Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Variable mark-space (pulse width) and frequency multivibrator

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,991
Helped
13
Reputation
26
Reaction score
14
Trophy points
1,318
Location
Greece
Activity points
35,880
Hi,
I would like to be able to make a variable mark/space multivibrator like this one from https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_6
However I would like the frequency to be variable from a few Hz to about 60Hz or maybe a bit more.
How can I accomplish this frequency variation?

On another point, how about "rounding" a bit the edges of the signal of it, is there anything I can to to accomplish this?
 

Attachments

  • NV_1203_Marston_Figure008.jpg
    NV_1203_Marston_Figure008.jpg
    117.5 KB · Views: 23

betwixt

Super Moderator
Staff member
Joined
Jul 4, 2009
Messages
15,571
Helped
5,036
Reputation
10,096
Reaction score
4,876
Trophy points
1,393
Location
Aberdyfi, West Wales, UK
Activity points
131,972
Rounding edges is easy, just add an RC filter at the output.
Making it variable frequency is not so easy, within a limited range you can make R5 and R6 a ganged variable resistor but ideally you would also make C1 and C2 variable as well. Not practical at those values.

Over a limited range you could consider using a variable frequency oscillator to trigger an adjustable monostable to produce the pulse width you need. Again, this is a trivial task for an MCU.

Brian.
 

    neazoi

    Points: 2
    Helpful Answer Positive Rating

BradtheRad

Super Moderator
Staff member
Joined
Apr 1, 2011
Messages
14,536
Helped
2,851
Reputation
5,712
Reaction score
2,836
Trophy points
1,393
Location
Minneapolis, Minnesota, USA
Activity points
108,523
Vary frequency and duty cycles using potentiometers. With some finagling you might manage to obtain a 10x frequency range.

Somehow the simulation puts slight rounding on leading edges.

classic astable 2 NPN 4 pot's vary duty cycle n Freq.png
 

    neazoi

    Points: 2
    Helpful Answer Positive Rating

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,991
Helped
13
Reputation
26
Reaction score
14
Trophy points
1,318
Location
Greece
Activity points
35,880
Vary frequency and duty cycles using potentiometers. With some finagling you might manage to obtain a 10x frequency range.

Somehow the simulation puts slight rounding on leading edges.

View attachment 177718
What are the potentiometer pairs for the frequency and the width?
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
22,244
Helped
4,613
Reputation
9,242
Reaction score
4,878
Trophy points
1,393
Activity points
146,954
Hi,

it´s easy with a dual comparator or a microcontroller.
But usually you don´t want IC solutions..

Klaus
 

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,991
Helped
13
Reputation
26
Reaction score
14
Trophy points
1,318
Location
Greece
Activity points
35,880
This circuit from that page https://www.electronics-tutorials.ws/waveforms/monostable.html mentions "the frequency of the output signal is always the same as that for the trigger pulse input, the only difference is the pulse width".

I am thinking that I could use this circuit with a potentiometer on Rt, to vary the width, and having this circuit triggered by a variable frequency multivibrator.
But before I experiment with this I need to know something about the trigger signal.
Author mentions that trigger signal must be a negative pulse. Negative, referenced to the ground? Or just a high to low transistion?
Because, if a multivibrator is used, its output signal (trigger input to the next stage) will be a square with +vcc and 0v levels. Can this +vcc to 0v transition be used as a trigger?
 

Attachments

  • waveforms-tim12.gif
    waveforms-tim12.gif
    12 KB · Views: 15

betwixt

Super Moderator
Staff member
Joined
Jul 4, 2009
Messages
15,571
Helped
5,036
Reputation
10,096
Reaction score
4,876
Trophy points
1,393
Location
Aberdyfi, West Wales, UK
Activity points
131,972
Author mentions that trigger signal must be a negative pulse. Negative, referenced to the ground? Or just a high to low transistion?
I would say the opposite. The trigger pulse is used to bias the left side transistor into conduction so it starts the monostable action. The diode blocks any negative voltages and is there to prevent leakage back into the trigger circuit.

When the trigger voltage (a square wave from the waveform shown) is low, that is ground or slightly negative, C1 discharges through R4. On the rising edge, as C1 charges, the current through it raises the voltage across R4 and if high enough makes the diode conduct. The waveform with positive and negative spikes is correct but it is only the positive going edges that trigger the timer.

Brian.
 

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,991
Helped
13
Reputation
26
Reaction score
14
Trophy points
1,318
Location
Greece
Activity points
35,880
I would say the opposite. The trigger pulse is used to bias the left side transistor into conduction so it starts the monostable action. The diode blocks any negative voltages and is there to prevent leakage back into the trigger circuit.

When the trigger voltage (a square wave from the waveform shown) is low, that is ground or slightly negative, C1 discharges through R4. On the rising edge, as C1 charges, the current through it raises the voltage across R4 and if high enough makes the diode conduct. The waveform with positive and negative spikes is correct but it is only the positive going edges that trigger the timer.

Brian.
So a square wave trigger input (0-5v) is enough to trigger tbe circuit every rising edge. Thanks Brian
 

LaTeX Commands Quick-Menu:

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top