QUESTIONS & ANSWERS
Mailbag continued ...
I have a wee problem with the
circuit you show in Figure 6 of the
August issue. Analysis shows that it
would send a continuous stream of
pulses to the servo at a rate that may
lock the servo.
In an R/C system using pulse
position modulation, one sends a frame
of pulses at approx. 20 ms intervals. On
the transmit side, each pulse triggers
the next pulse, etc. In an eight channel
system if all pulses are 2 mS, the total
time for the pulses is 16 mS. The remaining blank space is used by the receiver
to determine the start of the frame.
Hence, for Anonymous to control
the throttle, he would need the attached
circuit. The astable produces pulses at
20 ms intervals; the frame and the
monostable produces a pulse between
1 and 2 ms per frame. I don’t think a
■ FIGURE A
switchover is necessary if the engine is
a gas engine. It is easier to set up the
controls in such a way that a 1 or 0.9 ms
pulse effectively shuts off the fuel flow,
cutting the engine.
For a more complete explanation
of PPM, see N&V January ‘06, Auxilliary
I enjoy reading your column, keep
— Paul J. Weijer
Response: Thanks for the feedback. The R/C system is more complex
than I knew. Thanks also for your circuit
which is Figure A.
der. In the zero position, the display cycles normally but when
any cylinder is selected, the 4002 dual four bit NOR resets the
counter, stopping the cycling and blanking the display.
The 4024 counter divides by 128. The last two bits are
used to provide the four count, so dividing the 30 seconds
needed to cycle through the four cylinders, the period for
the 555 oscillator is 0.234 sec or 4. 22 Hz. The resistors, R1
and R2, don’t have to be equal and you can increase them
to extend the viewing time.
The parts list for the thermocouple multiplexer is
provided in Figure 4.
QI would like to use a Tektronix oscilloscope,
Model T922, to view the following signals:
1) Telephone line when my PC is utilizing it.
2) 110 VAC power line to view X- 10 control signals.
Both require some isolation, but I am not sure how to do this.
A1) The telephone company takes a dim view of
anyone connecting unapproved equipment to their
lines, so you will need an approved interface.
Commercial units are expensive, so I recommend
getting an old telephone — one that has a separate base and
handset. Unscrew the earpiece and connect your scope across
the speaker. The waveform may be improved by replacing
the speaker with a resistor ( 10 ohms?). Before you do this,
disconnect your scope from ground. I use a three prong to
two prong adapter that is available in most hardware stores.
2) The X- 10 signals are high frequency (120 kHz), so a
capacitor will separate the 60 Hz and the X- 10 signals. A
transformer will provide the safety isolation. The
transformer should be a pulse type which will further
attenuate the 60 Hz (see Figure 5). I looked for an ordinary
1:1, 100 ohm transformer but this 10/100 base-T unit was
as close as I could find. It is a dual; you only need to use
half of it. Mouser part number is 673-H1102. NV
November 2007 27