Am I being Stupid?

Forum 2005-2010 (read only) Hardware Interfacing
1

I'm simply trying to drive a SFH409 GAAS T1 Emitter from the 5v rail on the arduino which is powered from a 9v dc supply

I've put a 47ohm resistor between the 5v and the ir emitter to limit the current to under 100mA but for some reason the ir phototransistor (SFH309) just isn't responding to ir. I've checked the phototransistor is working ok and the emitter and phototransistor are only 1cm apart.

so the question is "am I being stupid?"

regards
Kev

2

Well, lots of things could be wrong. SFH409 is backwards? How is the SFH309 hooked up? How do you know it's "not responding to IR"?

3

sorry I was being ambiguous. The 309 phototransistor responds to ir using an alternative ir source but not when pointed at the 409 emitter so I am assuming the 409 isn't functioning. The 409 is connected correctly but I did recheck it :slight_smile: ,

I am assuming a Vf of 1.3v so the 47ohm should be limiting the current to around 80mA, is this sufficent to drive the emitter?

The 309 is being used in a simple voltage divider circuit with a 300k resistor in series and I'm picking off the voltage between them and feeding it to an analogue input pin (5v = beam is blocked, something less than 5v beam is not blocked, probably around 2 volts)

4

Only thing I can think of is SFH409 is dead. 80mA is more than enough to power it. Try reversing it? Or try a different SFH409?

Are you sure your resistor is 47 ohms and not 4.7 ohms?

5

sorted :slight_smile: the resistor was open circuit! that's the first dead resistor I've found lol.

didn't even consider that would be a problem. Thanks for the support it was doing my head in :slight_smile: