RC Time circuits with bs2.

rwgast_logicdesign · 2012-12-25 20:06

On page 145 of whats a micro controller there is an RC time decay circuit. Im curious about the acuall electronics. the schematic shows a 220 ohm resistor in series with P7 and a 470 ohm resistor parallel to to p7 the a 3300uF cap parallel to p7 also parallel to p7.

In electronics most of the time RC circuits are done with a resistor in series with a parallel cap correct? Well the parallel resistor in this circuit is the one setting the RC time, not the 220ohm series resistor. So first of all what is the point of the 220ohm resistor, the homework board already has a 220 ohm resistor in seires with every pin and this does not seem to effect the RC time when moved. Secondly why is the resistor in parallel with the cap? How is this slowing the current down before it gets to the 3300uF cap?

Tracy Allen · 2012-12-26 20:22

Have you read through rest of the chapter? It gives a pretty good rundown.

The capacitor charges up from the Stamp pin through the 220 ohm resistor, and as soon as the pin becomes an input, the capacitor discharges through the resistor in parallel. The experiment wants you to try four different resistors in parallel as you observe the time it takes to discharge down to the Stamp input threshold.

There are other ways to hook up an RC circuit. Maybe the arrangement below is what you are thinking of? The 10k and the 0.1µF are in series between Vdd and Vss. In this case, the pin would go low and discharge the capacitor thru the 220Ω, then the pin would become an input so that it can detect the change in voltage as the capacitor charges back up through the 10k. The point is that there are different ways to wire up an RC circuit, with different paths for charging and discharging.

[FONT=courier new]          10k                     0.1
Vdd -----/\/\/\------o-------------||-----------Vss
                     |
           220       |
pin -----/\/\/\------'
[/FONT]

RC Time circuits with bs2.

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