multivibrator is it suitable?

[whiz115]

Hi

i want to build a circuit so i can switch the output of a 3v/100mA power supply for short duration on and for a longer duration
off, also it is useful if i could regulate the on/off duration, can i use a multivibrator design for that purpose or is there any better
idea? (i want the circuit to be self powered from the same 3V source before the switching of course).

if somebody has a ready schematic which is exactly what i want to do, it is appreciated.

thanks.

 

[crutschow]

What on/off periods and frequency do you want?
Do you want single-shot or astable (continuous) pulses?

 

[Les Jones]

The classic astable multivibrator circuit will do what you want. To give variable on/off times put a variable resistor between each base and the positive rail. (Assuming NPN transistors.) You could also use a cmos 555 (LMC555) . I suggest using a P channel mosfet to switch the power on and off.

Les.

 

[whiz115]

hi

thanks for the reply guys!

What on/off periods and frequency do you want?

on <1.5'', and off 2-3''

 

[Les Jones]

I've not seen time measured in inches before !

Les.

 

[whiz115]

I've not seen time measured in inches before !

Les.

The prime symbol ( ′ ), double prime symbol ( ″ ), triple prime symbol ( ‴ ), quadruple prime symbol ( ⁗ ) etc., are used to designate units and for other purposes in mathematics, the sciences, linguistics and music. The double prime (″) represents inches (in), arcseconds (as), and seconds (s). However, for convenience, a (") (double quotation mark) is commonly used.


i'm not trying to offend you... just i wanted to reply your question..

 

[crutschow]

I agree with Les.
(") is commonly used for seconds of angular measurement.
I've never seen it used for seconds of time before.
In your referenced Wiki article, it states the abbreviation for seconds of time is s or sec, no mention of (").

 

[whiz115]

ok but we're getting off topic... you asked me "What on/off periods and frequency do you want?"

so it's 1.5 or less second the on state and 2-3 seconds the off state.

 

[crutschow]

Below is the LTspice simulation of a 555 circuit that should do what you want.
It uses the LMC555 CMOS verison of the 555, which will operate down to 1.5V.
The P-MOSFET can be just about any logic-level device (Ron resistance specified at a Vgs of 3V or less on the data sheet).
Pot U2 adjusts the duty-cycle and pot U3 adjusts the frequency.

 

[whiz115]

thanks for the complete response!!! most appreciated..

Edit...seems that most mosfets with low vgs
are smd versions which is not convenient in my case because I'll build the circuit on a
strip board and I don't have any smd adaptors...

does anyone know any to-220 for that purpose?

 

[crutschow]

Don't offhand know of any 3V logic-level in TO-220.
Just buy one with the short leads and solder longer leads to them. It's not that difficult with a small solder iron.
At 100mA there's no concern about heat-sinking.

 

[whiz115]

Don't offhand know of any 3V logic-level in TO-220.
Just buy one with the short leads and solder longer leads to them. It's not that difficult with a small solder iron.
At 100mA there's no concern about heat-sinking.

i was thinking to avoid a weird looking board, but if i don't have any alternative i'll use whatever i can find
by the way i'm not sure if the FDR840 is available where i am, but i guess i can order it.

thanks again

 

[crutschow]

i was thinking to avoid a weird looking board, but if i don't have any alternative i'll use whatever i can find
by the way i'm not sure if the FDR840 is available where i am, but i guess i can order it.

thanks again

Farnell and others sell a lot of logic-level MOSFETs characterized for Rds(on) at a Vgs of 3V, possibly cheaper and better than the one I used in my simulation.

 

[whiz115]

Hi again..

TTL mosfets proved not convenient solution to me, so plans changed...

since I want to switch on/off a 3v source
I will power the 555 from a 9v battery driving a buz11 mosfet or a BD139 which then it will drive a mcp1700-3.3v regulator

sounds ok?

P.S also the 1meg pots proved hard to find (I don't want to use trimmer)

 

[crutschow]

......TTL mosfets proved not convenient solution to me, so plans changed...

since I want to switch on/off a 3v source
I will power the 555 from a 9v battery driving a buz11 mosfet or a BD139 which then it will drive a mcp1700-3.3v regulator

sounds ok?

P.S also the 1meg pots proved hard to find (I don't want to use trimmer)

That would work.
You could also use a PNP BJT such as a 2n3906 or similar at 3V as shown below.

If you use a smaller value pot then you will need to increase the value of C2 by the same proportion (e.g. 100kΩ pots require a 22μF capacitor).

 

[whiz115]

the mcp1700-3.3v i said, was wrong idea for a 9v battery, also too many components.

 

[whiz115]

i've tried to import the LTspice simulation from your post into Eagle so i can make a pcb from it, (i know it sounds silly for such a trivial circuit since i could use a strip board), problem is there are some errors, probably missing component libraries... so i tried to make the schematic from the beginning and then import it to Eagle which then gave more errors... (i'm not experienced with such designing tools).

do you think your can help me with it? also i'm going to do it with through hole components

thanks for all your help.

 

[crutschow]

Sorry I really don't know much about Eagle.
What type of errors are you getting?
Perhaps someone else can help with that.

 

[whiz115]

hi

at LTspice "couldn't find symbol(s): pot"

i guess it's not serious, probably you added a library with more components
and the standard version doesn't have ( i didn't corrected it)

at Eagle there are more and they are important because i can't make the pcb
even when i did a new schematic...i didn't managed to turn it to pcb, there were
missing components, some of them were only smd versions, so probably also eagle needs
something.

"Ιmort line flag 400 -128 V+"
"could not find wires coordinates in order to place symbol"
"Device not found V+"
"Device not found trig"
"uknown part U2"

just a small pcb i want to build and they came up all these errors... , if everything
were all right with the schematic then making the pcb for me it is less effort than the strip board.

for some might look easier if i google astable 555 multivibrator pcb, but from a
quick look nothing matches what i want and they need correction before
i build any.

 

[crutschow]

Eagle can't work just from a schematic. It needs to know the exact part number for the package type you want or you need to select a package from the Eagle library for the part you are going to buy.
That includes all the parts including resistors, capacitors, pots and the 555.