Last friday I ordered 5 samples of a tiny smd op-amp from Texas Instruments. Very easy, no big fuss like the guys at Microchip are doing lately. I was very happy to see that they were shipping them with fed-ex and it would arrive on monday! They didn’t need to as samples are free, but well that was the only option :-)
They even put it in a huge box before they shipped it with fed-ex:
CO2 tax anybody?
(Oh yes, what did I order? A replacement for the AD8552 dual op-amp used in my thermophile sensor module. Analog didn’t give samples in the SOIC8 package. Texas Instruments did: the OPA2335
Note to self: + is Vss and – is Gnd ;-) )
This weekend, I made a bad PCB. The only option that was left, was creating a new one. But my PIC was already soldered on! After a search on the net, I didn’t find any approach with simple tools to desolder an SMD chip (in a way that it will still work afterwards!).
This and this are brilliant examples of how not to do it :-)
So I came up with my own solution:
- Put a lot of solder on the 4 sides of the chip.
- Turn an iron (to iron your clothes) upside down on maximum temperature.
- Put your PCB on it (no solder on the bottom).
- Now quickly heat up the 4 soldered sides with your soldering iron.
- The heat of the iron will make sure the solder remains liquid for a few seconds.
- You can use your soldering iron to push the chip off the PCB when you’re desoldering the last side.
- Do it quick so your chip won’t be burnt!
Good luck :-)
Analog devices has created a new inertial sensor, the ADIS16355. It includes a 3 axis high precision gyroscope, and 3 accelerometers. On top of that, it includes digital I/O with some on-board basic sensor filtering. It’s target market are dead-reckoning applications such as car navigation when a GPS signal is lost.
Being a cube with a side of 23mm, it seems kind of big. Unfortunately no weight specs on the datasheet yet. For the very small MAV’s this might be too big, but for most autopilot systems this sensor will improve navigation performance!
Samples will cost $350, +1000 will be $250 per unit.
Last month I finally bought a GPS module. I decided on the EB-85 because it’s cheap (thanks to mr. rc-cam!) and it has a 5Hz update rate!
However, this module has an UART that operates at 2.8V. My PIC runs at 5V. After some investigations I figures out that the TTL pins of my PIC would interprete the 2.8V as logical high, but the ST-pins (Schmitt-trigger) won’t. Because the hardware UART or PIC uses ST, I was left with 2 options: Use the TTL-pins and a software UART or continue looking for another solution :-)
What I need is a TTL chip that has en open-collector output. TTL accepts 2.8V as a logical high, and the open-collector port allows me to use any voltage as logical out if I use a pull-up resistor.
My solution uses the 7407 IC, which is a “Hex Buffers/Drivers With Open-Collector High-Voltage Outputs”.
This is the schematic:
On the left you see the PIC, on the right you see the connector to my GPS. In the center-top you see the pull-up resistor to 5V, in the center-bottom you see the pull-up resistor to 2.8V that consists of a voltage divider that yields 2.8V.
On my test-board it looks as follows:
This is what my test-program does:
- Send a message to the GPS so only the RMC (Recommanded minimum) messages come through:
- Every time a byte is received on the UART, an interrupt is launched and the received data is decoded.
- The main loop prints the data in the LCD
The overhead of the UART and GPS decoding is very low on my 4Mhz PIC. Perfect!
Here are some nice sites for electronics newbies like myself:
There was a slight error in my schematic: I played the voltage divider before the buffer instead of after it. It is corrected now.
This should also work (but my 7407 chip has 6 buffers so I didn’t care):
Unfortunately I’ve been busy with other things, so I haven’t added a new Kalman tutorial. One of the reasons is that I ordered 4 melexis sensors (IR) for an easier stabilisation for my first autopilot attempt. Almost 2 months after ordering, they finally arrived! Get them yourselves from Scale Robotics if you have enough patience to wait 2 months.
The PIC’s shown are some I bought really cheap while i was in Turkey :-)
My first goal now is to create an autostabilizing platform using a PIC and the 4 thermophile sensors.
It’s main features are
- Build in GPS (antenna not included)
- IMU (accelerometer + gyro) in 2 dimensions (pitch and roll)
- PIC 18F ready to program :-)
Selling at less than 300$, it’s pretty cheap!
Some remarks and stuff I’d love to see changed in the next version:
- The GPS isn’t the best available. uBlox GPS’s are a lot better.
- Not really optimised for weight…
- I wonder how easy it is to add other sensors.
- Is a 18F PIC powerfull enough?
- Doesn’t take advantage of the new 2-axis gyroscope in 1 chip.
I’ve been surfing the internet looking for ideas for my autopilot system. The most important sensor we need, is a GPS. Several important factors that influence my choice are:
- Update rate
- Easy to use
Right now I found two candidates:
a Sirf Star III based module (1Hz update, 10m accuracy, 17 gram)
Unfortunately, it uses some weird flex cable to connect, so purchasing it could be a risk when I wouldn’t find the right connector. In the the RCB-4H (basically a development board; 17 grams and requires an active antenna) might be a better choice…
To be continued :-)