Points Position Indicator PPI4-DC

BLOCKsignalling PPI4-DC

Advanced PPI with Adjustable Brightness & Simplified Wiring

  • Monitors the brief positive operating voltage across points motors when they are switched
  • Lights a corresponding led on a control panel to show the last operation of each set of points
  • No resistors required for the leds (although any already wired in can remain)
  • Saves all settings automatically to memory when the power is switched off
  • Monitors up to 8 sets of points
  • Opto-isolated inputs accept voltages from +5V to +60V

Wiring up a model railway control panel to show points positions can be a daunting prospect.  The BLOCKsignalling Points Position Indicator (PPI) monitors the brief switching voltage to either of the two coils of a points motor, and displays the last operation using coloured leds which can be mounted on a route mimic.

Manual methods to switch points include simple spring-loaded switches, push buttons, probe and stud, etc and the points coils have a common connection to the ground of the supply.

When points are driven from DCC Accessory decoders, the decoders most often provide a +12V supply to the common of the points coil, and then switch the other connections of the coils to ground to switch the points (use PPI5-DCC with these).

This PPI is designed for operation on systems where the coil common is GROUND.

The microprocessor controls the led brightness and so no resistors are required, simplifying wiring up.  Any resistors already wired in series with the control panel leds can remain.

Power Supply

The controller is designed for use with a 12V DC power supply.



The PPI has 8 channels, each channel with two inputs able to drive two leds on the output.

The inputs cause the associated output to switch when the input voltage rises above around 3V. Also, at this moment the other associated output is switched off.

In this way, only one of the output leds is lit at any one time, that being the one with the most recent positive input voltage pulse. This means that only one of the route leds for each channel will be lit at any one time.

Each time an input change occurs, it is stored in memory, so that when the power is switched off and on again, the led outputs are set automatically to their last recorded condition.

Connecting the Unit

Simply connect one of the PPI input terminals to one end of one of the points motor coils, the other input to the end of the other coil, and the common of the coils to the COM GND input to the PPI.

Only one connection from a coil common to the COM GND input of the PPI is required. This allows the coil voltages to be recognised by the PPI correctly against the ground reference.

Each time the points are operated the yellow led on the pcb will briefly light.


When powering up, the uP (red) led on the pcb will light when the microprocessor is running. Each time a point is operated, the IP (yellow) led flashes to confirm the input signal. If the points position has changed, the (red) uP led on the flashes to indicate that the updated status has been stored in memory.

Led Connection

When using leds it is important to connect them the right way around.

The negative lead (cathode) is identified by a flat on the side of the led body, and by having a shorter lead.

red led wiring


The red and green leds share the same output, so for instance, the first set of points are represented by a red and green led with their anodes connected to terminal A1+.

All the green led cathode connections are wired to the C1- terminal.

All the red led cathode connections are wired to the C2- terminal.

red led wiring



The high current wiring to the points motors can cause interference to the operation of the low power led wiring.

When building the control panel, take care to keep the low and high power wiring separate at the control panel and between the control panel and the PPI.

Following factory testing, sometimes more than one led will be lit. This will clear as soon as each set of points is first operated.


System Wiring

The diagram below shows the connections to one set of points.  In practice, connections to the PPI will normally be made at the control panel so that the wiring length can be kept to a minimum.

If using three-leg common-anode leds, they can be wired as per the following diagram.


Wiring 2-aspect Signals to Automatically Switch with Points Operations

A single common-aonde signal can be connected to operate when the PPI4 detects that the points have operated.




By wiring a pair of leds in series (one red and one green), a pair of signals can be automatically controlled to indicate which route is clear into the points.

The anodes and cathodes of the leds need to be accessible (ie the signal heads must not have a common anode or cathode).



Here is a close-up of the wiring necessary.




Wiring Kato Points

Kato points are often wired from an AC power supply (transformer), with a pair of diodes to create positive and negative rails to each points switch.

If double-pole (ON)-OFF-(ON) switches are used, one half can be used to feed the PPI from the positive rail.  These switches are sprung to always return to the centre position.

Wiring Kato Points to BLOCKsignalling Points Position Indicator PPI


We are also often asked about wiring Cobalt points motors to the PPI.  This is a stall type motor, and so can be wired in the same way as the KATO points.

It will be necessary to hold the points switch for a brief period to allow the Cobalt to complete its travel.

Wiring Cobalt Points to BLOCKsignalling Points Position Indicator PPI



Here are some quick tips if you are having trouble with the PPI.


Q. The leds on the control panel are all permanently lit, I cannot get them to change between red and green.

A. Check the polarity of the leds.  If the polarity is reversed, then they will stay lit and will be very bright.  The leds will be quickly damaged as will the PPI by this condition.  See also the following question.


Q. The leds on the control panel do not change when I operate the points.

A. The yellow led on the PCB should normally be extinguished.  It should light only when one of the points is operated.  If it is permanently lit, check that one of the points switches is not left set.  If the yellow led does not light at all, check the polarity of the connection to the common terminal.  This must be negative for the PPI4 and positive for the PPI5.


Q. When I change the points, the leds connected to the PPI flash rapidly and so does the red led on the PCB.

A. Just after operating the points the CDU recharges, and can draw a large current from the supply connected to it (generally a transformer), causing the power supply voltage to briefly drop.  If this same power supply is shared with the PPI this can result in it losing supply for a brief period and then rebooting.  It may be possible to cure this by arranging the PPI power feed to connect direct to the transformer, rather than doubling it up at the CDU input terminals (this may not always be a cure).  Another possibility is to power the PPI from a separate source.