555 monoshot-- loading effect

vinodquilon · Dec 17, 2012

alec_t said:
I've produced a design much as circuitspecialists describes but am presently unable to upload it because of the site's problems.

Can you sent it to my mail

One more point

{Don't worry about 0.5V steps, 0-5V continuous variation is required by manual turning of POT and the pulse at load is monitored for amplitudes set at 0,0.5,1,1.5,2,2.5,3,3.5,4,4.5, &5 volts.}

vinodquilon · Dec 18, 2012

modified design

Pls find the modified schematic.....Is it OK ?

Can I produce the low values like 0, 0.5, & 1.0 V by turning 5K ? (also 1.5, 2.0, 2.5, 3V medium values & 3.5, 4, 4.5, 5V high values at output of darlington)

alec_t · Dec 19, 2012

That should work ok, although there will be a slight (~5%) variation in voltage across the load as the load changes.

alec_t · Dec 20, 2012

Here's the schematic I mentioned:

vinodquilon · Dec 20, 2012

alec_t said:
Here's the schematic I mentioned:

Thanking you for posting informative schematic.

Here is one doubt..
Why can't we combine ten 100 ohms to one 1k and make use of 1k in the form of 1K POT and connect it at the output of 555 with CT of POT at +ve terminal of LM324. Other two terminals of POT, one at 555 output and other at ground.

And leave the 50K POT.

alec_t · Dec 20, 2012

Why can't we combine ten 100 ohms to one 1k and make use of 1k in the form of 1K POT

Because this circuit is for a stepped voltage setting, which is what you originally specified.

vinodquilon · Dec 20, 2012

alec_t said:
Because this circuit is for a stepped voltage setting, which is what you originally specified.

So, for a continuous voltage setting can I use the 1K POT connected as specified above by leaving 50K or 50K POT connected by leaving all ten 100 ohms ? (All other parts of schematic are intact.)

Which is better ?

alec_t · Dec 20, 2012

For a continuous variation you could modify the circuit I posted by eliminating all the 100Ω resistors and connecting the 50k pot (other values, e.g. 10k, would also work) ends between the 555 output and ground.
To limit the voltage adjustment range you might want to put a series resistor (e.g. 47k if a 50k pot is used) between the top end of the pot and the 555 output. The suggested opamp is an LM324 but almost any other opamp (NOT a 741!) with rail-to-rail (or nearly so) inputs and outputs would do.

vinodquilon · Dec 20, 2012

alec_t said:
To limit the voltage adjustment range you might want to put a series resistor (e.g. 47k if a 50k pot is used) between the top end of the pot and the 555 output.

If I put a series 47K then I have to adjust 555 supply to compensate for series drop at 47K. Hence is it necessary to use it ?

I think 50K POT alone I can continuously vary 555 output at OPAMP input from 0 to 5V by generating 555 output volt >= 5V

vinodquilon · Dec 20, 2012

555-opamp-bjt

555-OPAMP-BJT
Here is 555 oneshot with LM324 & 2N2222 for 0-5V continuous amplitude variation of 1ms pulse.

Any rectifications ?

alec_t · Dec 20, 2012

That should work for a 10V supply. For a lower or higher supply voltage reduce or increase, respectively, the value of the series resistor.

vinodquilon · Jan 18, 2013

Problem continues....

The following are the difficulties I have faced during the above circuit implementation,

1. 1ms pulse by pressing the trigger micro switch is hard to achieve as it depends on the duration of pressing the switch and is beyond 1ms (depends on the operating person)

2. To overcome the above difficulty I have designed one 3ms 555 astable and is fed to monoshot through Differentiator

3. In the 2nd case I cannot control the count of mono pulses at output as I only want a SINGLE 1ms pulse at 555 mono out.

dr pepper · Jan 18, 2013

The issue is that the trigger pulse is longer than the time out period, ie 1ms.
With it being such a short period a cap in series with the trigger switch might be impractical so I'd be inclined to arrange a circuit on the o/p of the timer that disables the trigger, ie make the timer non retriggerable until the switch is released.

vinodquilon · Jan 18, 2013

dr pepper said:
The issue is that the trigger pulse is longer than the time out period, ie 1ms.
With it being such a short period a cap in series with the trigger switch might be impractical

Here is it...

vinodquilon · Jan 19, 2013

optimized schematic

optimized schematic to overcome above mentioned difficulties..

vinodquilon · Jan 19, 2013

alec_t said:
Here's the schematic I mentioned:
View attachment 69289

When 50K comes in parallel with ten 100 OHMs steps, will it affect the 0.5V equal distribution of 10 way switch as net resistance get disturbed even if 50K>>1K.

alec_t · Jan 20, 2013

The 50k will be in parallel with from 1 to 10 of the 100Ω resistors, depending on the position of the 10-way switch. The maximum linearity error (assuming the resistor tolerance is <<1%) will be about 2% when the switch is at position '9' with the 50k bridging 900Ω. If that is too high you could replace each fixed resistor with a multi-turn high-stability (i.e expensive) trimmer (say 150Ω) and calibrate the system.

vinodquilon · Jan 20, 2013

alec_t said:
The maximum linearity error will be about 2%

Can I replenish this voltage drop due to the addition of 50K parallel by adjusting the 555 output above 5V through supply voltage variation ?
1. put switch at left side of R3 and adjust the 555 output through supply voltage variation for exact 5V across 50K pot
2. Do not disturb the above supply adjustment. We will get 4.5, 4, 3.5,.......etc at all other positions for this setup.
3. 0% linearity error

alec_t · Jan 21, 2013

Can I replenish this voltage drop due to the addition of 50K parallel by adjusting the 555 output above 5V through supply voltage variation ?

No. That would have a similar effect to adjusting the trimmer. It would vary the load current for all switch positions.

Edit: Increasing the value of the 50k trimmer to 100k would reduce the loading effect on the resistor ladder, but the input impedance/current of the opamp might then have a more noticeable error-producing effect. Just how accurate do you REALLY need the load current to be?

vinodquilon · Jan 21, 2013

alec_t said:
Just how accurate do you REALLY need the load current to be?

Here is the application context....

555 monoshot-- loading effect

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