QTR-1A Problems

I’ve got four QTR-1A sensors underneath the robot I’m building. Each supplying a different analog pin on an arduino. Will be using these for basic line detecting, however I cannot get the sensors to return anything but 4.8v.

Some notes about this project:

Running from USB power to the arduino OR battery power through 7805 regulator has the same returns.

All powers are tied together, all grounds are tied together, only thing separated is the signal return wires. I have even disconnected one sensor and tested it on it’s own with the same results. No matter a black surface, white surface, or any other surface, I get no change in return voltage.

Using a digital camera to look at the sensors while on, I cannot tell if the LEDs are even on, using other sensors I can, but I am unsure of how bright these LEDs are.

Could it be a bad batch that I have, or am I doing something completely wrong? Only one way to wire them up…lol. Maybe I’m tired and over looking something simple.


Rechecking some things its the IR LED is not turning on. My supply voltage drops from 5v to 4.3V across the LED. Isn’t one side of the LED supposed to be ground? I would imagine the anode would be fed from the 220 ohm resistor on board, while the cathode would be connected to ground, but testing across, the cathode is not connected to ground?

Tying the cathode of the IR led to ground, it works!! Not sure if this is a problem with the traces on the board or what, very strange.

Hello, necrolyte,

I am glad you figured out a solution. As shown on the schematic for the QTR-1A, the cathode of the LED should definitely be tied to ground, so it is strange that you observed otherwise. Was this an issue with all four of the sensors? Did you notice anything wrong with the solder connections or the traces on the board?

Can you show us some pictures of your QTR-1A boards? (If you have a camera with a macro mode, using it should help you take clear closeups.)

- Kevin

All four boards had the same problem. I noticed there is a ground via under the led portion, but without removing it I couldn’t tell if it was connected to the cathode. It looks like it SHOULD be, but being a multi-layer board I could not tell. I ended up installing a jumper wire directly to the cathode to the ground header pin. I don’t have smt equipment else I would have removed the led/collector and tied it back directly. You may want to test a few samples to see if its a bad batch.

I can try to get pics, but I don’t think it would help besides showing my wire jumper.

Chris Williamson


We have not had any other reports of problems like this (when did you purchase the sensors?), and we test all of the QTR boards before they ship, so if all of your boards are affected, it seems very likely that there is a problem with how you are using them. However, the interface is so simple that I don’t have any ideas as to what you could reasonably be doing wrong. Are you absolutely sure you are connecting power and ground to the proper pins?

- Ben

I purchased them on May 5th, and just now have gotten around to installing them. I too thought that I was doing something wrong, but it’s almost impossible to screw it up. I am very sure that I have them hooked up correctly, else by doing the ground jumper that I did, it should be dead shorting, and it’s working instead.

One thing that I am doing, but would not be affecting it, is that I’m using the 3 pin header you guys supply upside down, meaning the long side is going into the board because I’ve got the sensors mounted on the bottom of the robot (which is 1/8" thick), so I machined three matching holes through the bottom of the chassis and had to use the long end of the header to make it to the sensor board.

I’ll get a pic shortly and link it here.

Here’s a couple of pictures of the bottom of the robot. I don’t have a great macro filter, so I’m sorry about the quality. The first picture shows the jumpers that I installed from the cathode to the ground pin. The second is a picture showing that the LEDs are turned on with power on.

I know the screw heads look like they are messing up stuff, but they are plenty clear of the resistors on-board.

We have sold quite a few of these since May 5th without hearing of any similar problems, and it’s not realistic to think you would get (the only) four boards with the same rare defect, so I think there must be something that you are doing or initially did to the sensors that is systematically wrong. The ground pin should connect to the LED cathode through the board’s lone via. I’m not sure how easy it is to test now that you have added your wire jumpers, but it would be interesting to find out where the connection is missing on your four boards: between the GND pin and the via or between the via and the LED cathode. Do you see any issues with the solder connections between the LED and the SMT pads or between the GND pin and the through-hole? Does the trace from GND to the via on the underside of the boards look like it is cut anywhere?

- Ben

I can tell that the via is supposed to be tied to the cathode on the LED, however I cannot tell by eye that it is connected. Before I made my jumpers, by ohming out across the cathode and ground, they were not connected.

All of the traces look fine on the board. I did ohm out the via and ground pin before I connected the jumper, and they were infact connected, so I believe the problem was between the cathode and via. Without desoldering and removing the sensor itself, I can not tell if that trace is cut. If the trace IS cut, I see no way that I could’ve caused this on four boards.

If I had SMT equipment, I’d be happy to desolder one and check things out, but I don’t have anything small enough to do this with.

Not trying to say I’m not happy with the boards, I really love these things, just wanted to point out the trouble I’ve had. I agree that it’s not realistic that I would get the only four bad boards in the bunch, lol.

I initially had two sensors on the bottom of the robot, and left the other two in packaging. I changed my mind before powering up to add the other two sensors, and went directly to installing them. They literally went from package to robot, so I am unsure how anything may have happened to them.

Again, great product, I’m really happy with them, and I’ll be ordering more soon for further playing on other robots I hope, as well as more distance sensors. I’m really enjoying building this mini-sumo robot!

Well, I’m very glad to hear that you’re happy with them even though they did not work for you as they should. I’m still very curious about what the problem is; please let us know if you discover anything more about where the ground disconnect is on your boards. I’d also love to hear how your mini sumo turns out!

- Ben

Here’s some testing I was doing after I got the line sensors working. I have a SHARP distance sensor in the front and rear of the chassis as well.