phototransistor

[Murph]

I built a simple beam interrupt circuit with an ir LED and matching phototransistor.

Electrically it works perfectly.

If I measure the DAQ point with a multimeter it works.

Once I connect it to my DAQ Digital input it no longer drops the voltage enough to read it as a digital 0

Labview 2012. USB-6008 DAQ

See attached pic.

[apok]

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@Murph wrote:

I built a simple beam interrupt circuit with an ir LED and matching phototransistor.

Electrically it works perfectly.

If I measure the DAQ point with a multimeter it works.

Once I connect it to my DAQ Digital input it no longer drops the voltage enough to read it as a digital 0

Labview 2012. USB-6008 DAQ

See attached pic.

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is the ground on the transistor side connected to the daq gnd?

[Murph]

No.

The ground of the USB-6008 is connected to the ground of the circuit.

The collector of the phototransistor is what is connected to Din.

The light source is always on holding the collector low (with transistor on).

When the bean is broken the transistor should turn off allowing the collector to float up to close to +5v.

All this works with a voltmeter connected to the same spot.

Once I connect the DAQ, the low signal remains the same but it can no longer rise to anything even close to a TTL Hi.

[johnsold]

Murph,

Do you mean No or Yes? Apok's question referred to the emitter of the OP501A which you have shown as ground.  It that connected to DGND or any other ground on the USB-6008?

The USB_6008 has an internal pull-up resistor of 4700 ohms. Are you sure you have enough light to drive the phototransistor to Ic > 1 mA?

Lynn

[Murph]

I'm using an external power source to drive the OP280 LED, no worries there.

I am also using an external power supply at the collector of the OP501A, it then goes through a 100k resitor. The Daq is conneted there (like in the diagram) onto pin 17 (PO.0), the Dgnd (Pin 32) is connected to the ground of the external power supply (and emitter of the OP501A).

[johnsold]

What is the voltage measured at the DAQ input (using the voltmeter) with the LED on and off?

Lynn

[Murph]

With the DAQ disconnected using a voltmeter at the collector of the OP501A I get just under +5V with the LED off, around 0.25v when the LED is on.

After the DAQ is connected it goes from the +5V down to about 3.85, not enough to be read as a digital zero.

If I pass it through an opamp follower, it works and the voltages read the same as a meter. I'd like to avoid using circuits between the LED's and the DAQ.

[johnsold]

Murph,

Thank you. Those measurements tell me what is going on.

When you have the USB-6008 disconnected, the transistor goes into saturation and the current is limited to about 50 uA by the collector resistor.

With the USB-6008 connected the collector resistor is 100 kohms in parallel with the 4.7 kohm internal pull up.  This produces an equivalent resitance of ~ 4.5 kohms. The voltage you measure with everything connected, 3.85 V corresponds to about 250 uA of collector current.

I could not find any data on an OP501A. I found OPTEK Technology OP510 and OP501DA. Both of those appear to conduct poorly at low collector-emitter voltages.  If you are using one of these, you may need to use additional circuitry.

Lynn

[apok]

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@Murph wrote:

I'm using an external power source to drive the OP280 LED, no worries there.

I am also using an external power supply at the collector of the OP501A, it then goes through a 100k resitor. The Daq is conneted there (like in the diagram) onto pin 17 (PO.0), the Dgnd (Pin 32) is connected to the ground of the external power supply (and emitter of the OP501A).

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through your 100k resistor, you get 50microamps...the spec asks for 0.1milamp to 10milamp minimum collector current? change your resistor value....

[Murph]

It is indeed an Optek OP501DA http://www.optekinc.com/datasheets/OP501DA.pdf I am using.

What other way can I trigger a digital input with a phototransistor or diode?

I have looked at this in the past without success.