controlling the LED current using the DAQ assistance

[GoviRe]

Hello,

I am working with sampling the analog signal(voltage) of the photodiode.I am using a LED whose intensity will be sensed by the photodiode.Right now I am sampling the analog voltage signal of Photodiode using one channel of DAQ assitance.But now I want to controll the current of the LED and the LED power supply circuit is shown in the attachment,where the 3V battery is used as the supply I know that the voltage can be controlled using the GPIB pins when a power supply is used.

But I am using a 3V battery as the supply.I guess the LED current can be controlled in the two following ways

1)Using a constant resistor while varying the voltage of the battery

2)Or Using the constant supply of 3V while varying the resistance(uisng a potentiometer)

Can anyone suggest(explain) a way of controlling the LED current using the above mentioned or if any other possible way using the DAQ assistance.

Thanks.

[johnsold]

The DAQ Assistant is not a hardware device, only software to control certain DAQ devices. So, first we need to know what DAQ device(s) you may have available.

A DAQ device with an Analog Output could be used as a controllable voltage source. However, most DAQ AO lines do not have enough current capability to drive an LED to full brightness. So you typically would need to use an external buffer (amplifier or adjustable voltage regulator) to boost the current from the AO to a level suitable for the LED.

Lynn

[GoviRe]

Hi Lynn,

I am using the DAQPad-6015/6016.I also have this voltage regulator http://www.semiconductorstore.com/pages/asp/DownloadDirect.asp?sid=1387026628652

Is it possible to controll the voltage using this kind of voltage regulator through the DAQ assistance.

Thanks.

[johnsold]

The DAQ device should be fine. The link to the Semiconductor Store just brings up the home page. It does not link to a specific regulator. Please let us know which regulator you have.

Lynn

[GoviRe]

Sorry for the broken link but here I am attching the datahsheet of the Voltage regulator.

Can you explain how can I implement the required.

Thanks.

[johnsold]

You need an adjustable regulator.  I have used the LM317.  From a quick glance at the data sheet, it appears that the XC6216 Series Type C would work, although the minimum output voltage is 2.0 V. That may be too high, depending on the type of LED you are using. With the LM317 the minimum output voltage is 1.25 V.

You connect the output of the DAQ AO to the Adjust terminal of the regulator. For the LM317 the output of the regulator is then the DAQ voltage + 1.25 V.  The input to the regulator needs to be about 2 to 3 volts higher than the highest output voltage you want.

By adding an op amp and a few resistors to the LM 317 or XC6216 Series Type C you can make a current regulator which tracks the AO voltage.

Lynn

[GoviRe]

Thanks Lynn,

You mean if I need(for example) 4V (of which1.25Vminimum regulator voltage+2.75required voltage for LED) then input of the regulator should supplied with 4+2 or 3V

But I need a 3V for powering my microcontroller  also.So,should I use two voltage regulator one for controller and other for LED current controlling or is there any other for this.

Thanks.

[johnsold]

You need a fixed voltage for the microcontroller. You need a variable voltage (or current) for the LED. So you need two regulators. Any standard 3 V regulator should be fine for the microcontroller.

For the adjustable regulator input voltage the calculation is Maximum output voltage (~2.75 V) + Minimum input/output differential (or Dropout voltage). For the LM317 the dropout voltage varies with output current and temperature.  At 25 C the typical values range from 1.5 to 2.3 V. But the internal temperature may be higher so it is common to allow 3 volts above the Maximum output voltage. If the LED currents will be less than ~100 mA, a 5 V supply would probably work.  The minimum output voltage from the LM317 with the adjust terminal grounded is ~1.25 V. If your LED will turn on (dimly) at that voltage, you may need to add some circuitry.

Lynn