer than they are turned off so the total average LED current is 38mA. The base current for the transistor is 8.3mA and its average is 5mA. Then the battery current is an average of 43mA.

The current decreases as the battery voltage runs down.
The LEDs will be fairly dim when the 9V battery voltage drops to 6V.

A little Energizer 9V alkaline battery will power the flashing LEDs for about 25 hours getting dimmer and dimmer all the time.

 

At the risk of repeating a question that has been answered, what is the Vf of the LEDs?

 

At the risk of repeating a question that has been answered, what is the Vf of the LEDs?

In post #42 he said the red LEDs are 2.5V each.

 

Thanks for the reply.So it can flash couples of hours eh....quite a disappointment. I was hoping that it could flash fairly bright for couple of days if it is not for months. Maybe i should reconsider to use D cells. How about if i insert inductor to the circuit,would it help?

 

You have the LED lighting for 1.7 seconds then it is turned off for 0.7 seconds.
The little 9V battery does not last long because the LEDs are lighted for more than half the total time.

The 9V battery will last for months if you blink the LED on for only 30ms (it will look bright) then have it turned off for 970ms. You can do it if you change the circuit of the C555 oscillator.

 

The 9V battery will last for months if you blink the LED on for only 30ms (it will look bright) then have it turned off for 970ms. You can do it if you change the circuit of the C555 oscillator.

I see...it should be done that way,so it will be off in longer duration than blink on.For that, i have to change the value of capacitor C1 and resistor R2 to increase it frequency...right?

 

I see...it should be done that way,so it will be off in longer duration than blink on.For that, i have to change the value of capacitor C1 and resistor R2 to increase it frequency...right?

No.
Changing the capacitor or resistors will change the frequency but you need a different circuit using a diode to change the duty-cycle so that the LEDs light for a moment instead of lighting for most of the time like they do in your circuit.

 

I just did a little studying on this circuit. You have the LDR on the low side, connected to ground. This means the circuit will be active when there is light, opposite what I think you want. Reverse the resistors and put the LDR on the positive side of the power supply.

You have the flasher set up where it will be on for 1.8 seconds and off for .7 seconds. Are you sure that is what you want, or would you like this reversed?

The driver circuit is wasting 8ma just to turn the transistor on. A better driver (assuming you want the on/off times reversed) would look like this...

 

This means the circuit will be active when there is light, opposite what I think you want.

Yes,actually that is what i intended to do....circuit active when there is light.

 

You have the flasher set up where it will be on for 1.8 seconds and off for .7 seconds. Are you sure that is what you want, or would you like this reversed?

Actually it should be on and off around 1-1.5 seconds on 1.5-2.0 off. The circuit is just an experiment by testing different resistor to increase it frequency .But i don't really know how to get the exact frequency that i want.

 

OK, cool. How about durations?

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We cross posted there. I have some worksheets, as well as cookbook entries that make this simpler.

http://forum.allaboutcircuits.com/album.php?albumid=41

http://forum.allaboutcircuits.com/picture.php?albumid=41&pictureid=308

http://forum.allaboutcircuits.com/picture.php?albumid=41&pictureid=1233

My Cookbook

 

I already view a brief on the cookbook....there is a schematic on picture 12 which isMultivibrator with Diode. What is the function of the diode for the circuit?
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The conventional multivibrator (which is what you have in your schematic) can handle 50.1 to 99% duty cycles. The diode version can handle 1% to 99% duty cycles.

In the first R2 is used to both charge and dischage the capacitor. The diode "decouples" this, one resistor is used to charge the cap, the other to discharge the cap. Problem with it is, the diode hampers the accuracy of the calculations.

 

The 555 with the diode draws a fairly high and continuous supply current when the 555 is reset, through R1. R1 must have a fairly low value so that the duty-cycle of the output-high is low.

 

Agreed, on this design. The trick is to raise the values of the resistors as high as possible. It would be more correct to say the ratio of R1 to R2 must be as low as possible.

 

There some changes,with 40% duty cycle...Haven't try it though. Will there be any problem with the circuit?

 

I see it as 1.4 seconds on, 1.8 seconds off. Is that what you want?

If you want it to light while it is in the light then flip the LDR and variable resistor over again.

 

There some changes,with 40% duty cycle...Haven't try it though. Will there be any problem with the circuit?

Bill changed the output transistor to an emitter-follower so it no longer inverts. Then the circuit lights the LEDs continuously when the second Cmos 555 is reset (you want them to be off).

The duty-cycle is so high that a 9V alkaline battery will last for about 26 hours with the LEDs slowly dimming the entire time if the output transistor is fixed. The LEDs will look bright for only about 7 hours.

I blink my very bright LEDs for only 30ms and the battery lasts for months. But I use Schmitt-trigger Cmos logic ICs, not Cmos 555 ICs.