Nuts and Volts - February 2012

MAILBAG

Dear Russell: Re: Convert Logic Circuit to Microcontroller, November ‘ 11, page 23. In your column, you indicated that “a microcontroller can only do one thing at a time,” but the Parallax PX832A Propeller with eight parallel processors and 16 counters/frequency generators such as is used in the column by Jon Williams could easily handle the type of work Mr. Sellen was looking to do with minimum code.

—Gregg Erickson

Response: Thanks for writing, Gregg. I had no idea this was possible.

Dear Russell: Re: Figure 4, Solar Collector Schematic, November ‘ 11, page 24.

I don’t see how the circuit in the diagram shown can function. It would appear that any voltage developed by the solar panel would be blocked by the reverse biased Schottky diode D2. The circuit would not work without the diode either, as the - side of the solar panel would conduct to the - side of the battery continuously, through the MOSFET reverse diode. I know that mechanical contacts are not in vogue anymore, but couldn’t Q1 and D2 be

■ FIGURE A.

easily replaced with an appropriately selected relay?

— Bob Hibbard

Response: You are right. Oh, my, what was I thinking? I was treating the solar panel as a load instead of a source, and the diode makes no sense. I guess I should publish a corrected circuit with a P type MOSFET on the positive side of the solar panel (see Figure A). Thanks for writing.

COMPARISON OF 555 AND CD4047

QI’m weaning myself from relay circuitry and am curious about the pros and cons of a clock oscillator based on the 555 circuit and the one shown on the CD4047 datasheet.

I imagine the two different ICs are similarly priced, have much the same power consumption, and are more or less identical in size, so what would influence the oscillator designer towards one or the other of these two ICs? How come the CD4047 has such a bewildering array of input controls — what do they all do?

You’ve explained that the 555 can be halted by controlling the voltage on the reset pin. Do any of the CD4047’s inputs halt and restart oscillation like the 555’s reset input?

Relays sure are simpler. And they

26 February 2012

have nice big solder terminals.

— Stanley Froud

AI have not used the CD4047 but it is an interesting IC. This is how it works (I believe): When the astable-bar (pin 4) is low, it is in astable mode and it doesn’t matter what state pins 5, 6, and 8 are in.

When the astable-bar is high AND astable is low, it is in monostable mode and the +trigger and -trigger can be active. Looking at the logic diagram, I think that if you are using + (rising edge) trigger, the -trigger should be held low; if you are using the -(falling edge) trigger, the +trigger should be held high.

The oscillator out (pin 13) is a pulse — not 50% duty cycle — and the frequency is double the q and not-q outputs, which are 50% duty cycle.

The reset only sets the q output low and the not-q output high, but does not stop the counter. The retrigger input is intended to be used in the monostable mode to make the output time longer; I don’t know what it will do in the astable mode, but it may be possible to make one oscillator a slave to the other.