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Could someone come up with a
simple schematic for a blinking LED
(clear but blinks red)? The item would
use an NTE5405 SCR, a high intensity
(MSD) LED, and an LSR (light sensitive
resistor), all driven by a nine volt battery.
The purpose would be for the blinking
light to remain out during the day but
would come on at night and blink about
once every seven seconds. It would
then go out during the daytime.
#10111 Cy Fedor
What is the easiest or best way to
PWM a PIC16F690? I've seen Chuck
Hellebuyck's book, but he uses an
external pot to manually do it. I need
to do it in software.
What cheap or free Basic compilers
are being used for programming PICs?
#10113 Gregg Reber
Applications handbook by National
Semiconductor. It ensures that the
voltage across Rx is linear.
U1.2 is a comparator. Its output is
high when the voltage across Rx is
below the voltage set by VR1, and is
low (ground) when the voltage across
Rx is higher than the voltage set by
U1.3 will act as a follower with
gain when the output of U1.2 is at
ground. When the output of U1.2 is
high, the output of U1.3 will be at
U1.4 acts as an inverter since
Brian wanted +10V when the resistance is low.
The +10V can be derived from a
zener diode or a 10V reference IC.
Variable gain to set the voltage for
the desired maximum resistance value
for the circuit can be derived by
varying any of several resistors. I
would suggest making R8 a 5K potentiometer. Without knowing the actual
resistances Brian wants to measure, I
cannot suggest values for R1-R5.
However, the voltage across Rx should
be kept to < 5V due to the operation
of U1.3 which has a minimum gain of
one. If desired, an LED with series
limiting resistor can be added to the
output of U1.2. If the LED circuit is
connected to ground, it will indicate
when Rx is below the threshold set by
VR1. If it is connected to V+, it will
indicate when Rx is above the VR1
threshold and is in the linear range of
Get a programmable greeting
card at a pharmacy or Walmart,
replace the battery with a three volt
power supply, and connect the output
to a suitable audio amp.
[#8113 - August 2011]
Analog S Meter
I have a vintage Realistic DX-400
shortwave radio. The signal meter finally
I just received my September
issue and looked at the proposed
solution to problem 7113. The
solution proposed is quite simple and
the analysis of its operation is correct.
However, its output is not linear with
the value of Rx — as is indicated by the
formulas. I believe the request was for
a linear response. I have not built the
circuit in Figure 1, but it looks like it
should do what was requested by
Brian. U1 must be a rail-to-rail, single
supply quad op-amp. A TS914 should
work; +V should be between ≈
U1.1 is a current source which I
have used and found in a Linear
78 October 2011