hi, i need to make a circuit that extand the width of a short pulse, i'm using monostable circuit as you can see in the image below
the output is connected to a LED that suppose to blink every 200 ms, from reading the datasheet it is not clear to me if the LED will recive positive voltage and current, because the
IH=-16mA VH=3.4V
IL=0.4mA VL=0.2V
Datasheet
i connected the output to the \Q (inverted output)
will it work?
am i inverting both the current and the voltage?
A high on 5 will send 1 low. The output will be high (LED on) until the mono is triggered. It will then pulse off. (I have not checked the rest of the circuit).
Weedpharma
You can't use a resistor divider to generate a power rail - you use a voltage regulator for that.
You need a 0.1uF ceramic decoupling capacitor on the chip itself, however you power it.
Why not 74HCT series for the monostable? Same input voltage thresholds and less current.
weedpharma:
A high on 5 will send 1 low. The output will be high (LED on) until the mono is triggered. It will then pulse off. (I have not checked the rest of the circuit).Weedpharma
on HIGH the voltage is positive but the current is negative, i don't think the LED will work (or am i missing something????)
Look at the output transistor configuration. There are two transistors in series. If the top one is on, the positive rail is connected to the output while the lower transistor is off. When the reverse happens, the output is ground.
In your circuit when the top Tr is on, it outputs positive current to the LED.
The Q outputs are opposite to each other. When one is positive, the other is ground. It is not negative current.
Weedpharma
Oh dear!
There are just so many things wrong with that circuit!
The SN74121 became obsolete some 20 years ago. Where would you have obtained one? (Radio Shack?) Where did you find the funny circuit?
Please explain the purpose of your exercise, what is to trigger it and from what voltage and power source you propose to power it?
Then we can suggest a reliable way to do it.
Paul__B:
Oh dear!There are just so many things wrong with that circuit!
The SN74121 became obsolete some 20 years ago. Where would you have obtained one? (Radio Shack?) Where did you find the funny circuit?
Please explain the purpose of your exercise, what is to trigger it and from what voltage and power source you propose to power it?
Then we can suggest a reliable way to do it.
i can still order a few from Digikey.
the purpose of this circuit is to expand short time pulse generated from LVTTL signal so i could see some flashing (Blinking) light in a LED
i did change a few things, first this device is now powered with LDO and supplied with 5VDC, unlike that stupid mistake i made by trying to use voltage divider 
also the decoupling capacitor is now 0.1uF.
please suggest me ways i can fix this..
thank you all.
Your drawing's "R60" value is much too low. Try 1K.
I think that C_ext value of 100uF is too much (more than acceptable.) Use something <= 1uF.
Encyclopaedia of One-Shots
http://www.scarpaz.com/Documents/AN-366.pdf
Special Considerations and Notes (pg 21)
Furthermore, if a polarized timing capacitor is used on the
'221, the positive side of the capacitor should be connected
to the “CEXT” pin. For '123 parts, it is the contrary, the negative
terminal of the capacitor should be connected to the
“CEXT” pin of the device. (Figure 5).
74HCT family replaces 74, 74LS, 74ALS... If you had 5V input then the symmetric 74HC family
would be the right choice, if you can work all at 3.3V, then 74LCX is good (that family has 5V
tolerant inputs even).
74 and 74LS families are from the '80s, not used in new designs, more expensive, worse power
consumption, lower noise immunity, can't connect inputs to Vcc.
Also look up the pull-up current IOH of a TTL. It is only about 400ua.
TTL pull down harder but are week pull ups. Still, I don't think they can sink
20 ma. why not use the Q-bar as a pull down on the LED with a resistor to set
it to about 5 ma.
Dwight
What frequency are the pulses you wish to expand?
Weedpharma
ok i think i will use a simple 555 monostable like this one attached to this replay
will it turn on this LED? 151051VS04000
please see the output voltage and current in the datasheet:
Datasheet
Hi,
Remember with LM555, it triggers on negative going edge of pulse.
Tom....
I recommend using a CMOS version of the 555 like the 7555, since they don't need nearly
so much decoupling (the original 555 has shoot-through on the output drivers, the CMOS
versions don't) In The Art of Electronics they mention how they measured 200mA or larger
supply spikes whenever a standard 555 transitions, and recommend 470uF or similar as
decoupling for it when combined with sensitive circuitry.
MarkT:
I recommend using a CMOS version of the 555 like the 7555, since ...
It's the same as with the SN74xxx series - it is simply absurd not to use the incomparably superior CMOS version. By comparison, everything is wrong with the older bipolar version.
A bit like finding a reason to use an original IBM PC with an 8086 at 640k memory with a tape recorder. 
Hi,
A bit like finding a reason to use an original IBM PC with an 8086 at 640k memory with a tape recorder.
Commodore VIC20 with tape drive and speed cartridge.
Tom.....
i changed some stuff, now i use 555 as you can see in the screenshot
and also i use an inverter since the LVTTL pulses are HIGH and 555 require LOW (Trigger) to charge the capacitor as shown here:
mono stable (click here) explanation
now what do you think? will it work?
I doubt that the 3V signal from the inverter (4049) will satisfy the 555.
You could use a simple NPN inverter, much more versatile.
Paul__B:
A bit like finding a reason to use an original IBM PC with an 8086 at 640k memory with a tape recorder.
For those who don't know, the original PC did have a tape recorder interface.
I'm sure someone could find a reason to pull out a 80's era PC... but they would be reluctant to admit it in public... Like "I wanted to play a game of Zork the way God intended, on a Compaq Luggable 8088".
To which we would reply "You are a Silly Retro Geek"
