RF Sensing Alarm

RF sensing alarm is a device that would alert when it detects a continuous RF transmission that lasts more than 5 minutes. The device would have to be broadband (HF/VHF/UHF), be sensitive enough to detect a 5W transmission from inside the shack using a telescopic antenna, and produce a sound loud enough to alert me anywhere in the house.

The RF sensing alarm would also have to be self-contained, which means without any hookups to my radios. After a bit of reading and thinking, I came up with a solution that meets all the initial objectives. Here's circuit in detail.


The circuit shown above may look scary for some of you, but it is not. It can be broken down into four stages. Let's look at them one at a time. The first stage acts as a RF sensor circuit. It is made of U1C, one of the four operational amplifiers of a LM324 chip, and its associated input circuitry. U1C is used as a voltage comparator. Note that the two U1C inputs (plus and minus) have similar DC circuits connected to them. The plus input has R7, R8 and D3, and the minus input has R6, R10 and D5. In these two circuits, D3 and D4 are partly biased (about 200 mV of forward voltage) in order to better exploit the variation of voltage versus current that the diode produces. This translates into increased RF sensitivity.

In an idle condition (no RF detected), potentiometer R10 is set to make the voltage at the minus input of U1C slightly lower then the one at its plus input. This keeps the output of comparator U1C saturated to the "high" state (near supply V+). A strong RF signal present at the antenna terminal J1 reaches D3, a schottky diode, through C3, a coupling capacitor. The diode rectifies this signal and generates a drop of voltage at D3 anode. This makes the voltage at the plus input of U1C fall below the one set at the minus input. As a result, the output of comparator U1C flips down to the "low" state (near ground). In this state, the circuit is in RF detection mode and timer U1B is activated. Note that as soon as the RF signal disappears, the comparator immediately returns to its high state, resetting the entire alarm circuit. Ferrite bead FB1 and resistor R8 are used to block the RF from reaching comparator U1C. Additional RF decoupling is provided by capacitors C2 and C5.

The second stage acts as a timer. It is made of U1B, D4, C4 and R9. U1B is again used as a voltage comparator. When no RF is detected, capacitor C4 is kept charged by the "high" state of U1C. When RF is detected by the first stage, capacitor C4 is left "floating" and starts to slowly discharge through R9. When C4 is sufficiently discharged, after approximately 5 minutes, the voltage at the plus input of U1B falls below the one at the minus input which is set by D5 (about 200 mVDC). As a result, the output of comparator U1C flips down to the "low" state (near ground). In this state, the alarm is tripped. The timer stage drives piezo oscillator U1A and PNP transistor Q1. The latter is added to drive an external device with a voltage close to V+ when the alarm is tripped. The two diodes in series with the emitter provide a drop of voltage so that the base-emitter junction of Q1 is not biased when U1B output is in "high" state. The 2N3906 or equivalent transistor will safely supply a current of up to 50 mA.

The third stage is an astable multivibrator (square wave oscillator) and is made of U1A and surrounding components. It drives the piezo vibrator to produce a loud high-pitch sound. The oscillator circuit will operate only when the output of U1B presents a "low" state. Diode D2 serves as an isolating device between the two stages when U1B is in "low" state. The oscillator's frequency is set by capacitor C1 and resistor R5. The values chosen make the circuit oscillate at approximately 2600 Hz, a frequency that causes the piezo to generate the loudest sound.

The last stage is a simple buffer U1D that sinks current to turn on LED D1 whenever a RF signal is sensed by U1C. R1 limits the current to a safe level for the buffer, less than 20 mA.

The balanced input configuration of comparator U1C allows the supply voltage V+ to vary over a wide range and the entire circuit will still work. I designed the circuit for a 13.8 V supply, but I verified that the circuit works down to approximately 10 V without recalibrating it, and down to approximately 5 V if recalibrated. The high end of the range is more delicate to set since devices such as the LED, the piezo vibrator and polarized capacitors may be overstressed if component value changes are not made. Consequently, I would not recommend going beyond 20 V.

