5 on that page shows how I should have done it.
Speed control requires feedback. For a
vehicle, your eyeballs and the speedometer are
part of that feedback. If you want cruise control,
however, a tachometer is required.
In Figure 1, if Q1 is PWM, driving the motor
forward, then Q4 is turned on continuously and
Q2 and Q3 are off. When you want the motor to
stop, Q2 and Q4 are turned on and Q1 and Q3 are
off. This puts a short across the motor, slowing it
down quickly. To reverse the motor, Q3 is PWM
and Q2 is on. It takes time to turn Q1 off, so there
must be a delay before Q2 is turned on. You could
use a microprocessor to handle the details, but with a 555 timer
and some switches, a simpler (for me) circuit could be designed.
You will need some safety features. For example, it
should not be possible to switch from forward to reverse
when the accelerator is floored. Putting a short on the motor
for dynamic braking could throw the vehicle out of control,
so that needs to be addressed. I recommend using
only regeneration for braking and use the mechanical brakes
for stopping. Dynamic braking won’t stop the vehicle any
sooner and you have to have mechanical brakes anyway.
■ FIGURE 2
QI need a circuit to cycle four pairs of thermocouple
signals for a four cylinder air-cooled engine (CHT
and EGT) at a cycle rate of 30 to 40 seconds for the
four. As each cylinder cycles on, I need it to also
indicate the cylinder number (1-2-3-4) on a seven segment
LED. A selective hold switch would also be helpful for extended
viewing of any particular cylinder. The power source is 12 volts.
— A. F. Schwedler
AA microprocessor would be handy for this project,
but I am a newbie and can gin up a logic circuit
faster than I could figure out and debug a program.
In researching for this answer, I learned that CHT
stands for cylinder head temperature and EGT stands for exhaust gas temperature. I recall a similar question where I used
a Fluke model 80TK thermocouple module which outputs the
temperature directly in millivolts. The Analog Devices AD594
has built-in cold junction compensation and outputs 10 millivolts per degree C. It will be less expensive and since you will
have to have a printed circuit board anyway. I will go with the
AD594, which comes in two flavors: the A version is trimmed
to three degrees accuracy; the C version is trimmed to one
degree accuracy. The AD594 is compensated for type J thermocouples, good to 750 degrees C max (1,382 degrees F).
Figure 2 is the detail schematic for the AD594. You will
■ FIGURE 3
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