A Train Detection Clinic

Todays position detectors use either magnetics or light sources to register the passage of a train.

Magnetic detectors are in common use. Either to actuate a 'REED' switch installed just below rail height or to influence a 'HALL EFFECT' element buried in the track.The Hall Element is 1.5mm thick and 4mm square. It can be glued on top of a sleeper and hidden with ballast.
The leads, bent at 90( go through the baseboard.

Reed switches are encased in a glass tube with leads coming out of the ends.
They are a little fragile and best installed while laying the track by cutting out a sleeper and putting them in its place. Best used to drive a relay to allow the use of change over switching contacts.

The Hall Effect device does need considerably more electronics but can detect a much smaller magnet providing it is moving.

Of course the big disadvantage is that all locos and possibly rolling stock will have to have magnets attached and magnets tend to collect every loose track nail and spike in your layout.

CdS

Infra Red

Amplified

Detector

Optical detectors come in several types. They work by sensing a light shining on a detector and give an output change when this is cut by the train. These Light Sensitive devices are available in several styles. Most vary the resistance to the circuit as light falls on the surface. CdS cells (which are light-sensitive resistors), a resistive type, are not very sensitive and need a good near light source for reliability. Diode and Transistor types have a larger resistance change and will work in the general ambient lighting. Transistor types with inbuilt amplifiers are super sensitive and can work over long distances.

With photodiodes and transistors it is usual to have a small light bulb providing a source with the detector held in a small tube to cut out incident light. If you use the room light there will be problems with arms reaching over the track and low or varying intensity lighting.

A simple circuit - with light shining on a resistive or diode type adjust the variable resistor so about 3 volts is dropped across the photo cell. The output should be floating. When the photo cell is covered the voltage across it rises and this turns on the output transistor and the output is clamped to 0 volts. The two transistors could be replaced with an integrated circuit amplifier. If adjusting for a transistor type the voltage point is just below 1.8 volts.

The better way uses invisible infra-red beams crossing the track to detectors on the other side or may have a beam pointed upwards from between the tracks which is reflected down by the train's underside to the detector, also buried in the track. Mount the detector at an angle to the track as shown, so that the gaps between cars don't re-trigger the circuit continually.

Infra-red emitters and detectors are much more practical because they are not as easily effected by the room illumination, can be used over longer distances and are not visible.
Of course this also is a problem when trying to align them to the detector. It is also nice to have an indicator in the circuit so that while adjusting you can see immediately when it changes and so set the adjustment to just below that point.

For more reliable operation, long range or reflective detectors use a pulsed IR beam to eliminate any possible interference. IR can be detected over longer distances and with high current LEDs could even be used, pointing along a straight section of track, as an occupancy detector. For example, pointing down a long yard track.

A simple detector used to detect the passing locomotive consists of a short section of isolated track. As the loco crosses it, the wheels conduct the voltage from the live track onto it. A sensitive relay or opto coupler connected between the isolated rail and the opposite rail will thus be activated by the CAB supply as the train passes. An isolated section is very easy to cut into the track while building the layout but needs a little more care on pre-laid track. Epoxy cement can be used to hold the under side of the rail in position and the cuts need be no more than 2 sleepers apart. The rail height should not differ or else the wheels will not contact with enough pressure.

The most used method of block occupancy detection is by taking a small part of the current supplied to the train motor to actuate a detector. Called a CURRENT detector, there are several variations but all need a diode inserted in the current path to the track and so reduce the power available to the motor.
	Two diodes of sufficient current carrying capacity are used back to back so that whether a train is being driven forward or backwards one of the diodes is conducting. Diodes have a voltage drop of 0.6 volts and this is sensed by the detector. The detector circuitry needs an additional voltage supply which will be commoned to the CAB power.
The Bridge Rectifier configuration can be used to power a 1.5 volt bulb. The loco has to draw at least one tenth of an amp through the lamp to light it and it will only light the one bulb (and cannot control other devices, like trackside signals).	Some wiring problems can be caused by this common supply in particular with reverse sections. An alternate circuit can be used that has four diodes. This develops 1.4 volts across the two conducting diodes when the train is drawing power. This larger voltage can be used to light an LED which actuates through a light detector. This isolates the track electrically from the power supply for the detector. This is called an OPTICAL COUPLED detector. Its output could be used directly but it is more common to amplify it and have a delay so that it masks any loss of signal due to dirty track.

