is this true ?when i use this connection no output on pin 3 ^^As Alec noted, the trigger input cannot be low for a duration longer than the output pulse or the 555 will retrigger.
This can be prevented by using a capacitor and resistor, as he suggested, or making the generator pulse duty-cycle high so that the pulse is low for only a short time (a few microseconds minimum).
That can be done with a 555 astable by eliminating the resistor between the DIS output and the frequency determining capacitor.
The astable will then generate a series of negative going pulses of only a few microseconds duration, sufficient to trigger the monostable, but too short to affect its pulse width.
Edit: No, the capacitor should be in series with the pulse generator output, and should be a smaller value.
The one-shot period is approximately equal to Req*C3 where Reg is the equivalent resistance of LDR and R1 in parallel..........
when i choose c3 below 1u no output ^_^
the best result i got at 1u
The one-shot period is approximately equal to Req*C3 where Reg is the equivalent resistance of LDR and R1 in parallel.
Thus for C3=1μF, the maximum Req would be 1kΩ for a period of 1ms (the maximum allowed for a 1kHz frequency).
That's why it doesn't work for a value of R1 above 820Ω.
The one-shot should work fine with a value of C3 less than 1μF.
What value did you try?
sorry i put 0.1f instead 0.1u when i put 0.1u with 820no ghange with duty so i increase r1 to 8.2k it works fine ^^
What was the value of R1 when you tried the lower value of C3?
You need to put some thought into the parts values you choose.
Thanks alot ^^So now it's working okay?
So your next step is to add the 555 astable.
Note that when you reply to a post, put you answer after the copied post, not in the middle of it (unless you use a different colored font to make it clear what part is your answer).
You don't really need R2.is there any modification or notes must i do ?
You don't really need R2.
If you remove it (replace with a short) and increase the value of R3 to get 1kHz, then U2 will output a minimum negative pulse of several microseconds, which is long enough to trigger U1 but will not interfere with U1's output pulse-width.
If you do that, then you won't need to add a resistor and capacitor at U2's output to shorten the pulse.
You are getting pulse out of U2 so don't understand why they aren't triggering U1.
Could be a difference in the models used, since the one I have in LTspice works with that short a pulse.
Try putting R2 back in with a value of a few hundred ohms and increase if necessary until U1 triggers..
See how large R2 has to be to cause U1 to trigger.
Note: Please post your Oscilloscope picture as a separate graphic so it doesn't cover up the schematic.
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by Jeff Child
by Dale Wilson