Start
•
Basic
Principle
• What's
Measured?
•
How
to Measure
•
Measurement
Technique
•
Complete
the Circuit
•
Complete
Application
•
Proven
Reliability
•
Sources
and PDF
COMPLETE
APPLICATION
The
complete application is shown in Figure 4. In
the measurement circuit, the current source
is built from an adjustable linear regulator.
The current is set to a little higher than 200
mA. The main power switch Q1 is connected to
an additional transistor circuit to translate
the control voltages down to the 0- to 5-V range
the Freescale MC68HC908QT4 microcontroller is
capable of generating.
|
(Click
here to enlarge)
|
Figure
4—The power supply is the only part not
shown in the schematic of the soldering
station. The current source is powered by
8 V to reduce power dissipation. |
The
MC68HC908QT4 microcontroller is an inexpensive
8-bit HC08 device housed in an eight-pin DIL
package that’s easy to work with. The microcontroller
is connected to an LED that indicates whether
the soldering tip is below, equal to, or above
the desired temperature. A potentiometer regulates
the desired temperature. The remaining pieces
of the circuit are two trimmers that are used
to calibrate the offset and gain of the temperature
regulation.
A
schematic of the power supply I used isn’t shown
here. I used a small, lightweight custom switch-mode
power supply. However, the circuit will work
equally well with a mains transformer-based
power supply.
This
temperature measurement application is very
simple. As a result, the small amount of code
for the ’HC908QT4 is little more than 700 bytes.
The basic algorithm is shown in Figure 5.