Part Lists

C1 ------ .01uF, ceramic
C2 ------ 1000pF, ceramic
C3 ------ 1000pF, ceramic
C5 ------ 1000pF, ceramic
C4 ------ 100uF, 25V, tantalum or electrolytic (see text)
C6 ------ 10uF, 25V tantalum or electrolytic
C7 ------ .1uF, ceramic
D1 ------ LED, any color
D2 ------ 1N4148 or equiv.
D3 ------ 1N5711, schottky
D4 ------ 1N4148 or equiv.
D5 ------ 1N5711, schottky
D6 ------ 1N4148 or equiv.
D7 ------ 1N4148 or equiv.
FB1 ------ Ferrite bead
J1 ------ BNC, Female panel mount
Q1 ------ 2N3906, PNP or equiv.
R1 ------ 100K
R2 ------ 100K
R4 ------ 100K
R3 ------ 1K
R5 ------ 10K
R6 ------ 1M
R7 ------ 1M
R8 ------ 27k
R9 ------ 470K
R10 ------ 50K, 10-turn pot., Bourns 3006 type
R11 ------ 22K
U1 ------ LM324 Quad Op. Amp.
X1 ------ Piezo, muRata PKM-11 or Radio Shack 273-73 or equiv. External drive type.
Socket --- 14-pin DIP
Coax --- Short piece of RG-174 or equiv.
Box --- Hammond 1591A suggested
Ant --- Right-angle BNC, telescopic

This project is relatively inexpensive to assemble. If all the components are purchased, it should cost less than $25 to build it. Obviously, your junkbox's size will dictate the cost. I re-used components taken from old PCBs. This allowed me to build the project for less than five dollars, including PCB and box.

I admit, it is a rather compact design. The intent here is to make the design fit in a very common Hammond 1591A plastic box. Note that the circuit can be assembled using other techniques: universal PCB, veroboard, deadbug, wire-wrap and even a combination of these techniques. The layout is not critical except for the RF portion of the circuit: Components D3, C3, FB1 and R8 should be mounted as close of possible to each other using very short leads. This will guarantee proper operation up into the UHF spectrum. Also, decoupling capacitors C2 and C5 should be mounted as close as possible to pins 9 and 10 of U1C.

When assembling the PCB, use a soldering iron with a fine tip. Start by installing the LM324 IC. This is the hardest component to solder because of the pin spacing and overall PCB component density. An IC socket is desirable since the LM324 is rather sensitive to pin shorting compared to other operational amplifiers I've used in the past. With a socket, replacing it is a snap. Make sure you solder all components on the two PCB sides. Many component leads are used as vias to jump from top to bottom layers. Soldering all components this way guarantees you a functional circuit at the end. It's happened to me too often to forget to solder a component on the component side of the PCB.

Ferrite bead FB1 is inserted over a short piece of solid wire and the wire is soldered to the PCB pads. Potentiometer R10 is located on the PCB edge so that its adjustment screw can be accessed through a small hole drilled on one of the faces of the box. I've chosen not to put connectors for external connections to the LED, the piezo vibrator, supply line and the auxiliary output. Solder small gauge (#26 or smaller) wires directly to the PCB pads and connect the other ends to the external devices.

For antenna connection, I recommend using a female panel-mount BNC connector. It is small, reliable and will accept most right-angled telescopic antennas when mounted horizontally. Other RF connectors can be used if desired. Since the input impedance of the circuit is quite high, maintaining a constant impedance through the connectors is not an issue. In general though, it is good practice to avoid using UHF connectors (PL-259/SO-239) when working above 200 MHz, since they are not of constant impedance type. For coaxial cable, a short piece of RG-174 type or equivalent is preferred to limit overstressing of the PCB pads. This type of coax is much easier to route in a small box anyway. Solder the bare ends directly to the pads.