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Publication Number: FHWA-HRT-06-139
Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II


This appendix contains specifications for the following loop detectors (i.e., electronics units) and magnetic detectors (i.e., electronics units), as described in Chapter 5 of the Caltrans Transportation Electrical Equipment Specifications (TEES), August 16, 2002 Edition.(1)

As in Appendix J, the numbering system used in this appendix corresponds to that used in the cited specification, in this case—TEES, rather than this Handbook. The exception is that Tables K-1 and K-2 have been numbered and titled so that they can be added to the table of contents. TEES places Metric units first with the English units in parenthesis while the style of the Handbook is to place English units first with Metric units in parenthesis. Where definitions for advanced transportation controller (ATC)- and 2070- related terms are not provided, they may be found in the glossary.


5.1.1 – Channel Independence and Power Requirements

The sensor and isolator channels shall be operationally independent from each other. Each sensor or isolator channel shall draw no more than 50 mA from the +24 VDC [voltage direct current] cabinet power supply and shall be insensitive to 700 mV RMS [root-mean-square] ripple on the incoming +24 VDC line.

5.1.2 – Sensor Unit Features

The sensor unit or isolator front panel shall be provided with the following:

5.1.3 – Output Device Type

Each sensor or isolator channel output shall be an opto-isolated NPN open collector capable of sinking 50 mA at 30 VDC. The output shall be compatible with the controller unit inputs.

5.1.4 – Output Signal Characteristics

A valid channel input shall cause a channel Ground True Output to the controller unit of a minimum 100 ms in duration. An onboard two-post shunt jumper shall be provided to disallow this requirement when the jumper is in an OPEN position.

5.1.5 – Interface Requirements

The sensor unit or sensing element shall operate and interface successfully with an associated Caltrans Standard Sensing Unit or Element.

5.1.6 – Output Device Switching Time

The output transistor shall switch from OFF to ON state or ON to OFF state in 20 µs or less.

5.1.7 – Serial Output Definition

Each Enhanced (E) Sensor Unit shall provide a Tri-State EIA [Electronic Industries Association] 423 Serial Port for interface with through pins 19 and 21 of the unit connector to the assembly SIU [serial interface unit] Connector C03. Each sensor unit shall read in the Slot Address pins located at unit connector for unit slot Identification. Four onboard switches shall match the upper 5th to 8th bits associated Assembly name. Slot Pin 22 shall turn on the SIU Drivers allowing RXD to the ATC [advanced transportation controller]. Status and report messages shall meet SDLC [synchronous data link control] Protocol transmitting at 19,200 bps. Said Messages shall be as called out by the user.

5.1.8 – Electrical Surge Protection

Onboard protection shall be provided to enable the sensor unit or isolator to withstand the discharge of a 10 µF capacitor charged to ±1000 Volts directly across the input pins with no load present. With a dummy load of 5 Ohms, protection shall enable the sensor unit or isolator to withstand the discharge of a 10 µF capacitor charged to ±2000 Volts directly across either the input pins or from either side to equipment ground.


5.2.1 – Output Signal Definition

The sensor unit channel shall produce an output signal when a vehicle passes or remains over loop wires embedded in the roadway. The method of detection shall be based upon a design that renders the output signal when a metallic mass (vehicle) enters the detection zone causing a change of 0.02% minimum decrease in inductance of the circuit measured at the input terminals of the sensor unit.

5.2.2 – Malfunctioning Loop Signal Definition

An open, shorted, or otherwise malfunctioning loop shall cause the sensor unit channel to a constant signal state. A two-post shunt jumper or approved switch shall be provided to dictate output state upon malfunction. The jumper in closed position (Ground True) shall cause a Ground True Output to the controller unit. An open position (Ground False) shall cause a Ground False Output to the controller unit. An indicator labeled "EOS" (Event Output Status) shall be provided on the front panel. When in closed position, the indicator shall be ON. A second indicator per channel labeled "EVENT" shall be provided indicating a malfunctioning loop or tracking condition. The indicators shall be reset by the Unit RESET Switch.

5.2.3 – Supported Loop Configurations

Each sensor unit channel shall be capable of detecting all types of AGENCY licensed motor vehicles when connected to the loop configuration/lead-in requirements of Section

5.2.4 – Inductance and Q Range of Supported Loop and Lead-in Cable Assemblies

The sensor unit shall comply with all performance requirements when connected to an inductance (loop plus lead-in) from 50 to 700 µH with a Qparameter as low as 5 at the sensor unit operating frequency.

5.2.5 – Transformer Isolation of Loop Inputs

Loop inputs to each channel shall be transformer isolated.

5.2.6 – Number of Supported Operating Frequencies

Each individual channel shall have a minimum of 4 switch selectable operating frequencies.

5.2.7 – Drift Compensation of Tuning Circuits

The sensor unit channel tuning circuits shall be automatic and shall be so designed that drift caused by environmental changes or changes in applied power shall not cause an actuation.

