# 4 photo-interrupters in my circuit

Discussion in 'General Electronics Discussion' started by Charybdis, Nov 13, 2013.

1. ### Charybdis

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0
Nov 13, 2013
Greetings from a newbie!

I'm a 2nd year Electronics student working on a design project. I've designed a bird-counting bird-feeder, using a PIC16F887 chip and 7-segment LED display (4 digit). In order to detect a perching bird to count, I decided to use 4 slotted optical switches (OPB991T55Z), with a mechanical trip on 4 different perches.

My questions are as follows - 1) With the 4 switches in parallel going to the input ports of the chip, will I have enough current to run the detector side of the switches? I don't see where the data sheet for these components specifies a minimum current, only a maximum. I'm running a TTL logic circuit at the usual 5V. I was planning on using a 100-ohm current-limiting resistor. But I'm wondering is this is even necessary if I have 4 branches coming off of the main circuit, besides the current to the chip itself.

2) If I do still need the current-limiting resistor, will it be okay to tie all 4 of the switches to the same resistor off of the main line, or should each switch have it's own resistor at the branch? (It's doubtful that more than 1 input will be triggered at the same exact time, but I suppose it's not impossible.)

Here is the data sheet for the OPB99XXXX series:

http://www.optekinc.com/datasheets/OPB960-990_SERIES.PDF

I appreciate any insights or suggestions.

Thank you!

2. ### BobK

7,682
1,688
Jan 5, 2010
Look at the If(+) parameter and note 16 on page 5. It specifies that the LED current for detection must be > 15 mA. The datasheet also lists the forward voltage as 1.7V at 20mA, so that is probably the normal operating current. You should have a current limiting resistor for each detector since they will not necessarily share current equally when using only 1 resistor. Calculate the resistor to provide 20mA.

Bob

3. ### Charybdis

2
0
Nov 13, 2013

Just to clarify - So I'll need the current limiting resistors for the LED emitter to bring the If to 20 mA. But do I also need them for the sensors themselves? It appears the supply current (Icc on pg. 5) is to be no more than 12 mA, but doesn't show a minimum.

I'm trying to keep the circuit as simple as possible. Is there a way I can use one resistor for each line with the emitter and detector both tied to it without exceeding the current limit for both, or am I stuck with two different resistor values for each side of the 4 banks?

Thanks again!

4. ### BobK

7,682
1,688
Jan 5, 2010
You do not need to limit the current of the sensor. The Icc parameter is telling you the max current it will draw when connected to anywhere between 4.5 and 16V. You just need a power supply that can supply this much current.

You do need a pullup resistor on the output. If these are connected directly to the PIC, you can use weak pullups in the PIC and do not need an extra resistor.

Bob