A simple control loop and added
synchronous rectification could make
this circuit useful for driving LEDs.
Other applications await.
Note that another way to achieve a
similar result would be to use an op
amp monitoring the voltage across
a shunt in series with the load, and
using negative feedback to provide
the required drive voltage to match
the shunt voltage to a reference sinewave.
However, such a circuit may suffer
from stability problems, necessitating added compensation components
which would reduce its precision.
Mauri Lampi,
Glenroy, Vic. ($90)
Controlling model railway points with a servo
This controller was created to operate a set of points (or turnout) on a OO/
HO model train layout using a small
9g model servo (eg, Jaycar YM2758).
For simplicity, each set of points has
its own small microcontroller with
just three inputs. Potentiometers VR1
and VR2 set the two positions, while
switch S1 selects between them.
It runs from a DC supply of at least
9V and 1A. This is reduced to 5V by
7805 regulator REG1, which is adequate to operate a 9g servo. Power for
the microcontroller is decoupled by
schottky diode D2 and a 220µF filter
capacitor, to prevent motor current
surges affecting its operation.
The controller should be close to the
servo and the points, due to the weak
drive and to minimise power losses in
the wires. So switch S1 may be several metres away, and thus its wiring
is susceptible to interference.
Circuit
Ideas
Wanted
siliconchip.com.au
The 1kW/100nF RC low-pass filter
between S1 and the GP3 input of IC1
(pin 4) reduces the effects of EMI, and
it is advisable to use twisted pair wires
and/or grounded shielding to further
reduce the chance of interference.
Potentiometers VR1 and VR2 are
wired across the micro's supply, and
the wiper voltages are stabilised by
100nF capacitors which perform two
functions. They reduce the effects of
stray electric fields and also provide
the low source impedance required by
the micro's internal analog-to-digital
converter (ADC). The servo signal has a
330W series resistor to protect IC1 from
accidental shorts at CON1.
A heartbeat LED, LED1, flashes to indicate when the circuit is operational.
Setup is simple. With the power off,
set VR1 & VR2 to their mid positions
and the points also in their mid positions. Turn the power on and adjust
one potentiometer to set the points to
"ahead" or "turn". Then change the position of switch S1 and adjust the other potentiometer. The points are then
operational, controlled by S1.
Note that if the points are hard
against either end position and the
servo is trying to move the points
more, the servo will be destroyed in
little time. To prevent this, the mechanical link between the servo and
the points should not be rigid. You can
use an open-coil spring or provide a
U-shaped loop so that there is some
compression or extension of the link.
The software was written in PICBASIC Pro. The Servo_Dual_Posn_
SC.BAS and Servo_Dual_Posn_
SC.HEX files are available from
siliconchip.com.au/Shop/6/5638,
along with a PDF of the PCB pattern.
George Ramsay,
Holland Park, Qld. ($80)
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Australia’s electronics magazine
December 2020 89
