Category Archives: Electronics
At the Milford Hamfest, Debco Electronics was selling off grab bags. I bought two (and I'm calling them Junk Box Fillers)... of note:
- 74155 ICs - 2 line to 4 line decoder/demultiplexers
- LM613 Dual Op-Amp and Dual Comparator and Adjustable Voltage Reference
- M5229P Seven Element Graphic EQs
- 15 trimmer resistors
- 9 variable resistors (8 appear to be the same)
- Some 2N4861 JFETs
- Some IRL530 MOSFFETs
- Some other bipolar transistors
- Some touch tone decoders (one not on a circuit board, at least one on a circuit board)
- Some display boards
My work has taken me down the road of using Raspberry Pis as data collection devices. This means I need to power a Raspberry Pi in the field. I've had trouble finding a reasonably-priced 12V to 5V USB adapter that I could easily and safely fit into a box with a RPi. So I designed one in KiCAD and built it. The design is on my work github account.
I'm ultimately designing something that will connect to a battery, and batteries can explode if mistreated. Testing is critical, as is circuit protection (the fuse). I'm envisioning this to be in a box on the top of a pole with a camera, so the lead going from the battery (which will likely be on the ground) will be fused in case the wire gets cut. This is critical for the same reason it is necessary in a car - to protect the battery from short circuiting should something happen.
In putting these together at home, I tested these in every way I could think of, and assembly and testing went something like this:
- Solder SMD C2 and R1
- Test resistance from 7805 output to LED positive solder hole, should be 330 ohm (I used 330 ohm resistors instead of 310, since I don't happen to have any 310 ohm).
- Test continuity from 7805 output to ground via connected to C2. Should show no continuity.
- Solder USB connectors and C1
- Test capacitance from 7805 input to to ground via near C1. Should show a reading (mine all showed around 1000 uF, which is high, but my understanding is that multimeters are notoriously bad at capacitance)
- Add input headers, fuse holder, and LED
- Test continuity between inputs - should immediately beep, and then drop to no continuity after capacitors charge
- Apply voltage, LED should light, all magic smoke should remain contained in devices
- Test voltage from 12V- to 7805 output - should be 5.0v (mine showed something like 5.007v)
The one thing I was unable to test was the actual USB output voltages, but it seems to me that they should be okay.
I have five blank PCBs left for additional builds should I need it, although I'd have to have work buy more components. Maybe I could get some larger 7805s that would fit the ground pad...
I decided to build a Colpitts oscillator after watching one of W2AEW's videos.
The only issue I ran into was that I had the power supply hooked up backwards, so when I initially connected it to the scope, the +12V shorted to ground (and it burned a small hookup wire in the process). I should have paid more attention to the testing that W2AEW describes in his video - on a second try, I was seeing -7V where I should have been seeing +5V. And I shouldn't have trusted the two hookup wires I was using to get from a computer power supply output to a circuit.
I also didn't have 2N2222 or 2N5904 transistors, I used 2N3904s, which seem to work fine.
This is the second part of something that has been sitting on my workbench. Part I.
The video below shows the two traces of the input and the output of the second JFET.
The board is coming together as much as it could be for not being printed or having an enclosure.
The third part will be waiting for me to order a few pots, which I forgot to put on my last parts order.
With all the issues surrounding FTDI chips and drivers, I decided I'd look at a competing chip - the Microchip MCP2221. Unlike the "standard" FTDI FT232 (which is most similar to the MCP2200), the MCP2221 includes both a UART and an I2C interface.
Hookup and Configuration
Hooking up this is pretty easy. I sacrificed a USB cable and pulled the red to Vcc on pin 1, black to ground on pin 14, white to Data- on pin 12, and green to Data+ on pin 13.
After that, I checked things using the Microchip MCP2221 Utility from Microchip. One of the more interesting things in the utility was being able to configure four pins:
GP0 (pin 2), default is UART RX LED, but can also be SSPND (suspend?) or GPIO
GP1 (pin 3), default is UART TX LED, but can also be CLK_OUT (clock out?), ADC1, IOC, or GPIO
GP2 (pin7), default is USBCFG, but can also be ADC2, DAC1, or GPIO
GP3 (pin 8), default is I2C LED, but can also be ADC3, DAC2, or GPIO
The real reason I got this was to be able to interface to both I2C devices and UART (serial) devices, so I hooked a TC74 temperature sensor to it.
The hookup is straightforward, the TC74 uses four of it's five pins, Vcc, ground, SCL, and SDA. Vcc and ground are pretty self explanatory, SCL and SDA go to pins 10 and 9 on the MCP2221 (respectively). The SCL and SDA lines need pull-ups, I used 1.5k resistors.
To check things, Microchip provides an I2C/SMBus Terminal program.
Under "Advanced settings" you can scan the bus by giving it a range to scan...
Once I verified that the sensor was where I thought it was, I used the terminal tool to read it...
So the next steps are to figure out what I want to do with all this... I'm partly thinking a multi-function breakout board - UART, ADC and DAC
Many months ago, my brother-in-law brought over a guitar with the thought of "you fix it, I'll cover the costs, and we'll split the profits". It wasn't a bad deal, so I started fixing it. A month later, after I preceded to remove a large part of the partially-damaged finish, he called and told me that he got another guitar and he was no longer interested in pursuing it.
So it sat for many weeks.
And it sat for a few more weeks.
Finally, the same brother-in-law brought over a Marshall head that blew a fuse and a few diodes to repair. I needed a guitar to test it with, so I finished stripping it and ended up with something that looked decent and actually played (which I ultimately used to test the amp).
Around the same time, Hack-A-Day posted about the 1Wamp, a small, battery powered amp. The schematic looked like an easy build, and I was fairly certain I had all the parts lying around.
Amp Build Part 1: The Preamp
The preamp is very simple: 3 resistors and a JFET.
I skipped the Zener diodes, which are there for static protection. I'll probably add them in at some point in the near future.
It does work. I hooked it up to a 12V linear power supply and took a video of the input (from the guitar) trace and the output trace from the preamp.
The input is about 0.6 - 0.8 Vp-p. The output is about 1.6 - 2.0 Vp-p. That puts it somewhere in the range of 6-8 dBV. That's a little over spec, but I'm also feeding it 13.8V instead of 9.0 - 9.5V.
Next up: the Tone Control. zOMG 6 components!