View Full Version : Parallax USB Oscilloscope - Rock and a hard place

08-04-2006, 09:00 AM
I am an electronics newbie - sort of -·anyway, I got this idea of using a Basic stamp to build a controller for my electric trains.·Dealing with inputs with the stamp is a wiz. Programming· . . .· no problem. The hard part. Controlling 18v rms. I built a controller with an optisolator, a TLC555 chip, a TRIAC and a digital pot and a few transistors and caps. It worked great with a light bulb, but when working with an electric motor, I'm getting surges. So my research led me in the direction of a snubber circuit.·. . . . . And back in over my head.

I reasoned that an Oscilloscope would be valuable, because I am dealing with phase control, and the interference created by an electric motor. I found out about the Parallax offering and the lessons on how to use it. This is great. I know nothing about these tools.

I have 5 volts interfacing with the 18v rms·through the optisolator. I feel sure this scope could help me with the 5v part of my logic trace, but the 18v (36pp) is what is keeping me from buying this product, because it is rated at 20pp. I really want to see what is happening with the snubber.

Is it even reasonable to think that I could use this tool? ·Any wisdom would be appreciated.


08-04-2006, 07:51 PM

The scope is always a valuable tool to have. The capabilities generally lie in their max frequency input and the maximum input voltage.

To look at your circuit the Parallax scope will probably be fine with a simple voltage divider as a probe to keep the max voltage within limits. I haven't checked the 'scopes input arrangement, but a common industrial scope probe has 10x attenuation built in, which would help in your situation.

As for your original problem, searching for circuits with ability to drive a motor will turn up ones with more compensation than the usual light dimmer circuits. In fact, a good place to start might be a fan speed control from your local hardware store. They're triac-based and are made to work with AC motors. Having said that, your trains undoubtedly have DC motors which opens up the whole possibility of chopper controls of the PWM variety.


Tom Sisk


08-04-2006, 08:25 PM
That sounds like a Marklin set, and it really uses AC on the rails.
Direction-changes is done by signalling, somehow.(I'm not the train-nut in the family, my brother is)

The reason they use AC is that it gives them a much simpler system in the track, as the rails are one conductor, and the other is a set of pins sticking up in the middle.
On DC systems the rails are either Positive or Gnd, depending on the direction of travel, which works OK as long as thee track is either open-ended, or simple circle. As soon as you have a loop in the track which means the train can change to the opposite direction without you touching the controls, you'll have to make certain there's a break in the tracks somewhere in the loop, and then manually reverse the polarity as the train passes that point. Messy...

Don't visit my new website...

08-04-2006, 10:56 PM

Thanks for the responses.

So the voltage divider would consist of 2 resistors that go from the hot to neutral on the AC part. The scope measuement would be taken between the two, and the scope's grouund would be contacting the neutral side. Have I got that right?

RE the trains . .· nothing special, just Lionel 0-27. They run on AC.·Most of th solutions I found used diacs to fire the triacs, which required more voltage than I had to play with. Plus, my soltion was to controll the AC with f volts, using solid state. ·I settled with a TRIAC driver and the TLC555 timer and a digital pot.

I have snubbed the circuit 66 ways from Sunday, and I'm still getting surges on my test motor. I'm wondering ·if I need to backtrack. This scope would probably help.

Chris Savage
08-05-2006, 01:15 AM
Another thing to keep in mind on our scope...Your acceptable input voltage range will be from -10V to +10V for DC, so you won't really be able to measure the 18V without a divider.

Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)

08-05-2006, 02:09 AM
I knew about the -10/+10 constraint.·that is why I have reservations about the product. I also know that however I implement the divider, it should not be too invasive to the circuit I am testing. Assuming 18 volts would a divider of 750k and 250k resistors be appropriate? That would limit the voltage and provide 1M of total R from hot to neutral.

Chris Savage
08-05-2006, 02:59 AM
As long as you're within 20V you can just divide in half, using a simple divider (use resistors with really good tolerance, like 1%) and then you only need divide your readings in half.· That would be easier.· If you're going to go any lower you could use a trimmer pot to get 1/4 division.

Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)