Input devices

This week, I set out to do 2 sensors : a magnetic field sensor and a proximity sensor based on Neil's step response.

Magnetic field sensor :

This board uses the hall effect sensor described in class.

This took a while to figure out because i had to understand all the vocabulary in the datasheet. And if i have to give a piece of advice, it would be : RTFM!!!. I designed the circuit based on the resolution I wanted using the 1.1 V internal reference then realised that the output of the hal effect sensor was biased so it output 2.5 V when B=0.

The sensitivity of the sensor is 5 mV/G so using Vcc as a ref would have given me really bad resolution for smalle fields. Some quick calculations showed that I wanted the voltage to vary between 2.4 & 2.6 V to get the really high resolution.  I decided then to make a voltage divider on one pin and put the sensor on the other and measure the difference. It is bascially the same idea as the wheastone bridge.

I then saw in the datasheet that the noise was about 40 mV and I was trying to measure up to 500 mV so I was worried about the noise. I decided to do a test run of the device by measure +- 0.55 V (ie +- 110 G)  by using the voltage divider and the 1.1 V internal reference.

So on the voltage divider side, the voltage is 1.95 V. this means I will measure 0 V difference between the sensor and the divider when B= -110 G, i will measure 0.55 V when B =0 G.

Here is a schematic, the board layout and the png for milling :



An here is the final board :


I modified Neil's temperature sensor code to work for this and it programmed correctly (according to avrdude) .

However, whenever I try to run the python side of things, the application freezes and crashes. A bit of testing showed that I cam not actually sending or receiving any bits.
I will try with another computer & cable to see if it is hardware related or software related.

Proximity sensor :

I was really impressed with Neil's step response sensor and wanted to try it for myself. I also wanted to try programming it differently (using Rehmi's suggestions) to see if I can get more range out of it.

I have only had  time to make the board.


Avrdude said it programmed correctly using neil's code but i currently have no way of verifying things since the serial communications is also freezing for this.

 I found that for the proximity sensor, the problem was that i had the FTDI header backwards and I had the 2 cables connected which shorted everything. This one works fine now, although it is mainly a touch sensor.

After spending alot of time debugging the magnetic board, I found 2 problems : first, in the bridge, there is a pull down resistor which is unneeded and caused trouble. I was getting things which looked like this on the serial line but good signal on the inputs :

After a long time, I realized it was the ground which was the problem, the solder at the ftdi wasn't good and the traces on the top weren't connected properly to ground. Now it works fine but it seems slower than the other sensors: ))