Long-Range Wireless Gate Annunciator

Microwave Motion Sensor

Alternate (or Supplement) to PIR Sensor

Max Carter

Read first the original article for background.


This page describes a different way to detect a vehicle or pedestrian entering through the gate: the microwave motion sensor. While the installed PIR sensor works 99% of the time, there are occasions when it doesn't, usually during precipitation events (heavy snowfall, drizzle). This gave me an incentive to investigate another interesting approach. The device described uses the HB100 microwave motion sensor. These are incredibly cheap: available on eBay for as little as $3. A microwave motion sensor differs from a PIR sensor in that it's an active device. Whereas the PIR sensor monitors ambient (IR) radiation impinging on the device, the microwave sensor radiates a low-power microwave beam and monitors the signal reflected from objects.

HB100

Antenna Side Other Side
Note installed header pins.

The HB100 consists of a 10.525 GHz dielectric resonance oscillator (DRO), transmit antenna, receive antenna and Schottky diode receive mixer/detector. If an object within range happens to be moving toward or away from the sensor, the reflected signal will differ slightly in frequency from the transmitted signal (the "Doppler effect"). The frequency shift depends directly on the target's speed as seen by the sensor - the greater the speed, the greater the shift. The received signal is mixed with a sample of the transmitted signal to derive a difference signal. The difference signal appears at the sensor's IF output pin.

IF Amplifier/Filter/Rectifier

The sensor requires an amplifier/filter/rectifier to complete the system. Figure 1 shows the circuit, derived mostly from this application note. It consists of a sample-and-hold input stage, variable gain filter/amplifier, full-wave rectifier and output buffer transistor.

Figure 1

The motion sensor is operated in pulse mode to minimize power consumption, important when the power supply is a battery. The 7555 (U2), a CMOS version of the 555, generates 20 µS pulses at 0.5 mS intervals (2 kHz 4% duty cycle). The sensor radiates only when the output pin (3) of U2 is low. This results in a 25:1 reduction in average power consumed by the sensor module, and a reduction (by 14 dB) in average radiated power. After adding in the power consumed by U2, average current draw is about 5 mA, compared to 50 mA the sensor would draw in continuous mode.

Pulse mode could be thought of as a form of spread-spectrum, a technique used in wireless communications systems to minimize interference. The synchronous sample-and-hold input to the IF amplifier operates as a 'matched filter' to de-spread the received signal, making it appear to the amplifier/filter as a continuous, low-frequency signal. This compensates for the reduced transmitter power and may provide some interference rejection. Note that the sample-hold transistor (2N2222) operates in reverse active mode during part of the sample cycle. [Google: "bjt reverse active mode"]

The IF signal from the sensor is amplified and filtered by opamps U1a and U1b. Gain can be adjusted from about 3000 to 10,000 (~70-80 dB). Frequency response is ~5-50 Hz, making the sensor most sensitive to speeds in the range of 0.15 to 1.5 MPH (0.25-2.5 km/h), walking speed. (For more on the speed range issue, see 'Lessons Learned' below). The output of U1b is fed to U1c and U1d. When the signal exceeds 1 volt peak-to-peak amplitude, one or both 1N914 diodes conduct, charging the 100 µF capacitor and turning on the NPN output transistor. The output is pulled to ground when a moving object is detected.

IF Amplifier Gain vs. Frequency

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IF amplifier as-builtMated to HB100

Note the pin headers.
These accept the pins on the HB100.
  
Installed in weather-tight boxMounted on the gate post opposite the PIR sensor


Connecting to the Existing System

The output of the microwave sensor is an open collector, as is that of the existing PIR sensor. So.. incorporating the microwave sensor into the system is a simple matter of paralleling its output with the output of the PIR sensor, creating an either/or situation. A ground on either output turns on the Gunn driver.

Figure 2 (copied from Figure 1 of the original article) shows the Gunn driver and the connections. The 12-volt line, ground, and the input to the driver (2.2k resistor) are all paralleled. It could hardly be simpler.

Figure 2



You May be Wondering

Since the HB100 sensor and the microwave link (Gunn osc./radar det.) operate on the same frequency, why don't they interfere with each other?

Microwave Sensor to Microwave Link:

There are several reasons, any one of which probably would be enough to prevent interference - trees obstructing the path, lower effective radiated power from the microwave sensor, cross polarization discrimination (XPD) in the receiver - but the killer reason is this:

  • The speed radar detector (the receiver) is set to its CITY mode. In this mode it becomes blind to the signal from the HB100. The HB100's transmissions are apparently too short in duration (20 µS) and/or spaced too infrequently (1/0.5 mS) to be detected. I don't know the designers' rationale for the receiver's CITY mode, but the effect is to make the Doppler sensor's signal invisible to the device.

Microwave Link to Microwave Sensor:

Possibilities include path loss, the directional nature of the two systems' antennas, and XPD, which together provide 75-80 dB of isolation. The main reason however, I think, is this:

  • While the two systems nominally operate on the same frequency, they are not actually on the same frequency. The receiver in the microwave sensor is extremely selective, having a bandwidth on the order of 50 Hz. The chance that the Gunn transmitter is operating within 50 Hz of the microwave sensor is virtually nil. The matched-filter/sample-hold IF input may also provide some interference rejection.

Belt and Suspenders

The gate is now monitored by two motion sensors employing completely different methods. If one sensor doesn't detect an incoming vehicle or pedestrian for whatever reason, the other surely will. I will report on how well this works out after the dual-sensor configuration has been up and running for awhile.


Update: Lessons Learned

  • The sensor should be mounted horizontally (as shown in the photos above) for best performance. In this position, the sensor radiates a horizontally polarized signal.

  • The sensitivity control [SENS] should be set no higher than what is necessary for reliable detection. The optimal sensitivity setting for this installation ended up near the lower end of the pot's range.

  • Beware of nearby metallic objects moving in the wind. A bouncing strand of barbed wire on the rear side of the sensor caused false alarms until I secured it.

  • Installed at its initial location, the sensor was exquisitely sensitive to pedestrian motion, but was virtually blind to vehicle motion! Why? I soon learned that the sensor was discriminating against the speed of the targets' motion. As explained above, the IF amplifier discriminates against speeds above about 1.5 MPH. The fix was to relocate the sensor so as to place its axis at a less acute angle to the direction of travel. The angle was increased from about 10° to about 80°. See illustration below:


    The sensor "sees" the target's speed multiplied by the cosine of the angle between the target's moving direction and the axis of the module (see app note). At the new angle (80°), the ratio of actual speed to apparent speed is increased by a factor of about 5.7 [cos10°/cos80°] over the original angle, an improvement of nearly six-fold. Given the ±45° width of the sensor's beam, vehicles passing through the gate travelling at almost any speed now fall within range of the IF amplifier's frequency response and are easily detected.

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Schematics produced with DCCAD.


Parts

HB100                 -   Open Impulse I ordered two, got two, but one was Dead On Arrival. Ya takes your chances. Google "HB100"
   
IF Amplifier    -   All components were obtained from Futurlec. Cheap but slow (2-3 weeks).
    
Weather-tight Box -   Amazon (also has HB100)


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