Infrared – Proximity Detector
The detectors are designed for short range – reflected light operation but can also be used for short range, ‘broken beam’ detection as well.
This circuit was designed primarily for Between The Rails detection of model trains.
In this circuit the TSOP4830 Infrared receiver is used as a sensitive detector that does not require shielding from room lighting. The detector will work under almost any lighting condition from complete darkness to full sunlight.
As a guide, if a TV remote control will work in a particular area then so should this circuit as the TSOP4830 would normally used in conjunction with a remote control for a television or VCR.
This circuit will work in almost all light conditions but it can be affected by infrared remote controls. For its designed use this sould not be a problem.
This circuit is more complex than typical infrared or visible light sensitive detectors at this site and elsewhere but will work in a wide range of light conditions.
Basic Infrared – Proximity Detector Circuit
Basic Circuit Operation
The 556 – LED driver circuit produces an approximately 10 millisecond burst of 30 kHz light 3 times per second via the infrared LED.
The lower portion of the circuit is the IR detector, time delay and output section. The output of the voltage comparator, IC 3, produces a steady LOW – open collector output when infrared light of the correct frequency is detected by the TSP4830 receiver IC.
- When the IR Receiver Module receives a sufficiently strong 30 kHz infrared pulse from D1, its output, pin 3, will go LOW for the duration of the pulse. (About 9 milliseconds.)
- When the output of the IR detector goes LOW, the voltage at the PLUS input of comparator, IC 3, will be made LOW and the comparator’s output will go LOW. As long as the infrared pulses are seen by the receiver module the output of the comparator will remain LOW.
The 4.7uF capacitor provides a time delayed resetting so that the output of the comparator is steady between the IR pulses.
- When the 30kHz infrared signal is no longer detected, the 4.7uF capacitor will charge and the output of the comparator will go HIGH.
In typical circuits the output of the comparator would control a LED or drive a relay.
Typical Installation Of The Proximity Detector
The LED is encased in a section of heatshrink tubing to prevent the light from reaching the receiver directly from the sides of the LED.
The top of the tubing is flush with or slightly higher than the top of the ties. The bottom of the tubing should extend about 1″ below the bottom of the infrared receiver IC.
The IR receiver module is mounted through a 1/4″ hole drilled through the roadbed.
The top of the receiver module is at the bottom of the ties with the lens of the receiver facing the IR LED.
Across The Track Installation
Proximity Detector Notes
- This circuit uses the TSOP4830 infrared receiver module in a manner for which it was not designed. However, for model train detection the circuit worked very well and is able to sense the uncoupling pin of a Kadee® type automatic coupler.
- The infrared LEDs pulse at a rate of approximately 3 times per second. The slow rate also allows the green LED D2 to be seen flashing by the user but is fast enough for model train detection. The slow flash rate can also be helpful for trouble shooting.
- The circuit as shown has an output release delay time of approximately 2.5 seconds. The delay can be made shorter by decreasing the values of capacitors at the input of the comparators.
- For between the rails type detection the infrared LEDs need very little current as the distance between the LED and the receiver is short.
- The circuit as shown is designed to control LEDs connected at the outputs through 1K current limiting resistors mounted on the circuit board. The output resistors can be replaced with jumpers or their values changed to drive other devices.
For example; The Base of a 2N3906 PNP transistor could be driven through an external 3.3K resistor mounted on the circuitboard. The transistor would be used to drive a low power relay.
There are other TSOP48xx series infrared receivers that operate at carrier frequencies between 30 and 56 KHz. that could also be used with this circuit if the LED driver’s carrier frequency was changed to suit. Also there are other receivers with different case styles that should work as well.
Using The Infrared – Proximity Detectors
- The TSOP4830 has a wide field of vision and can receive light through the back of its case. In some installations it may be necessary to shield the receiver from stray signals from other emitters.
- The infrared LEDs will have to be mounted in a light blocking sleeve. A section of closely fitting heat shrink tubing slipped over the IR emitter is ideal for this purpose and does not NEED to be shrunk.
Moving the IR LED deeper into its heatshrink sleeve will reduce the sensitivity of the detector by reducing the amount of light available without having to change the current through the infrared LED.
- This system is not particularly suited to use in tunnels as the infrared may be reflected from the roof of the tunnel and into the receiver causing a false detection. (Inside tunnels, an infrared emitter/detector pair across the track would be more suitable.)
- The proximity detector could also be used across the track. In this case the output of the circuit would be LOW until and object breaks the infrared beam. In this case, the time delay would be reversed causing the output to go HIGH about 2.5 seconds after the beam is broken. (Decreasing the value of the 4.7uF timing capacitors would shorten the detection time delay.)
- In the beam blocking mode, the distances can be quite large. For long distances it may be necessary to increase the current through the infrared LEDs by reducing the value of its series resistor.
- The proximity detector is not limited to model railroad use. Any object that reflects or blocks enough infrared light could be used to trigger the circuit. The proximity detector could also be used by robots to locate walls or obstacles.
- This circuit may be affected by some TV or VCR remote controls depending on their operating frequency and might not be suitable for layouts where infrared controlled throttles are being used. On the other hand, a TV remote might be used to see if a particular receiver is working.
Is The IR LED Working?
The image may not be very bright but close-up or in a darkened area the light should be visible on the camera’s display screen.
Proximity Detector – 30KHz Carrier Frequency Adjustment Notes
METHOD – A uses five fixed resistors to set the 30KHz frequency of IC 1B.
METHOD – B uses a potentiometer. This is easier to calibrate but can be easily be put out of calibration.