	The Twin-T or Twin Transistor type of detector is popular because it uses few simple components and does the job. There are many variations on this type but all have common features. Two back-to-back diodes sample the current used by the motor or any other resistance across the track. This develops 0.6 volts across the diodes and this is enough to activate one of two transistors. These drive the relay driver and pull in the relay A 'tickler' resistor can be used to supply a small current to the opposite track and allow a train to be detected when the CAB is switched out.
A more modern version of the Twin-T uses Integrated circuit amplifiers. The current makes a 0.6 volts drop across the diodes which is then fed via resistor voltage dividers to the two amplifiers. One of the amplifier's outputs changes and is then used to pick a relay or opto coupler. A delayed drop of the relay driver is used to stop it from chattering on dirty track. Note the track common and the power supply common are the same. It is advisable to have a relay or other isolated output to avoid signal or other circuits being commoned to the electrically noisy track supply.
If the track is running DCC or an oscillator is used for car lighting it can also be the voltage supply for the detector.	Becoming more common now is the current TRANSFORMER detector. This works with DCC or a pulsed CAB supply by sensing the changing current flow to the motor. The track connecting wire is used as the input winding of a transformer that has a secondary winding with a high number of turns . This transforms the current pulses of the input into larger detectable changes at the secondary. Its immediate advantage is that the detector part is isolated electrically from the track power. It needs a constantly changing voltage and this can be provided by inserting a high frequency oscillator of low current in with the track power. This also lets it detect on non-powered track sections. If you use DCC it provides this alternating source automatically.

Comparison Table	Reed Switch.	Hall Effect	CdS or Diode	Photo Transistor	IR beam	Rail Gap
Sensitive to room lighting	No	No	Yes	Yes	No	No
Detects What	Strong Magnet	Magnets	All	All	All	Loco
Reaction Distance	1 - 2cm	2cm	8cm	15cm	15 + cm	Spot
Installation	moderate	easy	moderate	moderate	moderate	moderate
Operation cleanliness	metal bits	Metal bits	good	good	good	good
Circuit Complexity	low	moderate	low	moderate	most	low
Available as	Parts	Parts/Kit	Parts	Parts/kit	Commercial	Parts

Type	Simple Indicator	Opto Coupled	Twin-T transistor	Twin-T I.C.	Transformer
Diodes in circuit	2	2	1	1	None
Voltage loss	1.4 volts	1.4 volts	0.6 volt	0.6 volt	None
Track Power needed	1/10th amp	10 ma.	small	small	None*
Isolated output	No	Yes	yes/no	yes/no	Yes
Sensitivity	Low	Low	High	High	High
Direction detect	No	No	yes/no	yes/no	No
Noise Interference	Yes	No	Yes	No	No
Installation at trackside	Diodes	Diodes	Diodes	Diodes	All at track
Best used on	DC	DC	DC	DC/Pulse	DCC/Pulse

Position or Spot detection.

Block Detectors

Position Detectors

Drawings and text taken from a clinic presented by Rod Murgatroyd at NZMRA Convention, Easter 2002. If you want more information get in contact
Preamble: From time to time the magazines discuss various types of detection systems. This is an overview of common types which will help you toward a better understanding and assist you in making the right choice to suit your requirements.	A train can be detected at a POINT on the track or while within a BLOCK of track. These detectors would be used for different purposes. Although two point detectors along a section of track could be joined to indicate that a train was within the area, this is not usually done.