Accessory decoder
Turnout Decoders
The first accessory decoder I am building is for controlling turnouts. This accessory decoder controls four turnouts. The turnout motors are Peco and are twin coil solenoid types. These need to be briefly activated to switch the turnout. Using DCC to control layout accessories simplifies the wiring considerably. The previous design I had for turnout motors controllers needed several connections - 5V, 16V, Gnd, one control line for each motor. With the DCC accessory decoder, all I need is two connections from the nearest rail feed. Features
The turnout decoder is PIC 16C84 based like everything else. Some of
the features listed here will be implemented right away while others are
just wishful thinking.
Circuit Description
Circuit diagram in Postscript(48K)
A voltage doubler turns the DCC signal which is approximately 12V to
about 23V. This higher voltage is necessary to drive the turnout motor
with a capacitive discharge. The current requirements are that of the capacitor
charger(100mA) and the PIC. The PIC 16C84 running at 10MHz is supplied
5V by a 7805. A voltage divider running directly off the DCC line provides
the PIC with the DCC input. The internal diode on the PIC clamps the voltage.
The PIC outputs drive the two coils in each turnout motor. The eight outputs
go to BD679 Darlington transistors. These transistors are activated to
discharge the capacitor through the turnout coil. The diode on the collector
protects the transistor from voltage peaks. The capacitor is charged using
a constant current source built around a PNP low power transistor and an
LED as a voltage reference. Using a constant current charge source speeds
up the charging of the capacitor while keeping an upper limit on the charge
surge current.
The RTCC is not preloaded and interrupts when it overflows. With a 10MHz
crystal, the interrupt occurs once every 102.4us. The timer ISR
stores the 'overflow' in two more bytes. The first byte will overflow
once every 256 * 102.4 = 26.2ms. The second byte will overflow once every
256 * 26.2 = 6.71 secs. The DCC signal is routed to RB5 which is configured
to interrupt whenever the DCC signal changes polarity. This is timed and
the bit period determined. This is then decoded into bit streams and then
to DCC commands.
The commands are used to update the desired state of the coils
stored in a nibble. The current state of the coils is stored in another
nibble. The coil activation routine has three states - idle, activating
a coil and charging up the capacitor. In its idle state, it looks for
differences between the current state and the desired state. If it finds
a difference the most significant differing bit is extracted and the
coil corresponding to it is activated. Each motor consists of two coils
and a 0 implies one coil being activated and 1 implies the second one.
Once the activation times out, the drive to the coil is deactivated.
The routine moves to the capacitor recharge state. Once this state
times out, the routine looks for more differing bits. If there it finds
one it does the whole thing again - otherwise it goes back to idle state
and keeps polling for differences.
The process above ensures that only one coil is activated at a time as
the capacitor may not have the charge to activate two coils at once. It
also ensures that the capacitor has had sufficient time to charge before
it is used to activate a coil.
As this was the second component I was designing and building, the first being the command station, I decided to design a proper PCB for this circuit. The PCB layout and the legend overlay are available as Postscript files. I made the PCBs myself but that's another story.
