555 monoshot load design

[vinod]

I want to give 5V-1ms pulse to a resistive load using 555 monoshot.
First I am connecting 33 Ohm, then 68 Ohm and finally 100 Ohm as load.

Here is my final schematics....(one using OP AMP and other using BJT).
Kindly give me necessary advice on selection between the two ones that is better?

Is there any mistakes in my design and selection of components ?
After getting a clear answer I will go for hardware implementation.

[Robert Reed]

You did not mention pulse requirements. So looking at the whole range of possibilities and for starters:
Forget the op-amp. It will not be able to source/sink enough current thru those loads except at very low levels. Also the rise/fall times of the pulse will be markedly slower.
Now as to the BJT circuit. First eliminate the 1K base resistor as the follower will have a high impedance up to a point*. Bypass the the collector directly to ground with a 0.1 & 100 mf cap in parallel. Reduce the collector resistor to 22 ohms so as to increase this stages dynamic out put range. Install the attenuator directly to the 555 output and wire as a potentiometer with CT wired directly to the base. The * refers to the pulse amplitude versus load. At lower levels of drive (Ie <30 ma) the circuit will work as stands, however consider the extreme of 10 V pulse amplitude with a 33 ohm load which will require 300 ma thru it. The higher power levels will require an added power transistor and using the existing transistor to drive the power transistor in Darlington fashion to reduce the load on the 555 and associated potentiometer. As to using a common emitter stage instead of a common collector stage for driving the load - a completely different story.

PS- did not look up the specs for 2N705 and am assuming it is a small signal transistor. Also did not calculate the time constant components for the one shot, but off the top of my head they look to be in the ballpark for the pulse period you show.

[dyarker]

In BJT circuit, what is the intented purpose of the 10K variable resistor?

In its present location it will do nothing compared to the values of the load resistance. At the very bottom end of its range the variable resistor would have an effect, but probably burn out.

Used as potentiometer in base circuit it would adjust output voltage. Depending on Q1 gain, I think a lower value pot would be needed.

Cheers,

[sghioto]

The schematic shows a 2N706 which is rated at only 50ma max.

Steve G

[vinod]

You did not mention pulse requirements.

5V-1ms pulse

[vinod]

Does this works !!!!!

[sghioto]

No. The pot won't have any effect due to the high input impedance of the darlington configuration. See my modifications in the attachment. In this setup the values chosen will allow you to adjust the output voltage from 0 volts to 5 volts depending on the load, but this will be a completely variable adjustment not .5 volt steps as stated in your schematic heading! Also as you vary the load the output voltage will vary somewhat for a given position of the pot.
The 2N3904 will handle the current (200ma max) and the power dissipation for such a short pulse width.

Steve G

[vinod]

but this will be a completely variable adjustment not .5 volt steps as stated in your schematic heading!

Steve G

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.

[Robert Reed]

If I recall correctly,the 555 output has a rather high rating (maybe 200 mA). This would certainly be sufficient to drive any BJT directly without the need for a Darlington arrangement. As Steve mentioned you could use a 3904, but better would be a 4401 in the same TO-92 case (it has a higher current rating and similar Hfe). Like the 3904, its dirt cheap and very available. BUT- in either case it depends on the pulse repetition rate.
What is the highest rate you expect to see??

[vinod]

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, 3 medium values & 3.5, 4, 4.5, 5V high values at output of darlington)

[sghioto]

Looks good to me. The resistor values were chosen assuming a 1.3 volt drop on the base-emitter junction of the darlington configuration. This voltage drop may differ using the 2N4401. If the output voltage won't go all the way to zero then lower the value of the 1.3K resistor. Of course this will change the limit at the other end so you may not be able to get to exactly 5 volts. If that happens you will have to lower the value of the 3.7K resistor. As you can see changing one value will effect the other limit. You may have to experiment with the values to get the results your looking for.

Steve G

[vinod]

Looks good to me.

Steve G

But as per one of my friends opinion...

"The circuit does not produce accurate voltages. The output voltages change when the load current changes and they change when the temperature changes. The voltage even changes as the output transistor warms up.

The output high of a 555 is a darlington that has a voltage drop of 1.3V at low current or 2.5V at high current. Its datasheet also shows how its voltage changes when the temperature changes and when it warms up.
The two transistors making a darlington will perform the same. "

[Robert Reed]

The Darlington arrangement is almost universal for driving the pass transistor in stable linear power supplies for 40 years. The 555 timer output is a totem pole arrangement and not a Darlington pair. I think you need new friends!

[sghioto]

That may be true of the NE555 but looking at the specs on the TLC555 (the cmos version) the voltage drop is typically only .1 volts for a 1ma load at 15 volts Vcc and the load on the 555 with a 10 volt supply will always be just about 1 ma with the resistor values chosen. Changing the output resistor will not effect the 555 output.
You never mentioned how accurate this circuit has to be or what the operating conditions (temperature) might be. Might as well just try it out and see if it works.

Steve G

[vinod]

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

Any rectifications ?