PEKit: Cannot make RCDecay to work

Dimitri

So far, Propeller Edution Kit was fun and easy. Currently I am stuck with RCDelay example:

The lab expects the following schematics:

My circuit is as following:

I use 103 capacitor which should be 0.01 μF.

The spin code is from lab "TestRcDecay.spin" (unchanged) (I attach it here as well)

[ATTACH]92215[/ATTACH]

When I run the program it returns me constant 'time = 0". Turning potentiometer knob does not help...

Any ideas are appreciated.

Thank you,
Dimitri

kuroneko

Figure 3-3 of the PE-Kit tells me that each group of 6 sockets is electrically connected. This would suggest that your capacitor isn't doing anything useful and you effectively wired pin 17 to ground which is not a good idea.

StefanL38

Hi Dimitri,

I highly recommend to insert a 150 Ohm resistors between each propeller-IO-pin and the thing you want to connect.

I mean this way

Prop-IO-Pin-No 0

---

150-Ohm

---

anything you former connected to the IO-Pin
Prop-IO-Pin-No 1

---

150-Ohm

---

anything you former connected to the IO-Pin
Prop-IO-Pin-No 2

---

150-Ohm

---

anything you former connected to the IO-Pin
....
Prop-IO-Pin-No 31

---

150-Ohm

---

anything you former connected to the IO-Pin

Why this?

This will protect the prop-IO-pin from sourcing or sinking too much current if you accidently connect them to either +3.3V or 0V.
This short peace of wire has a resistance of 0,1 Ohm.
Now imagine you have programmed the IO-pin as output and set him to +3.3V and connect it through 0,1 Ohm to ground.
current 3.3V / 0.1 Ohm = 33A (theoretically). As the current rises something inside the propeller-chip will burn through.
You are a lucky guy if this hasn't happened. Now with 150 Ohm inbetween

3.3V / 150 Ohm = 22mA. Which is a save value. (Each IO-pin can drive 30 mA).
Additionally I recommend examining carefully how each whole on your breadboard is connected to the others.
Reading basic materials about electronics is a good idea too.
This is a website that explains the things in a quite good to understanding way. (which is seldom on the internet)
http://openbookproject.net//electricCircuits/DC/index.html
best regards

Stefan

Dimitri

kuroneko wrote: »

Figure 3-3 of the PE-Kit tells me that each group of 6 sockets is electrically connected. This would suggest that your capacitor isn't doing anything useful and you effectively wired pin 17 to ground which is not a good idea.

Thank you, kuroneko! You were exactly right it was not doing anything. Once I changed layout, RCDecay started working just fine:

Re: Stefan's suggestion:

StefanL38 wrote: »

I highly recommend to insert a 150 Ohm resistors between each propeller-IO-pin and the thing you want to connect.

I agree with your suggestion as a rule of thumb. However, PEKit documentation explicitely addressed this approach for potentiometer exercise. Here is a quote from page 127:

A 100 Ω series resistor can instead be placed between the I/O pin and the RC circuit. The main benefit
of this approach is that it reduces any potential effects of even a brief current inrush when the I/O pin starts
charging the capacitor. The main drawback is that a protection resistor at the I/O pin forms a voltage divider
with the potentiometer’s resistance, which in turn prevents the capacitor from charging to 3.3 V. Instead, the
capacitor charges to 3.3 V × R ÷ (R + 100). As the value of R gets smaller, the voltage the capacitor can
charge to before the decay measurement starts also drops. Although Δt would still vary with R over most of
the potentiometer’s range of motion, the measured decay time would no longer be directly proportional to
resistance...

I guess I was just like that I didn't burn my Propeller chip.

Thank you for your suggestion.

dimitri