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I just wanted to understand the process of measuring battery...etc. voltages using the DrDAQ device. In their PDF broshure they say:

Thanks to the power of your DrDAQ you can also use it as an oscilloscope or spectrum analyzer. Just run the supplied PicoScope software and your DrDAQ becomes a singlechannel scope with a 100 kHz bandwidth, 8-bit resolution and the ability to measure voltages of up to ±10 volts.

Do I need to have the oscilloscope probe, or would I be able to measure voltages using the "screw" terminal connectors? How would I go about doing that?

Disclaimer: I have read their user manual more than once, but they do not have any instructions for measuring voltages. Their website is full of experiments, but none of them detail the process of measuring battery voltages using DrDAQ.

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Yes - you can measure battery voltage directly using the DrDAQ USB datalogger.
However, the oscilloscope channel will measure voltages up to 10V directly and the 3 x RJ11 analog input channels (on the right hand side, labelled Ext 1 ... 3) will measure up to (only) 2.5 Volts directly. You will be able to use a "voltage divider" using two resistors to measure higher voltages than these. See below.

enter image description here

The specification sheet provides information on voltage levels and general capability.

You do not need an oscilloscope probe. You can either use the oscilloscope channel with a BNC connector wired to whatever wires you want or you can use any of the 3 x RJ11 connectors (aka FCC68 4/4) using an RJ11 plug. RJ11 is used for telephone cables and many other applications and you can obtain one for little or nothing.

The "pinout" of the RJ11 connectors is not shown on the specification sheet (reference above) and there is also power out on these. Due to the cost of the board you will no doubt want to be a little wary when "playing" You may have a full manual that shown pinouts. If not then

  • Turn PCB over. Are two pins of each RJ11 connector joined together If so, these will be ground.

  • The RJ11 will probably use two pins for signal_in + ground and two pins for power_out + ground. The grounds will probably be joined.

    With a voltmeter measure between each pair or pins to try to locate power and ground. Probably 0/5 or 0/3.3 or perhaps 0/3 or ? Pin combinations are 12 13 14 23 24 34 so you only need 4 measurements max. If you gt -5 or -3.3 etc swap leads to get +5 +3.3 etc. Then positive lead is + psu and -ve lead is ground.

    Signal in is two leads that are not psu. Grounds are probably common at connector or elsewhere on PCB. It is better to use ground actually made for Vin if not commined at connector but should work well enough using either if there are two grounds.

    They MAY have some other unction on the last pin eg signal_in, +V, ground, ???.

    Asking thenm for pinout if not in manual would be a good idea [tm]

Using the oscilloscope input is the safest start. You will very very probably NIT hurt it with up to 10 V_DC applied (or quite a bit more you'd hope. Scope input has 4 ranges which are software switched - hopefully using supplies cope software will allow you to choose range.

Using resistors to make a voltage divider.

Two resistors can be used to scale battery or other voltages down to the level acceptable to the DrDAQ board. In the diagram below the battery to be measured is Vin and the output to the DrDAQ is Vout. The voltage scaling down ratio is

  • Vout = Vin x R1 / (R1 + R2)

  • Vin = Vout x (R1+R2) / R1

R1 + R2 could be a "potentiometer" (aka pot, variable resistor ...) if you want to play. A 10K pot would (probably) be a suitable value. Note that using a pot or divider of your choive you can apply more voltage to the input than its maximum rating. I'd hope that DrDAQ were competent enough to protect their inputs to well above rated value BUT you need to see what they and their data sheet says.

If you make R1 = 10k then if R2 = 2.2k the scaling range is 3.3:1. If you make R1 = 1k and R2 = 6.8k then the scaling factor is 7.8:1 etc.

I'd recommend making R1 = 1k to start playing then later try R1 = 10k. It MAY be you can use R1 = 100k but that will probably introduce inaccuracies.

enter image description here

The above diagram is from here Worth a read for you I think.

Software access:

I do not know how you accss the EXT channels using the supplied software. Using the BNC scope input it should be aasy to use the supplied software. The Ext inuts may be easily accessible with the supplied softwae, or not. If not then using the scope BNC input would be a good starting point. The resistor scaling applies, but the R1=100k is liable to work more accurately than with Ext inputs.


Read. Process. Ask more questions ... .

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  • \$\begingroup\$ That is what I needed to know. Thank you very much. Also, I have found the pinout diagram for their RJ11. Its on page 28 of this document. Pin1-Signal input. Pin2-Ground. Pin3-Autodetect. Pin4-VCC. And also. I have just confirmed with PC-OSCILLOSCOPES that it is not possible to measure voltage using screw terminal on DrDAQ USB. \$\endgroup\$ – Temperage Aug 18 '11 at 0:31
  • \$\begingroup\$ @Temperage - See page 7 in the manual. The scope buttons show EXT1 2 3 along the top - you should be able to use them. See page 33 - its shows external voltage scaling exactly as I have showed above. (They use R1 = 1K. The terminald at rear are digital outputs. The scope should be able to display the Analog inputs via the RJ11 jacks at right (when oscilloscope BNC connector is facing forwards.) See page 13 for input/output specs. Read and re-read pages 28-33 until uoi understand them well. Read 34-38 & understand (harder?). \$\endgroup\$ – Russell McMahon Aug 18 '11 at 3:29
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I want to describe how I ended up measuring voltages using DrDAQ. I have a USB version of DrDAQ which does not allow you to measure voltages using the screw terminal. The serial version of the board does have that capability.

The easiest way to measure voltages up to 10V is to use the external passive oscilloscope probe. I just plugged in the probe BNC connector to the middle "scope" port of DrDAQ and used the crocodile clips to connect to the (anode) negative battery end, and connect the (cathode) positive battery end to the tip of the oscilloscope probe. I was able to monitor voltages in real-time using either the PicoScope or the PicoLog software. You will have to select the Oscilloscope channel without making any further adjustments.

Another possibility is to develop your own sensor that can be plugged into the EXT1-3 RJ11 plugs on the side of the unit (kindly suggested by @Russell McMahon). Here is the pin diagram from their manual.

PIN Diagram of DrDAQ External sensors

I was able to measure voltages up to 2.5V by connecting the Pin1 to the cathode, and Pin2 to the anode of the battery. It is also possible to measure higher voltages by using a current divider. DrDAQ manual has this diagram for constructing a voltage measuring sensor.

DrDAQ Voltmeter Sensor

It should also be noted that DrDAQ should not be subjected to high voltages. Their manual says the unit is protected up to 30V, but I would not risk frying my USB ports and destroying my laptop.

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