# I need to design a schematic that produces an asymmetrical square wave with a pulse width of 260 us

#### Buildacircuit

Joined Jan 11, 2022
6
I need to design a schematic that produces an asymmetrical square wave with a pulse width of 260 us, Output 0-45mA, pulse rate 1-16Hz from a 9v battery. Any ideas where I can start trying to figure this out?

#### MrChips

Joined Oct 2, 2009
30,501
Welcome to AAC!

For a fixed pulse duration, you can use a monostable multivibrator such as CD4528.
They are available as different part numbers, CD4528, CD4538, MC14528, MC14538.

MC14538B

#### DickCappels

Joined Aug 21, 2008
10,123
Those are dual monostable vibrators so you can use one of the monostables fixed at 260 microseconds, the end of its output will trigger the other mono to make a [(1 to 1/16)-260 microsecond ] pulse, the end of which can trigger the 260 microsecond mono again , thus starting another cycle.

#### Alec_t

Joined Sep 17, 2013
14,227
Since this is Homework, what ideas have you had so far?
What are the allowable tolerances on your pulse timing?

#### Ya’akov

Joined Jan 27, 2019
8,976
Please ignore me but I can't resist the impulse...

Schematics don't produce waveforms, circuits do!

#### MrSalts

Joined Apr 2, 2020
2,767
I need to design a schematic that produces an asymmetrical square wave with a pulse width of 260 us, Output 0-45mA, pulse rate 1-16Hz from a 9v battery. Any ideas where I can start trying to figure this out?
All of the above circuit suggestions are good but don't address the unusual requirement of ZERO to 45mA output. @Buildacircuit , can you confirm the variable current output? Can you provide a circuit of what this device connects to?

#### crutschow

Joined Mar 14, 2008
34,055
don't address the unusual requirement of ZERO to 45mA output
I assumed that the load resistance would vary to give a zero (no load) to 45mA output.

#### Buildacircuit

Joined Jan 11, 2022
6

#### crutschow

Joined Mar 14, 2008
34,055
Yes, since this is Homework Help, and we are supposed to help with your problem, not provide the complete solution, I removed the completed schematic.

What was the question?
The circuit is a 555 astable circuit, triggering a 555 one-shot (monostable).
There are many circuits on the web showing those two types of circuits.

#### Buildacircuit

Joined Jan 11, 2022
6
Yes, since this is Homework Help, and we are supposed to help with your problem, not provide the complete solution, I removed the completed schematic.

What was the question?
The circuit is a 555 astable circuit, triggering a 555 one-shot (monostable).
There are many circuits on the web showing those two types of circuits.
Yeah, I don't know where folks got the idea that this a homework problem. I'm flattered but long since out of school. I'm trying to build a circuit as a covid project and trouble shoot a device that we use in our physiotherapy clinic (I'm a physiotherapist). If you don't mind putting that schematic back up again I did have a question about it. Are all the v+ symbols connected? I'm trying to reproduce your schematic on texas instruments program t.i.n.a. so I can look at the wave output on the oscilliscope.
Thanks

#### MrSalts

Joined Apr 2, 2020
2,767
Yeah, I don't know where folks got the idea that this a homework problem. I'm flattered but long since out of school.
Umm...

#### ericgibbs

Joined Jan 29, 2010
18,661
Yeah, I don't know where folks got the idea that this a homework problem.
Hi Build.
Moderation uses the information you have registered as, when joining AAC.
Mod.

#### crutschow

Joined Mar 14, 2008
34,055
Ok, if this is not homework (for school), here's the circuit:
Pot U2 adjusts the pulse frequency, and pot U4 adjusts the pulse width.
Are all the v+ symbols connected? I'm trying to reproduce your schematic on texas instruments program t.i.n.a. so I can look at the wave output on the oscilliscope.
Yes, all the V+ nodes are tied together.
In LTspice all nodes with the same name are connected together (makes the schematic less cluttered).

Does Tina have a good 555 model?
The one I'm using is transistor based, not just a functional model.
The waveforms shown below should be essentially the same as what Tina's "virtual oscilloscope" would show.

Last edited:

#### ScottWang

Joined Aug 23, 2012
7,397
If you are not a student now, please modify the contents of your information.

#### Buildacircuit

Joined Jan 11, 2022
6
Ok, if this is not homework (for school), here's the circuit:
Pot U2 adjusts the pulse frequency, and pot U4 adjusts the pulse width.

Yes, all the V+ nodes are tied together.
In LTspice all nodes with the same name are connected together (makes the schematic less cluttered).

Does Tina have a good 555 model?
The one I'm using is transistor based, not just a functional model.
The waveforms shown below should be essentially the same as what Tina's "virtual oscilloscope" would show.

View attachment 257510
I really appreciate your help again. I've attached a screen shot of my simulator circuit (t.i.n.a) and the voltage readout over time analysis (virtual oscilloscope). I don't understand why I'm not getting a the waveform that I want. Any ideas why this isn't working? There is a good chance that I made a mistake making the schematic (there is a pretty steep learning curve for a non-engineer). Again I appreciate any help you can give.

Thanks,

#### ericgibbs

Joined Jan 29, 2010
18,661
hi Build,
Looking at your sim image, the timescale is wrong, its Secs not nSecs, also a couple of component errors. ie: 10n not 10u and 150R to 0V
E

BTW: delay the start of the plot as per Carl's image ~1Sec

#### crutschow

Joined Mar 14, 2008
34,055
As Eric noted, the time-scale is wrong. Try 2 seconds.
150R to 0V
150Ω to ground is correct (RLoad in my schematic).

#### ericgibbs

Joined Jan 29, 2010
18,661
hi Carl,
The TS's image shows his 150R in series with his measurement.?
E

#### Buildacircuit

Joined Jan 11, 2022
6
This is all working great now. Thanks everyone for your help!

#### Buildacircuit

Joined Jan 11, 2022
6
As Eric noted, the time-scale is wrong. Try 2 seconds.
150Ω to ground is correct (RLoad in my schematic).

I had one more question. I'm trying to learn, via trial and error, the role of the potentiometers. The one on the left changes the frequency of my pulses, great, and but I don't really know what the one on the left does.

Thanks