Q&A
MOSFET is just as simple (see Figure 5).
Note that there is no capacitor
filtering the voltage to the lamp
because doing so would increase the
average voltage and cause the lamp to
burn out sooner. The zener diode
and C1 provide 15 volts DC to the
regulator circuit. The LM34 output is
10 millivolts per degree F, so 700
millivolts equals 70 degrees. You can
use a voltmeter to measure the set
temperature at the test point.
The feedback resistor, R6,
provides about one degree hysteresis
so the lamp does not start flickering.
You might want to put the lamp in an
opaque jar so the animal is not
subjected to the light going on and
off. Maplin only stocks the LM35
which is calibrated in degrees C, so
you will need a conversion chart if you
use that. The values of R2, R3, and R4
should be: 130K, 2K, and 2K.
for Engineers
MAILBOX TIMER
QWhen the mail arrives in my
box, it transmits a signal to
the house. I need a circuit to
stop the signal if the box is
left open — maybe to let it ring four or
five times at the most and then stop so
it doesn’t drain the battery. I am
using a six-volt battery #476A for the
transmitter. Can you help me?
— Anonymous
AThe circuit shown in Figure 6
is necessarily connected to
the battery as long as the
mailbox door is open. The
battery is rated 105 mAh and the
circuit drain is about 1/4 mA so the
battery life is about 16 days with the
door open (not counting the transmitter current).
In Figure 6, when the door is
opened, the switch closes. The charging
of C2 turns on Q1 which pulls the
trigger low and starts the 555 timer. The
output goes high which turns on Q3,
which then turns on Q2 and applies
power to the transmitter. When C1
charges to 2/3rds of the battery voltage,
the timer resets, turning Q2 off. It stays
in that state until the door is closed and
opened again. C3 is just an RF bypass
for the transmitter power. NV
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January 2008 27
