Can QTR1A reflectance sensor operate on 3.3V? Alternatives?

Will the QTR-1A reflectance sensor operate at 3.3V? (I’m guessing not, otherwise would say so, but one can always hope.)

I’m looking for a reflectance sensor to use with the Arduino Pro Mini 328 - 3.3V/8MHz
( … ts_id=9220).

I note that the Pololu wheel encoder ( can be modified for 3.3V operation. Is this my only option?



The QTR-1x sensors are unlikely to work well at 3.3V because the IR LED will be quite dim. If you are comfortable with surface-mount soldering and have access to 0603 resistors, you could change the current-limiting resistor to allow for 3.3V operation, but the best solution might be to use our QTR-8X sensor array, which has a jumper that lets it work at 3.3V. The 8-sensor board can be broken into a 6-sensor board and a 2-sensor board, both of which can be easily modified to work at 3.3V. The encoder you linked to will not work as a general reflectance sensor.

- Ben


Another thing you could potentially do is use a soldering iron to remove the 47k resistor and replace the 150-ohm resistor with a solder short. You would then basically have a breakout board for the reflectance sensor that you could use with whatever values you like.



I think it could work decently at 3.3V for some applications, though your sensing distance wouldn’t be as big as at 5V. So, if you’re comfortable with the things Paul and Ben suggested, you could consider them as a backup plan and just try the modules first without any modification.

- Jan

Thanks guys for the quick and helpful responses.

I’m getting moderate output differences with the sensor “as is”. Probably not quite good enough for my needs though. To get good output lows requires placing the sensor very accurately with respect to my reflective strip (I’m using this as a cheap and dirty rotary encoder). I think modifying for 5V is easiest given that I’ve got someone here with surface mount soldering experience and a supply of 0603 resistors. In which case, what value should I replace the 150 ohm resistor with? Assuming the LED drops 2V, ~65 ohm to limit current to 20mA, yes?

(I’m going to go ahead and order some of the 8x strips regardless.)

That sounds about right. If you have some 150Ohm 0603 resistors around, you could solder one on top of (i.e. in parallel to) the one that’s already on the board, to cut the current-limiting resistor to 75 Ohms.

- Ben

2V is really high; it’s likely more around 1.2V. With a target current of about 20mA, that leads to about 95 ohms. Your 65 ohms would probably still be okay, but the current would be on the high side.

Also, we’ve been assembling these with 220 ohm resistors lately, so you should take that into consideration if you try some kind of stacked resistor approach.

- Jan