Lap Triggers

Application Note

Optical Lap Trigger System

The optical system consists of the LTX-400 transmitter and the LRX-400 receiver.

The LTX-400 transmitter has an array of infra-red emitters which produces a focussed beam to give an operating range of about 30m in optimum conditions. LTX 400 is powered by an external battery connected via a flying lead. LTX-400 transmits one of 256 codes, selected by switches inside the unit. The codes are split into 16 groups and each group is split into 16 channels. The LRX-400 receiver is set, by an internal switch, to respond to only one of the 16 groups, signals from other groups are ignored. When a matching group code is detected, the LRX-400 outputs a TTL level pulse as well as an analogue voltage to identify which of the 16 possible transmitter channels was transmitted by the LTX-400. An LED on the rear of the LRX-400 indicates when a transmission from the selected group has been detected.

LRX-400 can discriminate between up to 16 different lap triggers, transmitting different channels, around the track. For example, transmitters could be placed on the start/finish line, in the pit lane and at other key locations to create timing split points and speed traps.

LRX-400 is a compact, cylindrical unit that can be readily accommodated on a car. It has a wide dynamic range and integrated filters to minimise interference from sunlight or other heat sources. It should be mounted in the car in such a position that it has a clear and direct view of the transmitter as it passes. If a window is used, it must be transparent to infra-red at 875nm. If possible, the mounting should be in a location which does not get wet or dirty.

Two variants of the LRX-400 are available which either combine the timing pulse and channel identification voltage on a single output or provide two separate outputs. In each case the channel identification voltage has 1 of 16 values according to the channel code received from the transmitter.

In the Combined Output Version a short (10ms), TTL level, active high, digital pulse is superimposed on the channel identification voltage when a valid lap-trigger signal of the selected group code has been received.

In the Separate Output Version the timing output is a short (10ms), TTL level, active low, digital pulse which indicates that a lap-trigger signal from the selected group has been received. The channel identification voltage is output continuously on a separate wire and only changes if a signal is received from a differently coded transmitter using the same group.

Setting up the LRX/LTX-400 system

Transmitter – LTX-400 needs to be set up to send a code which corresponds to the desired group and channel. The settings are by a DIL switch inside the unit. The switch is accessed by removing the rear lid of the unit (held by four screws). The switch is at the bottom of the board near to the tripod mount and is marked with the ON position and the individual switch numbers. When replacing the lid, use a new O-ring and sealing washers.

Switches 1 to 4 define a group number (from 1 to 16). Table 1 lists the switch positions for each group. Switches 5 to 8 define a channel number, which is used to identify the LTX-400 to the receiver. LRX-400 generates an analogue output corresponding to the channel number. Table 2 lists the switch positions for each channel as well as the LRX-400 channel identification voltage.

To avoid interference, ensure that no other team at the circuit is using the same group as you.

Receiver – LRX-400 needs to be set up to receive the same group as is being transmitted by the LTX-400. Remove the screw on the back of the receiver with an Allen key and use a small Phillips screwdriver to turn the recessed PCB mounted switch inside. Rotate the switch clockwise to select from group 1 to 16. The switch has no stop, the two arrows on the back cover show the allowable movement. Set an LTX-400 transmitter to the desired group and point it at the receiver. Rotate the switch slowly until the LED on the receiver flashes at approximately 1Hz. This indicates the receiver is correctly set to receive that group. Replace the cover screw with its sealing washer.

Table 1: Optical Transmitter Group Setting
GroupSwitch 1Switch 2Switch 3Switch 4
1OffOffOffOff
2OffOffOffOn
3OffOffOnOff
4OffOffOnOn
5OffOnOffOff
6OffOnOffOn
7OffOnOnOff
8OffOnOnOn
9OnOffOffOff
10OnOffOffOn
11OnOffOnOff
12OnOffOnOn
13OnOnOffOff
14OnOnOffOn
15OnOnOnOff
16OnOnOnOn
Table 2: Optical Transmitter Channel Switching
ChannelChannel Identification Voltage (V)Switch 5Switch 6Switch 7Switch 8
14.562OffOffOffOff
24.287OffOffOffOn
34.012OffOffOnOff
43.737OffOffOnOn
53.462OffOnOffOff
63.187OffOnOffOn
72.912OffOnOnOff
82.637OffOnOnOn
92.362OnOffOffOff
102.087OnOffOffOn
111.812OnOffOnOff
121.537OnOffOnOn
131.262OnOnOffOff
140.987OnOnOffOn
150.712OnOnOnOff
160.437OnOnOnOn

Microwave Lap Trigger System

The microwave lap trigger system consists of the LTX-310B transmitter and the LRX-310B receiver.

In the LTX-310B transmitter, a specially selected horn antenna gives a focussed narrow beam of microwaves at 10.6GHz. Careful choice of the E and H plane pattern minimises multipath effects caused by scattering from the ground and other objects. The LTX-310B has an internal sealed battery that provides approximately eight hours operation when fully charged. An external battery can be connected for longer operation.

LTX-310B modulates the transmitted microwave beam at one of 36 possible frequencies, allowing six teams to each operate up to six LTX-310Bs at the same track. The group code for each team is set at the factory by an internal switch. An external 6 position rotary switch selects a channel which allows the receiver to discriminate between different transmitters on the same group code. LTX-310B has a key locked power switch, preventing accidental deactivation.

The LRX-310B receiver is a small X-band microwave module with a specially-designed waveguide antenna in a small, lightweight assembly, ideally suited for car mounting. It can be mounted by applying self adhesive hook and loop or Dual-Lock type fasteners to any face other than the front.

The slotted waveguide antenna covers the entire front face of the LRX-310B. The antenna is most sensitive perpendicular to the front face which should have an unobstructed line-of-sight path to the transmitter. The receiver may be mounted behind a window which is transparent to radio at 10.6GHz. Kevlar, glass-fibre, polyethylene and PTFE are suitable window materials. Carbon-fibre or metal are unsuitable. To prevent distortion of the RF beam shape, the thickness variation across the RF window should be less than 0.5mm.

Maximum detection range is approximately 25m. To achieve this, it is important to match the polarisation of the transmitter and receiver antennas to within ±30°. If the transmitter is mounted with its feet downwards, then the receiver must be mounted with its long axis horizontal.

LRX-310B outputs trigger pulses as it passes through the beam of an LTX-310B transmitter. The pulse repetition interval is inversely proportional to the modulation frequency and the amplitude of the pulses is proportional to the strength of the received signal. LRX-310B detects any of the 36 channels transmitted by LTX-310B and does not need to be set up. The transmitter channel is identified from the Pulse Repetition Interval shown in Table 3.

Table 3: Microwave System Settings
Transmitter settingReceiver Output
Factory Switch Group SettingRotary Switch Channel SettingPulse Repetition Interval μs
11
2
3
4
5
6
1575
1543
1513
1479
1447
1416
21
2
3
4
5
6
1383
1351
1319
1289
1256
1224
31
2
3
4
5
6
1192
1160
1127
1096
1064
1032
41
2
3
4
5
6
1000
968
936
904
872
840
51
2
3
4
5
6
808
776
744
712
680
648
61
2
3
4
5
6
616
584
552
520
488
456