Part Description Part No.
All parts are from Allied Electronics ( www.alliedelec.com)
unless otherwise noted.
AD594AQ Thermocouple IC
AD594CQ Thermocouple IC
Quad analog switch
One of four decoder
BCD to seven segment
Seven stage ripple count 735-0907
Quad two input OR
Dual four input NOR
1 µF, 50 V
R4, 5, 6, 7
One pole, adj positions
■ FIGURE 4. Thermocouple multiplexer parts list.
■ FIGURE 5
need eight of them; one for each thermocouple. It is
important that the thermocouple is grounded, but if
grounding the junction is not convenient, you can put a
10K resistor from the negative input to ground.
In Figure 3, the AD594 outputs are routed to a digital voltmeter (DVM) using a pair of quad analog switches (4066). The
analog switches are controlled by a 4555 one-of- 4 decoder. I
originally considered feeding the two bits of the 4555 input to
a seven segment decoder but that would count 0-1- 2-3 and I
want it to count 1-2-3-4. I put the 4002 four bit adder in cascade with the BCD to seven segment decoder to provide the
1-2-3-4 count. I never used an adder before — I hope it works!
The switch, S1, allows continuous viewing of one cylin-
Congrats on a very innovative and
flexible charger (July ‘07, page 33). Do
you know if the voltage sag at full
charge is also present with alkaline
cells? I recharge AA and 9V batteries
frequently and get quite a bit of extended life. I plan to build one of these
chargers for my lab.
Also, congrats on being a PICBASIC
PRO user. Except for floating point math,
I have not found anything that I wanted
to do that I could not accomplish with
PBPro. It’s a real genius piece of work.
In your code for the charger, I
believe that you need to initialize the
value of Oldvolt on the first pass
through to zero volts or some low value
just to make sure that some flakey
value is not stored at runtime and you
inadvertently shut down on the first
comparison with Volt. It may happen
automatically but I like to be in control.
Also, I plan to add a few sequential
measurements with averaging just to
make sure that I don’t get bit by a one
time noise glitch.
Thanks for picking up the reins
from Mr. Byers — he left some big
shoes to fill. I never met him, but I will
always remember him.
— Bill Jorden
Response: I don’t know about
alkaline cells, my knowledge is limited
to what I have read on the Internet. It is
possible that the cell will gas and have
reduced terminal voltage. If you try it,
let me know if it works.
Thanks for the feedback on the
code. Your points are well taken.
I’m very new to electronics and
trying to educate myself. What is the
purpose of Figure 14 on page 34 of
the July ‘07 issue? Can IC 12F675 be
programmed or something? Also, what
is A/D input (last paragraph)? Does
someone make a PCB or entire kit for
the charger? What does the reset switch
— Bill Higgins
Response: Figure 14 is the PICBASIC PRO program for the microprocessor, PIC12F675. You can get more
information from www.melabs.com.
You can also download the datasheet
for the PIC12F675 at www.excelpoint.
There is a great deal of information in
the datasheet; it will take some time to
digest it all. There is a forum that will be
helpful at www.picbasic.co.uk/forum/.
The PIC12F675 has a built-in
analog-to-digital (A/D) converter you
can read up on that is in the datasheet.
There is no kit; I did build a breadboard
to verify that it works. There is no reset
switch. I suppose I could have had one
because there are unused pins, but I did
not think it was necessary.
The figure you have for the mixer
output in the August ‘07 issue does not
seem to be correct to me. The two
signals are indeed added. The description of a mixer often does state that the
output contains sum and difference
frequencies. However, the equations for
a mixer are of a multiplicative relationship. Please see the ARRL Handbook for
a picture of what a mixer output should
be. The figure you have shown could
easily be the output of an operational
amplifier “adding” two sine wave signals.
— Jlcdlc via email
Response: You are correct that the
operation of a mixer involves multiplication; the multiplication occurs in the
non-linearity of the amplifier. The math
is beyond me; I just know that it works.
If both signals are small, no mixer action
occurs and there is no sum or difference
at the output. But if one signal is large
and the other is small, the small signal is
run up and down the non-linearity of the
amplifier producing a multiplication
effect. Note that in Figure 2 (August,
page 25) the period of the waveform is
0.1 µS = 10 MHz, the difference frequency, not the addition of the two inputs.