Optical interrupter connections

w4fej

I trying to use a Honeywell HOA6991-L51 device with the Prop development board. (P/N 32111).

I have connected the device per the datasheet (at least I think I have) as follows:

Red - IR diode anode (+3.3vdc)
BLK - IR diode cathode (ground)
Grn - Goround
Wht - VCC (+5.0vdc)
Blue - Output. Pin 20 on the prop pulled up to 3.3vdc via 300 ohm...at the junction of the collector and pin 20

It makes no difference if the path is open or clocked the output side remains at or near ground the the pin 20 side remains at 3.3 vdc. By the way I removed the connection to pin 20 and no change..

Anyone see the flaw in this setup??

Mike B.

Phil Pilgrim (PhiPi)

Okay, first of all, you need a current-limiting resistor in series with the IR diode to avoid frying it. Its forward voltage is 1.6V and is rated at 20 mA. So (3.3V - 1.6V) / 0.02A = 85 ohms. A 100-ohm resistor would work fine there.

Next, 300 ohms seems quite stiff as a pull-up resistor, unless you need a very fast response.

Finally, what are you blocking the IR path with? Often, things that seem opaque to the human eye are transparent to IR. Try using a thin piece of metal to make sure you're blocking the path.

-Phil

Harprit

Its worth clarifying for beginners that the specific device that was initially used is probably no longer viable.
H

Mark_T

Typical pull-up resistor values on the output might be more like several k or several 10's of k, not 300 ohms.

Phil Pilgrim (PhiPi)

Mark_T wrote:

Typical pull-up resistor values on the output might be more like several k or several 10's of k, not 300 ohms.

Those were my thoughts, too, as expressed above. However, in the part's datasheet for the open-collector version, propagation time and turn-on/turn-off delays are characterized with a 390-ohm pull-up. But unless the OP needs a 70 ns rise time, a much higher valued pull-up would work fine without the extra current draw.

-Phil