Monday, March 16, 2015

Tinker-Box á la Bob Pease

I made a tinker-box, as recommended by Bob Pease in Troubleshooting Analog Circuits. It's simply a box of banana jacks connected to various resistors and capacitors. There is one rotary switch connected to resistors of values 10-66kOhms. There is a second rotary switch connected to capacitors of values 10p-1uF. Finally there is also a 100k potentiometer and a ferrite bead. Not much else to say about it other than that it's of great use when experimenting with op-amps. I used it when  i built last posts EMC-meter.
Inside of tinker-box.

Panel-side of tinker-box.

Saturday, March 14, 2015

EMC sound detector

I saw this youtube video a while ago. It shows an EMC meter using a piezoelectric summer, a coil and a fet.I thought that instead of  coil, I could do the same thing using a semi-rigid cable for a magnetic field probe, like the ones Doug Smith describes. I have previously written about a field probe that I have used with an oscilloscope. The signal from the probe that I have seen has been on the order of 10mV, max. This is far to low to make a summer sound. The summer needs a at least a few volts of signal to sound enough.
So to make this circuit work I needed to produce a gain of at least a few hundred. I started out trying to do this with an uA741. With a gain-bandwidth product of 1MHz, I could at most produce a gain of 100 at 10kHz, which was a bit weak. After that I tried using a . It had a better gain-bandwidth product of 3MHz, but I had a lot of problems with lock up and oscillations.
Finally I solved the gain-bandwidth issue using two op-amps, the LM358 (two amplifiers in one package).  Since the signal is low, and we are interested in the AC-signal, I AC-coupled the feedback loop in both amplifier stages. Final circuit looks like this:
EMC sound detector
EMC sound detector, actual build.

There was some trial and error and usage of what components was at hand. To determine the maximum gain of the second stage, I used a potentiometer where the 270 ohm resistor is now. I turned the potentiometer to just below the point where the circuit started to oscillate. The circuit is sensitive enough to sound out fields from, for example a cell phone, but the sound volume isn't that high. I think the circuit can achieve higher gain with a smaller cap on the output (22uF now). I would need to put a higher resistor after it though, to keep this stage from filtering out low, but audible frequencies.

Tuesday, March 10, 2015

New PCB's and Books

Got the new PCB's for the MIDI2VC+ v2 from OshPark yesterday. I'm pretty happy with how they look. I managed to cram in everything on the top layer. The bottom layer is only ground, not a single trace or component.
New PCB's from OshPark
Also, I got some nice books from dad for my birthday. It's:"Build your own transistor radios" by Ronald Quan and "Experimental methods in RF design" Wes Hayward et. al. These will most certainly give some nice ideas for electronics projects. I've already started reading Quan's book, and it's really interesting.
Some nice birthday books from dad

Sunday, March 1, 2015

Velleman KSR6 kit, error in silkscren

Last post I talked about the KSR6 Velleman kit and that the robot didn't turn when closing in on objects. After troubleshooting the robot for a while, I finally found the error. The voltage divided node at U4 +in and U3 -in lay at nearly 6 volts, instead of the expected 3 volts (which in retrospect is odd, this voltage should instead be closer to 2V, only explanation I can come up with is that that R8 had a bad connection, but who knows?). This made the integrator U3 integrate only voltages over the node, which was almost nothing... After inspecting the schematics and the layout, it was obvious that the silkscreen component markings had mixed up R8 and R10. Switching them made the robot work the expected way.
This might not be true for all PCB versions. This PCB was marked 21-885. If you are uncertain, measure the voltage of pin 3 or 13 of IC1. It should be  close to 3V. If resistors are placed wrong, voltage will be closer to 6V.