speakers if you want. You may want
to increase the bass boost in the
audio system because the
transformer is rated 300 Hz to
QI just finished a sound alarm which was described to be equipped with a photocell, but discovered
that it came with a clear
phototransistor (LED style: T1 3/4).
Instead, I’d like to use a real
photocell but don’t know what
ratings (ohm values) I should use
(for light and dark).
Can you specify optimum ohm
values (dark/light) for such a
AYou should be able to just wire the phototransistor in place of the CDS cell that was on the schematic. The
difference is that the transistor has
polarity while the CDS cell does not.
The transistor collector goes to
positive and the flat on the T1 3/4
package should denote the emitter.
If the phototransistor has three leads,
leave the base lead open. CDS
photodetectors are hard to find;
RadioShack does not carry them
anymore, which is
probably why the
QI have a question on an ETASIS redundant
power supply model
#EFRP-250A. I found
a datasheet on their
website that mentions TTL
compatible signals. Is
there any way I can test
this power supply? When I power it
up, the LED status turns red; it is
supposed to turn green. I assume it
has something to do with the way
the output connector is wired.
— Jeff Miller
AThe power supply should turn on when +5V is applied to the PSON terminal. If the status LED is
turning red, that indicates there is a
fault and the supply needs repair.
MORE ON THE
QI bought a complete receiver kit which includes the SI570 oscillator, microprocessor, and LCD
frequency display; thanks for your
answer on the oscillator. Now, my
question is: How is the application
circuit — shown for ECL (emitter
coupled logic) operation — to be
modified to work with TTL signals?
The output is to be from 2V to 10V
to the varicaps.
— Charlie Fenech
QUESTIONS & ANSWERS
AIf your circuit is similar to Figure 12 in the datasheet (except when operated from +5V), the connections
to the AD9901 and the filter circuit
will be as in Figure 1. The AD9901
output is from 3. 2 to five volts, so
there is a large offset which has to be
adjusted out via R7. The signal to the
varicaps is eight volts p/p and the
AD9901 output is 1.8 volts, so the
required gain is 8/1.8 = 4. 44.
The gain can be adjusted with
R11; the controls don’t interact, so
the adjustment will not be difficult.
The RF filter can only have one
capacitor or the loop will be
unstable. The series resistor is for
loop stability and I assume that
the 30 ohms used in the datasheet
QIn your answer to “making circuit boards,” you mentioned using pre- sensitized copperclad
board that can be exposed using
ultraviolet light. My question is: What
ever happened to Kodak photoresist
and developer where you made your
own sensitized boards? I can’t find it
— William E. Kenyon
■ FIGURE 1.
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