Blog Posts

Often the question is asked, what is the simplest way to get modular synth control voltage (CV) into an analog input pin?

This simple circuit using only 3 resistors and 1 capacitor converts the -5V to +5V CV signal range to the 0 to 1.2V ADC input range.

For Teensy 3.2, 3.5 and 3.6, you would use analogReference(INTERNAL) to configure for the 0 to 1.2V range.  Then you can read the CV signal at any time with analogRead().

While this circuit is the simplest and easiest way, it is not necessarily the best way.  The input impedance, or load placed on the CV signal, it 31K ohms, rather than the standard 100K impedance normally used for modular synthesis systems.  This forum discussion goes into the circuit’s limitations and suggests more advanced ways using opamps.

 

Originally this article was published on the DorkbotPDX website.  Since that time, the DorkbotPDX blog section has vanished.  I’m reposting it here, to preserve this info.  A copy of the original can also be found at the internet archive.

Years ago, in my slow quest for better photography of electronic projects, I built a light table to eliminate shadows.  Most of the white background photos you see on the PJRC site are shot with this light table.

This is how it turned out.

The build used boards from OSH Park (then “Laen’s PCB group order”), materials from TAP Plastics, 15 white LEDs and parts I mostly had laying around.

This view is inside a cheap 2 foot sized light tent I purchased from some ebay vendor, and a couple bright lights outside the tent on both side.

The LEDs are Cree CLM3C-WKW-CWBYA453, which are supposedly the same 5500K color temperature as the CFL lights outside the tent. Maybe that matters, maybe not, but it seemed like a good idea.

The Cree LED is a surface mount part, but fortunately Lean’s PCB group order made it very easy to convert to something I can solder wires onto. All the PCBs mount with double sticky mounting tape.

As you can see in this LED photo, there’s a bit of shadow. It’s a soft shadow due to the light tent casting light from many directions, but it’s still very present.  This is the type of shadow I’m hoping just a little bit of under side lighting will eliminate.

This little board is a constant current regulator. It takes a 0 to 5 volt input and regulates a 0 to 20 mA output current to a string of 5 LEDs. I wanted to make sure the current was perfectly constant since the camera might choose a quick shutter time.

Here’s the schematic for that circuit. At the time, it seemed like a good idea to sense the current using a resistor between the NPN transistor’s collector and the LEDs. The idea was any small change in ground potential between the board 0-5V control signal wouldn’t matter, if I ran separate signal and power ground lines.

But I didn’t consider the current draw though those resistors around the upper opamp. As you can see in the schematic, I change the values to about as high as I reasonably could. It still have a tiny bit of the lowest part of the range where the LEDs won’t completely turn off.

The 0 to 5 volt signal just comes from this potentiometer on the front panel.  Because it’s driving only the inputs to opamps, it doesn’t have any substantial load.  I still used a 1K pot anyways, though a higher value would have consumed a little less current.  I guess I didn’t care about an extra 5 mA.

The power for everything comes from this simple little power supply, which is just (approx) 24 volts unregulated, and a 5 volt regulated output from the pot, which is from a 7805 regulator.  Simple.

Of course, the opamp circuit isn’t perfect. After putting this together, I decided to try a different approach, sensing on the emitter side, and no current sensing path to add to the LED current! I also included 4.7K resistors on the feedback looks, and the positive inputs see about a 1K impedance. Any errors from the opamp’s input (PNP) bias currents should be small, and should be more on the negative than positive, so hopefully any tiny error will tend to reduce the LED current, not increase.

Then again, the original boards might work out ok, but Laen’s PCB group order makes it so very easy to just quickly draw up a (small) board.  Because the cost is so low, it’s easy to just send it off without all the worry the goes into a normal board order.

I still haven’t actually put this thing to use… the top plastic cover turned out to be just a bit too small, so I need to go shave it down to size on my table saw (which currently has a bunch of other project stuff piled on top of it). But soon, with a little luck, I’ll be able to take pictures of electronic stuff and adjust the light table to null out or at least soften away most of the little shadows that I still get, even with the light tent.

