Go Down

Topic: Problem with Op Amp (Read 7813 times) previous topic - next topic

andrewferguson

Hi everyone,

An update to what has happened so far. I have tried tapping the 5v of the Arduino against the 10k resistor, however for some reason it is not making the stepper pulse. I checked to make sure the grounds of the chart recorder and arduino are connected (they are), but I still cannot get it to work. Using a multimeter I measured the voltage between the 10k resistor and Ground whilst the chart recorder was pulsing the stepper by itself, it displayed 5.6v, so I assume that the Arduino can control it. The voltage coming out of the 10k resistor and into the base of the transistor is 0.7v as expected.

Does anyone know why this is not working? Could it be because I have left the OUT of the 555 timer connected to the 10k resistor whilst testing?

Thank you,
Andrew

polymorph

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

andrewferguson

The schematic is the one provided by Leo (Wawa) in an earlier post - 2N3704.jpg . For convenience I have attached it to this post.

Andrew

Anders53

Funny thing is, that op-amps do not really need a dual power supply.
But that is the principal way to explain it.

Its the input signal you will have to control, to be symmetrical around half the supply voltage.
That way the op-amp thinks the input signal is around the ground level.

Its done very easy in AC signal applications, where a resistor divider sets the midpoint to bias the op-amp input.
A signal capacitor is then used as a DC level shifter on the signal input.

In cases with non critical signal applications, like digital signals, you can use the op-amp as a simple signal comparator, where you only control the gain, to prevent the op-amp from oscillating.

Set one opamp input to half the signal supply voltage of the signal sourcing supply, and then amplify as usual.
The op-amp will output a steady low level, until the input signal exceeds half the signal supply voltage.
Then the op-amp output swings high to its local power supply level.

andrewferguson

Hi,

Anders53, thanks for the information. I am sure that it will be very useful when I am using Op Amps in other projects.

I have now eliminated the problem of too low current in Leo's circuit, by using a PC Power Supply with 5v. It can supply a large current, but again, nothing happened when 5v was tapped against the wire going into the 10k resistor.

Can anyone help me resolve this frustrating issue?

Thank you,
Andrew

polymorph

Quote
...nothing happened when 5v was tapped against the wire going into the 10k resistor.
Where? If directly to the transistor base, it'll burn it out. Do you mean you have a freehanging 10k resistor with one side on the transistor base, the other side to nothing, and tap it to 5V?
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

andrewferguson

Not quite. The 5v is connected to one side of the 10k resistor, the other side being connected to the base of the transistor. In addition to the 5v being connected to the first leg of the 10k resistor, the output of the 555 timer is also connected.

Andrew

andrewferguson

Hi everyone,

Just a quick update to say that I have now measured the Vcc of the 555 timer and have determined it to be around 6v. I then used a voltage divider to half the voltage on my 12v power supply in case it would be triggered by 6v rather than 5v, but no success.

Does anyone have any suggestions for what I can try next?

Thank you,
Andrew

Wawa

#38
Apr 10, 2015, 10:26 pm Last Edit: Apr 10, 2015, 10:36 pm by Wawa
I have not disconnected the 555 from the 2N3704 because one of the goals of this project is to have the chart recorder work with the Arduino, but also to preserve its original functionality. Do you foresee any problems with leaving the 555 connected to the 2N3704? (There will be no output from the 555 at that time.)
Yes. If the 555 outputs a "high", even a few volts, the transistor will always be "on".
The Arduino resistor can only turn the transistor 'on", not off.
It is sort of an "OR" port. If 555 OR Arduino inputs a voltage, the transistor is "on".
If the timer would output a constant "low" there would be no problem.


What value for the resistor between Vcc and pin 3 do you recommend? The one the chart recorder uses for pin 15 is 2k2, the closest I have are 1k or 3k3. Would any of them be suitable? (Vcc is around 15v, the SAA1027 registers HIGH at >7.5v, LOW at <4.5v)
3k3 is fine.
Leo..

andrewferguson

Hi Leo,

Thank you very much for your suggestion. I have just checked on the chart recorder, and yes the 555 timer is always high (5v)! This is almost definitely the source of the problems I have been having. Aside from cutting the connection from the 555 timer, do you know of a way round this (I really would like to keep the original functionality of the chart recorder).

Thank you,
Andrew

Wawa

I think the only way around that is to use a switch.
You could also use some sort of switching socket (headphone socket).
So you can connect/disconnect the Arduino with a jack plug.
As soon as you plug that in, it switches the 555 off.
You can use a 3.5mm or 6.25mm TRS jack plug for that.
Leo..

andrewferguson

Hi Leo,

Would it be possible to use a logic inverter to invert the HIGH to a LOW? (see attached photo).

IN would be connected to the output from the 555 timer, V+ to 5v, and OUT would go to the 10k base resistor. Would the HIGH signal from this circuit (when the transistor is off) be enough to turn the other transistor on, with its 10k base resistor? And is there any other problems you can spot with this idea?

Thank you,
Andrew

Wawa

#42
Apr 10, 2015, 11:05 pm Last Edit: Apr 10, 2015, 11:28 pm by Wawa
If you are sure the timer will always be "high", there is another way.
You can use a transistor to turn the transistor off.
Give me 10 minutes, I'll draw up the diagram.
Leo..

Here it is.
Note: logic is inverted, so adapt your code. A low to high signal will step.
edit: this circuit is not affecting the original function of the plotter.
If you remove the Arduino, that transistor is always "off".

andrewferguson

Hi Leo,

Thanks for that circuit, I will try it and let you know how it goes.

I have one question, you say 'If you are sure the timer will always be "high"', do you mean that the timer must always be HIGH when the extra transistor in your circuit is switched on? And if so, how bad would the consequences be of switching on the transistor when the timer was LOW? Would it break the chart recorder, or just not work?

Thank you for your help,
Andrew

Wawa

#44
Apr 11, 2015, 12:13 am Last Edit: Apr 11, 2015, 12:30 am by Wawa
Nothing will go wrong if the timer output is "low".

The Arduino input just won't work anymore.
The supply for the first transistor comes from the timer....

Timer "low" = base Tr2 always "low" = SAA pin 15 always "high".
Leo..

Go Up