Tutorial: Connecting a Simple Circuit to the EspoTek Labrador

Chris Esposito edited this page Feb 6, 2018 · 1 revision

A Few Important Notes

There are some things to keep in mind when using your Labrador board:

  • Labrador is a collection of independent tools that happen to share a single USB connection and software interface. Apart from small amounts of unintended crosstalk, there are no internal connections between Labrador’s different instruments!

  • All voltages are GND-referenced, with the exception of the multimeter which references CH1 against CH2. If your oscilloscope measures 5V, it means that the voltage at the node your oscilloscope pin is connected to is 5V higher than the Labrador’s GND pin. Make sure you connect the GND node of the circuit you are testing to Labrador’s GND pin!

(NOTE: In Electrical Engineering there is no such thing as an "absolute voltage" at all; it's only possible to measure differences in voltage between two nodes. A 1.5V battery doesn’t “have” 1.5V; it just means that the positive terminal is 1.5V higher than the negative terminal.).

  • The AC coupled pins remove the DC component from your signals, and are meant mainly for audio purposes and dual-ended opamps. For general use, you will almost always want to use the DC-coupled pins, as they do not alter your signal in any way.

  • Labrador is idiot-proof. You should be able to short any two pins together without damaging your board, and even if you do something like connecting a 12V external power supply directly to one of your digital output pins, they’ll nobly sacrifice themselves to ensure the rest of the board will still function (and you can fix it by replacing a single resistor). Experiment a bit! You can’t do too much wrong.

  • Labrador is idiot-proof, but not complete-idiot-proof! If you start touching live wires to random components on your board, I cannot guarantee what will happen. Please make sure you only connect your circuits to the external header pins, and don’t dangle live wires near the microcontroller!

A Simple Circuit Example

The simplest circuit I can think of is the voltage divider:

image_2

It transforms the input voltage Vin to the output voltage Vout according to the equation: capture

For example, if you used a 1k resistor for R1 and a 10k resistor for R2, the voltage at Vout would be equal to 10/11 of the voltage at Vin.

I tested this theory in real life by building the voltage divider and connecting it to Labrador!

You can do the same. If you’ve not dealt with oscilloscopes or signal generators before, it’s the best place to start.

image_3 Above is a picture of my breadboard. (NOTE: If you’re not familiar with how the connections in a breadboard work, I recommend checking out Adafruit’s tutorial. I also recommend tearing the backing off of a breadboard at least once in your life. Just make sure you have duct tape handy to put it back together again.)

The top yellow wire connects Vin to both the signal generator CH1 as well as the oscilloscope CH2 pin. The lower yellow wire connects Vout to oscilloscope CH1. The black wire connects the circuit's GND pin to Labrador's GND pin. The resistors used were both 6.8k, meaning that the voltage at Vout should be approximately half the voltage at Vin.

(NOTE: I say "approximately half" because analog components aren't 100% accurate. The resistors I used have a tolerance of +/- 1%, meaning that the actual resistance could be anywhere between 6.73k and 6.87k. This can lead to slight imbalances in your circuits. If only there are a $29 tool that could measure such things...)

image_4

To test the theory, I loaded up the Labrador's software interface and generated a 3V sin wave using the signal generator CH1. As per above, oscilloscope CH1 (yellow trace) measures Vout, and oscilloscope CH2 (blue trace) measures Vin.

By eye, it appeared as though the yellow trace was about half the height as the blue trace. I looked at the max and mean voltages to confirm it, though; 0.681 is almost exactly half of 1.38, and 1.465 is almost exactly half of 2.92. Vout is almost exactly half of Vin.

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