flip flop multivibrator using transistor

[neptune]

frequency is given by 1/rc i have only 100uF capacitor available. to increase frequency when i decrease the value of r to 330 ohm from 10k ohm both of the leds are on.
i also tested value for 1000 ohm still no change.
i want to have frequency of 40Hz. what should i do???

 

[audioguru]

You must use capacitors with a smaller value.
The LEDs will appear to be lighted all the time if they are driven at 40Hz.

 

[neptune]

But the formula says we can increase frequency by decreasing resistor value.
also at what frequency does human eye can not see led's blinking.

 

[ericgibbs]

But the formula says we can increase frequency by decreasing resistor value.
also at what frequency does human eye can not see led's blinking.

hi,
The typical persistance of vision for a blinking light is about 25Hz to 30Hz, any slower it will appear to flicker, 16Hz is the considered minimum.

 

[neptune]

thanx............

 

[gregsmithcts]

New Member...

Hi Guy's

I'm new here and just (re)starting out in electronics... (last time was over forty years ago).

I simulated this circuit (livewire) and found that it was very unstable (using BC108's) as the switches. I also noted (and here is the question) that resistance's less than about 33k in the RC network failed to activate the transistors.

Is this correct? With too low a base resistance, the transistor fails to switch off? And if so, is this because the base is at the wrong polarity with respect to the emitter. Or have I totally got this wrong?

I also noted very big (intermittent) transient voltages across both caps. Would this be due to the simulation or would this occur if I actually built the circuit?

Sorry for so many questions in my first post, but I'm very eager to learn.

 

[Sceadwian]

ericgibbs, the 25-30hz is incorrect, it depends on the modulation depth and off time vs on time and the relative motion of the observer and object, many people can easily see flicker at 120+hz if it's 50-50 duty cycle and on off modulation, such as with LED arrays run directly off AC power. or PWM'd with a high off to on ratio. For LEDs to avoid flicker I wouldn't use anything less than 200hz, higher if available. The flicker is especially noticeable if the object is moving.

 

[neptune]

. BC108 are low power transistor 300mW . resistance less than 33k is failure.maybe because it requires less current to open base emitter, in that case both LED's will remain open.
use high power like shown in diagram. i always test circuits physically and its running smooth.

 

[audioguru]

.maybe because it requires less current to open base emitter, in that case both LED's will remain open.

Doors and windows open and close.
Transistors and LEDs turn on and turn off.