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pH output to control pump

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pH output to control pump

Postby Boards14 » Tue Jan 05, 2016 3:41 pm

Hello

I'm trying to use the output from the pH probe on the DrDAQ, (I can output values to excel) but want to turn a pump on when the pH exceeds a set value?

Is there software available (old threads i saw were from 2004 so i'm hoping something exists since then...)?

or some info on using the output pins to control a solid state relay? I've got some limited programming knowledge using an Arduino but no idea how to write/upload a program onto the DrDAQ so far or how to use the digital input/outputs...

Any advice would be great.

Thanks
Dan
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Re: pH output to control pump

Postby AndrewA » Thu Jan 07, 2016 2:38 pm

Hi Dan,

Yes you can control the digital outputs with picolog software.
In picolog goto the edit channel window, you wish to trigger the digital O/P on.
From the 'edit Channel' window, click options - opens the parameter options window - click Alarm - tick enable alarm. Set the Lower and upper thresholds. If the measurement goes out of this range, the alarm will sound.
Digital Output 1...4. - You can make PicoLog drive one or more of them when an alarm condition occurs.
Tick the boxes next to the outputs that you wish to activate.

The DrDAQ output, toggles from 0 volts (Alarm Off) to 3.3V (Alarm on) and is current limited with a 2.2 kΩ internal series resistor. Around 1.5mA.
Regards Andrew
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Re: pH output to control pump

Postby Boards14 » Thu Jan 07, 2016 2:53 pm

Thanks Andrew, so it's just limited to on/off? And then with a transistor or relay I should be able to power a pump once it hits a threshold. Should do the job :)

Cheers
Dan
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Re: pH output to control pump

Postby AndrewA » Fri Jan 08, 2016 8:48 am

Hi Dan,

The alarms in Picolog software can only control the digital outputs. Yes you need to use a transistor/Solid state relay that would switch on with 1.5mA or less.

The digital outputs 1 and 2 on the DrDAQ can be set as PWM outputs. But you can only control these with the PicoScope 6 software or the SDK.
The DrDAQ, PWM outputs with resolution of 1us and the period can be set to 65535 µs or less.
Using low pass RC filter on the output will convert this to dc voltage.
A period of 65535 µs is around 15Hz, a filter capacitor on the output would need to be a large value, say 100uF, as there is already a 2k2 series resistor.
In picoscope 6- I set the period to 1ms (1kHz), and connected a 1uF cap on the output, I had still had 500mV ripple voltage, you may want to use larger value.
At 1ms period you still have 1000 steps in voltage level. You would need to buffer this output to drive anything that draws current.
You could try using the alarms feature in Picoscope if you wish to control the PWM O/P.
NOTE: If you are using PicoScope software you can only set the duty cycle from 0-100, and not say 50.2, so there are only 100 steps. You can then set the period to 1us, and you will have no ripple will a 100uF cap.
Regards Andrew
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Re: pH output to control pump

Postby Boards14 » Fri Jan 08, 2016 12:08 pm

I think i'll stick to the relay to start with, with a variable power pack to control the pump speed.

Thank you for all the help.

Dan
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Re: pH output to control pump

Postby Glovisol » Fri Feb 19, 2016 7:51 am

MATH SERVO

Hi Dan,

If you wish to control the pump flow rate, in order to have a flow PROPORTIONAL to the PH value measured by the Data Logger, you can use a simple Math Servo, driven by the ON/OFF outputs of the DrDAQ, as the one described in:

HIGH PRECISION PROGRAMMABLE VOLTAGE REFERENCE - PART 1 & PART 2, which you will find in the Picolog section.

Kind regards,

glovisol
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Re: pH output to control pump

Postby Boards14 » Fri Feb 19, 2016 8:23 am

Thank you all for your help but I've got a satisfactory solution now using a simple feedback loop triggering a higher voltage relay to run the little peristaltic pumps and with a 5 second delay for the alarm I get suitable precision.

Yours
Dan
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