USB Oscilloscope

Fancy new USB oscilloscope in hand, I approached the now slightly charred CPU board in its sad and disconnected and kind of neglected state and promptly realized: I know arse-all about how to use this thing.

So I tapped on my chin a bit and hrm'd and harumph'd.

Then, not one to be stopped by something so trivial as not knowing what I'm doing, I started woking on the perfect project to learn the scope with.

It's not pinball, but what the heck... read on.

I make little gadgets. Cobbled together with various bits of electronics I've collected over the years. Knowing this, a friend asked if I could make a simple blinking red LED heartbeat for her halloween costume. Well if that doesn't sound like a job for the 555 timer chip, then I don't know what does. And if the 555 timer chip doesn't sound like the perfect thing to hook my scope to, I don't know what will.

The hearbeat I'm shooting for is a THUMP THUMP PAUSE THUMP THUMP PAUSE. Each THUMP lights an LED, one for each half of the heart. To get the exact timing for the duration of the lights and the pause I elected to use a 3 stage cycling timer circuit. I based my work on this sample circuit.

Output A and Output B are hooked up to red LEDs, and Output C is used only to restart timer A. All that was needed was to determine the correct values for all of the resistors and capacitors to make the lights blink at the correct rate. In the schematic, the 100K resistors should be considered variables R2A R2B andR2C. So along with R1A and C1A, R2A will determine the the amount of time that Output A is high, then how much time elapses before timer B is triggered.

Determining the values of the components is a simple matter of math. And while I recommend that you be able to do the math, we can also rely an any number of timer circuit calculators to help us pick our values.

I knew that I wanted each LED to be on for about 1/4 second, or 250ms. Then the pause before blinking again would be about 1/2 second (500ms) to give about 60 beats per minute. A bit of work showed that 3000K and 1000K resistors with .1 uF capicitor would be pretty close to what I'm looking for, and low and behold, I also happen to have those components in my box of stuff. (double the 3000K to get the 1/2 second pause for timer 3). Also, I elected not to add any extra circuitry for reseting the timers, instead when the batter is applied, we get all timers starting then the circuit corrects itself due to the longer duration of timer C. This keeps things smaller and simpler.

All wired up with a handy 9V battery and we have a nice blinking hearbeat. And we have three timer circuits creating some very simple and accurate square waves for us to hook our scope up to.

Next step, a quick stop at parallax.com to download and install the scope software. Then we are ready to see how this thing works. Our goal is to get the two probes to show the square wave generated at the output pins for timer A and timer B.