 

Originally this article was published on the DorkbotPDX website.  Since that time, the DorkbotPDX blog section has vanished.  I’m reposting it here, to preserve this info.  A copy of the original can also be found at the internet archive.

Some time ago, I needed to actually “see” a current waveform in the 100 uA to 5 mA range with at least a couple MHz bandwidth.  This extremely expensive probe would have been perfect, but instead I built something similar for about $30 using the amazing Analog Devices AD8428 amplifier.

The first step was cutting the power trace and adding a resistor.  I used two 1 ohm resistors in parallel.

At 5 mA, this makes only 2.5 mV.  My scope’s supposed resolution is 1 mV, but the truth is there’s plenty of noise down in the 1 mV range.  That’s pretty common for most scopes, even pretty spendy ones.  So it’s just not feasible to measure this signal directly (not to mention using 2 probes and subtracting them in the scope).

That incredibly expensive Agilent probe probably has a couple really nice amplifiers inside…. so I went searching for an amplifier.  After a bit of searching, I found the AD8428.  It has a fixed gain of 2000 and a bandwidth of 3.5 MHz.  That’s a gain-bandwidth product of 7 GHz !!!  It’s also an extremely well matched instrumentation amp with an amazing CMRR of 140 dB.  So it gets rid of the power supply voltage and outputs the amplified signal referenced to ground.

The AD8428 is perfect.  It’s so very easy!  Of course, such amazing performance costs money: about $20.  Here’s that expensive little amplifier, and a 5V to +/- 15V power supply (about $10) to power it.

The one trick with measuring such tiny voltages is twisting the 2 sense wires together.  Honestly, I didn’t try it running them separately, but since this thing is getting voltages in only the microvoltage range for the lower measurements, I didn’t want to risk picking up noise.  I also put a 100 ohm resistor on the output, just in case I accidentally short the output or do something clumsy that might blow that little $20 part.

Here’s a scope screenshot using this little amplifier to “see” the current (the blue waveform).

In this test, the microcontroller is running in its slowest mode at only 10 kHz, drawing about 120 uA.  Then when the chip’s internal oscillator is started, the current jumps to about 600 uA.  Later, the CPU switches to actually clocking from that oscillator.  There’s an on-chip clock divider which is switched in and increased gradually.

The bottom trace (red) is the voltage on the chip’s 1.8V linear regulator.  It turns out that sudden jumps in current cause pretty substantial downward spikes on the regulated voltage.  This more gradual startup approach really helps.  This sort of thing is impossible to see with a slow multimeter, but with a reasonably good bandwidth measurement of the current, it’s easy to see what various code actually does to the current.

I tried connecting my multimeter to the amp output.  Sometimes it’s just a lot more convenient to look at a single number on a meter than fiddle with the scope.  I had been using the current mode on the meter before building this.  One thing I was surprised to find it my little meter updates its screen much faster while measuring about 125 mA than it does when measuring 125 uA.

Another interesting thing I’ve been noticing is patterns within the blue current waveform.  This Agilent scope has a “digital phosphor” rendering of the huge amount of data it collected.  This static screenshot can’t really capture the interactive experience of adjusting the waveform intensity, where various regions within the data change brightness differently, indicating there’s something interesting/different going on.  Even so, you can see several areas in the screenshot where interesting things are happening once the CPU is up and running.  It’s interesting how the current waveform changes as different code executes.

I know this isn’t anything terribly impressive… basically just buy a high-end amplifier and use it with a series resistor.  Maybe it even reads like an Analog Devices ad?  I’m not affiliated with Analog Devices… I just bought this part at normal qty 1 pricing from Digikey.

Still, this is the first time I’ve ever really looked at such low microcontroller currents with a few MHz bandwidth, and I’m guessing not many people have ever bothered to really measure such currents, so I thought I’d share.

 

Years ago I originally posted this article to the DorkbotPDX site.  Hackday published an article about it at the time.  Since then, the DorkbotPDX blog section has vanished.  I’m reposting it here, to preserve the info… for the next time I or others might need to do this sort of current measurement on a budget.