Detector Theory of Operation

From SPCoast

Contents 1 Circuit Description 1.1 Power Supply 1.2 DCC Isolated Detection 1.3 Theory of Operation 1.4 Remote Sensor Interface 1.5 Test Card 1.6 LocoIO Loconet Interface

[edit] Circuit Description

[edit] Power Supply

The Detector is powered by an external 1 Amp 8-15 VAC source and has an internal 5v DC regulator which filters out any ripple and noise.

[edit] DCC Isolated Detection

The core of the Detector is a pulse transformer mounted on a remote sensor board that isolates the detector circuit from the track power circuits. Because it relies on an AC signal being coupled through the transformer, it is for use with DCC systems only - it will not work with pure analog DC throttles.

The signals from the remote sensor are processed by half of an LM556 IC that buffers the signal and provides visual occupancy feedback.

The Coilcraft VT-5 Current Transformer was available from All Electronics Corp, but as of Nov, 2005, was out of stock and unlikely to be reordered. Other similar pulse transformers should work as well, but they have not been tested. The low cost of the VT-5 ($0.35 in quantity) made this circuit much more economical than traditional diode type detector circuits.

Features of the pulse transformer based detector include

• Instant detection
• Controlled release delay - Approximately 2 seconds, can be changed on a per detector basis
• Less than 1 Milliamp detection current sensitivity
• 5 Amp track current rating
• Complete isolation between track power and logic circuits
• No "diode losses" in the track voltage

[edit] Theory of Operation

When no trains are detected, the transistor does not conduct, the capacitor charges to 5v over a period of about 2 seconds, at which time the current thru the resistors stops flowing. With no current flowing, the TRIGGER voltage is pulled to ground by the 1Meg resistor, causing the open collector output (!DETECTED) to go high and the Occupied feedback LED to go dark. (This is also the default state if there is no remote detector connected to the base unit)

When current flows in the detected track section, a voltage is induced in the pulse transformer. When a sufficiently large voltage (> 0.7v) is induced, current will flow through the Base to Emitter junction of the transistor and it turns on. This shorts out the capacitor, which causes current to flow thru R1 and the 1M resistor. This pulls TRIGGER up close to 5V, which causes !DETECTED to be pulled to ground and the Occupied LED to turn on.

The sensitivity of the detector can be changed by increasing or decreasing the number of turns on the primary side of the transformer. The default suggestion of "2 turns" gives a sensitivity of less than 1mA. Keep in mind:

• a "turn" is simply a wire passing thru the hole in the middle of the transformer. Discussions about partial turns (one and a half turns...) are somewhat meaningless - they are concerned with wire routing in areas other than the hole - where it doesn't really matter to the transformer.
• More turns means more voltage is induced, so less current is needed to trigger the detector.
• More turns means more sensitivity to wiring and track capacitance that may result in false detection.
• More turns also means that at high currents (i.e., when the track is shorted), excessive voltage may be generated that could disrupt the circuit. (Don't exceed the working voltage of the transistor!)
• Turns that are tightly wound are more efficient than loose turns. (Fewer turns are needed.)

NOTE: Block occupancy detectors for DCC systems do not need to sense current flow in both directions due to the high rate of direction change (thousands of times per second). The time delay [given by R(1Meg+10k) * C(2.2uF) = 2.2seconds] is more than enough to compensate for the few miliseconds when the track current is flowing in a reversed direction.

Diode D1 protects the transistor from any reverse voltage biases.

[edit] Remote Sensor Interface

The RJ-12 jack on the board carries the signals and power to the Remote sensor board. The pinout is symetrical so no damage will occur if the cable is twisted between the connectors.

Pins 1 and 6 are connected together (Vcc), as are pins 3 and 4 (Gnd). The TRIGGER signals are active high; they are pulled down to ground thru a 1Meg resistor on the base unit.

[edit] Test Card

The Test Card is used to verify the proper operation of the base module and the interconnect wiring to a remote detector.

The card contains a power indication LED and a toggle switch. The LED lights if the cable is properly wired and connected to a base unit; the switch mimics the presence of a train in a detected block.

[edit] LocoIO Loconet Interface

John Jabour's LocoIO provided 16 digital I/O conections that could be accessed via Digitrax's LocoNet. Selected Loconet packets could control outputs; alternatively, inputs would generate Loconet messages. John made kits available for a while; after he went on to other pursuits, Hans DeLoof took up the project, revised the circuit board, and produced updated versions of the firmware. Both these boards required cables of some sort (ribbon or telco) to connect them to sensors and devices.

Since the Detector project was designed to utilize all 16 I/O lines on a LocoIO, it was possible to reduce the cost by eliminating 8 telco connectors, jacks and cable assemblies and including a "prewired" LocoIO core on the same circuit board.

This is a simple PIC design - it uses a LM311 Comparator to read the Loconet bitstream and a NPN transistor to write to it. There are a couple of pins that are used to monitor configuration jumpers, one to drive a "Loconet Activity" LED; the rest are directly tied to the "Occupied" outputs from the detection circuitry.

See the LocoIO documentation for more information.