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Universal buffer for the DrDAQ

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Universal buffer for the DrDAQ

Postby Glovisol » Sun Dec 14, 2014 7:13 am

EasyCapture1.jpg
PC screen
Closeup buffer board.JPG
Close up
Complete system.JPG
Complete system
All considered the DrDAQ is an amazing piece of equipment. In a small package, practically the size of a credit card, together with the Pico supplied software, it packs an Oscilloscope/Audio Spectrum Analyser, a PH /Oxygen meter, a Multi-waveform Function Generator, 4 input/output digital ports, a resistance measurement facility and 3 pre-settable analogue (voltage) inputs, not conting the built-in sound and light sensors. One of the most useful features is the Data Acquisition capability when working with Picolog software.

Unfortunately the three EXT. inputs have pull-up resistors to the positive supply. Presence of the resulting voltage on the inputs poses a problem in most measurements. This drawback can be completely overcome by using suitable operational buffers which drag to zero the DrDAQ inputs: to be able to really go through 0V, the operational amplifiers need a negative supply. In this post I am presenting a complete buffer interface working off the 5 V taken from the DrDAQ itself and generating the required negative voltage at -2.5 V.

The buffer connections are all plug-in: therefore when sensors other than voltage sources are used, the corresponding plugs are removed and the sensor is plugged into the telephone type connector of the DrDAQ. (NOTE: in my DrDAQ the telephone plug EXT. 1 had a defective contact and I had to remove it, so check these sockets if you have erratic operation.)

This buffer provides the following facilities.

1) Eliminates the unwanted voltages at the EXT. inputs. The new buffered inputs are at a very high impedance and can go trough zero Volts thanks to the built-in negative supply feeding the operational amplifiers.
2) The extra operational amplifier can be used to increase the input dynamic range of any one EXT. input from 0 – 2.5V up to 0 – 10 V, but still providing a nearly infinite input impedance and also reading the input through zero Volts.
3) The extra operational amplifier can be used to provide other features, as shown below & in diagram.
4) Precision 2.49V standard voltage source for one point routine calibration of the DrDAQ. Also a three point calibration facility can be easily provided as shown.

The buffer works with the + 5V supply voltage supplied by the DrDAQ and normally does not require an external supply. It can also work with an external + 5V or + 12V power supply by simply removing a jumper. Reverse polarity protection is provided.

The unit is built on a small prototype perforated board. After assembly the board is secured to the wooden base with double edge tape. A subminiature in-line 24 pin connector (S1/J1) is used on one edge of the board for quick removal. External connections are made with miniature screw terminal M1.

Brief description

The LM324, IC2, provides the four operational amplifiers. Amplifiers A, C and D work as unity gain followers for the three EXT. inputs of the DrDAQ. Resistors R3, R4 & R5 provide the Autodetect function for operation with .DDS files. The output of the astable square wave multivibrator IC1 (NE555) is rectified by voltage doubler D1-D2. The negative voltage has a value of -2.5V for a supply voltage of 5V. When the jumper between pins 11 &12 of M1 is removed and +12V is supplied to pin 12 by a external supply, negative voltage reaches the value of -9V.

The 2.49V precision voltage is supplied by special zener IC D4 (LM336). Specification for D4 calls for an initial voltage of between 2.390 and 2.590 V. Resistor R6/R7 must be selected with the help of a precision DVM to obtain 2.49 V. Then long term calibration of this source will keep within a few PPM for ambient temperatures 15 to 25 °C.

To calibrate a voltage channel, just connect it to pin 8 of S1 and read the voltage on Picolog. The channel can then be calibrated by writing a .DDS file or by writing table/equation in Picolog. For three point calibration just connect the precision resistor voltage divider shown to pin 8 of S1 to obtain 1.5 V and 0.5 V. Then a scaling table can be prepared.

The PC screen shows the DrDAQ reading the voltages of a Microprocessor controlled PSU on EXT. 1, 2 & 3.
Attachments
BUFFER 4 Jp.JPG
REVISED SCHEMATIC & ERROR CORRECTED
Glovisol
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Re: Universal buffer for the DrDAQ-ERRATA CORRIGE

Postby Glovisol » Sat Dec 27, 2014 5:10 am

On a routine verification of the schematic of the Universal Buffer I REALISED THERE IS A MISTAKE in VERSION 2 OF 12.12.2014. PINS 2 & 3 OF THE NE555 IC1 ARE INTERCHANGED. I have now amended the schematic drawing VERSION 2.4 of 24.12.2014 and inserted this schematic in the original post.

I apologise for any inconvenience caused.
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Re: Universal buffer for the DrDAQ

Postby alan » Sun Dec 28, 2014 5:42 am

Many thanks for this!! The DrDAQ is a great building block that can be at the heart of many projects.

I am sure your work will help many others.
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Re: Universal buffer for the DrDAQ - Further information

Postby Glovisol » Wed Jan 14, 2015 3:58 pm

Working in the lab with the DrDAQ + Buffer combination, I found the following.

1) To avoid unused channel overload by stray noise it is advisable to load any Buffer's unused input with a 1 KOhm resistor. This is easily done by mounting the resistor on the terminal board between input to be loaded and common ground.

2) Switching the Picolog software on, four different logging channels are available:
a) the SCOPE channel with a bipolar dynamic range of +/- 10 V.
b) the EXT 1 channel with positive input only, 0 to 2.5 V.
c) the EXT 2 channel with positive input only, 0 to 2.5 V.
d) the EXT 3 channel with single or differential input, by using the extra OP AMP. True bipolar differential input is possible by returning the input load resistor of AMP B not to terminal 3, ground, but to negative pin 8. Some care must be taken to have output of AMP B always in the positive quadrant.

3) Switching the Picoscope software on, four different, low frequency scope channels become available. By exploiting each channel's scaling and offset features is is possible to read down to a few millivolts: this is very good and useful sensitivity.
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Re: Universal buffer for the DrDAQ ERRATA CORRIGE

Postby Glovisol » Wed Jan 21, 2015 4:31 pm

I picked up a further error in the diagram: connections to EXT.2 and EXT.3 inputs were omitted. The corrected diagram with added connections in VIOLET is enclosed. Apologies again.
Attachments
BUFFER 5 Jp.jpg
Connections added to Ext. 2 & 3
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Re: Universal buffer for the DrDAQ - ERRATA CORRIGE

Postby Glovisol » Tue Feb 24, 2015 5:10 pm

BUFFER - ERRATA CORRIGE

There are errors in the list of connections of S1, schematic right side. The revised buffer schematic, Ver. 3.0 / 24/2/2015 is enclosed. More apologies.
Attachments
BUFFER 6.JPG
Buffer - revised schematic Ver. 3.0
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Re: Universal buffer for the DrDAQ

Postby Glovisol » Sat Feb 28, 2015 11:57 am

BUFFER - ERRATA CORRIGE

Buffer schematic Ver. 3.2 of 25/02/2015 - new schematic attached.

Corrected denominations of AUTODETECT resistors.

NOTE / THIS POST EDITED ON 18/11/2015 and buffer schematic has been replaced with Ver. 3.3 to correct diode D6, which was previously wrongly placed - APOLOGIES!!
Attachments
BUFFER 8 JP.JPG
Version 18/11/2015
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