5.2.8 – Pulse and Presence Mode Support

Each sensor unit channel shall have PULSE and PRESENCE selectable modes. Pulse Mode Description

In the PULSE MODE, each new vehicle presence within the detection zone shall initiate a sensor unit channel output pulse of 125 (±25) ms in duration. Should a vehicle remain in a portion of the detection zone for a period in excess of 2 seconds, the sensor unit channel shall automatically "tune out" the presence of said vehicle. The sensor unit channel shall then be capable of detecting another vehicle entering the same detection zone. The recovery time to full sensitivity between the first vehicle pulse and channel capability to detect another vehicle shall be 3 seconds maximum. Presence Mode Description

In the PRESENCE MODE, the sensor unit channel shall recover to normal sensitivity within 1 second after termination of vehicle presence in the detection zone regardless of the duration of the presence. The channel sensitivity settings 2 and 6 shall provide presence detection of a vehicle in the detection zone for a specified time period and inductance change as shown in Table K-1.

Table K-1. Presence detection time for detector sensitivity settings of 2 and 6.
SettingMinimum time duration (minutes)Detector Input Inductance change (%)
630.02 or more
100.06 or more
241.00 or more

5.2.9 – Sensitivity California Standard Plan ES-5A and B Loop Configurations (California Department of Transportation Standard Plans) are listed below:
  • – Single Type A, B, Q, or Round Loop with a 250 foot (76 m) lead-in cable.
  • – Single Type A, B, Q, or Round Loop with a 1000 foot (305 m) lead-in cable.
  • – Four Type A, B, or Q Loops connected in series/parallel with a 250 foot (76 m) lead-in cable.
  • – Four Type A, B, Q, or Round Loops connected in series with a 1000 foot (305 m) lead-in cable.
  • – One 50 foot (15 m) Type C Loop with a 250 foot lead-in cable. Sensitivity Settings – Each sensor unit channel shall be equipped with 7 selectable (digitally read) sensitivity settings (shown in Table K-2) in presence and pulse modes to accomplish the following under operational and environmental requirements of this specification.
Table K-2. Sensitivity settings for presence and pulse modes.
10.64% delta L50.04% delta L
20.32% delta L60.02% delta L
30.16% delta L70.01% delta L
40.08% delta L0Channel OFF – All sensitivity settings shall not differ by ±40% from the nominal value chosen. Each sensor unit channel shall not detect vehicles, moving or stopped, at distances of 3 feet (0.91 m) or more from any loop perimeter, in all configurations listed in paragraph

5.2.10 – Response Time

Response time of the sensor unit channel for Sensitivity Settings 1, 2 and 3 shall be less than 5 ± 1 ms at an approximate loop frequency of 40 kHz. That is, for any decreased inductive change that exceeds its sensitivity threshold, the channel shall output a ground true logic level within 5 ± 1 ms. When such change is removed, the output shall become an open circuit within 5 ± 1 ms.

5.2.11 – Normal Operation Time

The sensor unit channels shall begin normal operation within 2 seconds after the application of power or after a reset signal of 30 µs.

5.2.12 – Tracking Rate

The sensor unit shall be capable of compensating or tracking for an environmental change up to 0.001% change in inductance per second.

5.2.13 – Tracking Range

The sensor unit shall be capable of normal operation as the input inductance is changed from ±5.0% from the quiescent tuning point regardless of internal circuit drift. The sensor unit shall be capable of normal operation as the input resistance is changed from ±0.5% from the quiescent tuning point regardless of internal circuit drift.

5.2.14 – Temperature Change

The operation of the sensor unit shall not be affected by changes in the inductance and/or capacitance of the loop caused by environmental changes with the rate of temperature change not exceeding 1 degree C (1.8 degrees F) per 3 minutes. The opening or closing of the controller cabinet door with a temperature differential of up to 18 degrees C (32.4 degrees F) between the inside and outside air shall not affect the proper operation of the sensor unit.



Each sensing element shall be designed for ease of installation, repositioning, and removal. The sensing element shall be 57 mm (2.2 in) maximum in diameter, have no sharp edges, and its length not exceed 450 mm (18 in). The sensing element shall be constructed of nonferrous material and shall be moisture proof. The element shall contain no moving parts or active components. The element shall have a 15.4 m (50 ft) lead-in cable. Leakage resistance shall be a minimum of 10 megohms when tested with 400 VDC between lead wire, including lead wire entrance, and the fluid of a salt water bath after the device has been entirely immersed in the bath for a period of 24 hours at 20 ±3 degrees C (36 ± 5.4 degrees F) . The salt water bath concentrate shall contain one fourth ounce of salt per gallon (2 gm per liter) of water. – DC Resistance

Each sensing element including lead-in shall have a DC resistance of less than 3500 ohms and an inductance of 20 Henry ± 15%.


When resident in an active cabinet input assembly and attached to one or more Model 231E Sensing Elements resident in conduit under the travelway, the sensing channel shall output a Ground True Output to the Controller Unit when sensing an induced voltage caused by a California Licensed Vehicle passing within 1.83 m (6 ft) from an element with a 305 m (1000 ft) of lead-in cable at all speeds between 5 and 130 km/h (3 and 80 mi/h). The sensing channel output shall be continuous as long as the vehicle is detected. A digital reading switch with 8 selected step positions for gain (0 to Full) and a momentary test switch providing a voltage test input shall be furnished for each channel on the front panel.


  1. Transportation Electrical Equipment Specifications (TEES). California Department of Transportation, Sacramento, CA, August 16, 2002. Available at http://www.dot.ca.gov/hq/traffops/elecsys/ (Accessed March 28, 2004.)

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