THEORY OF OPERATION
___Two components stand out that allow the
RF-Vibe circuit to be concise and effective. They are the
universal operational amplifier, LM741, labeled as IC1 and the
small inexpensive dc motor which we shall fabricate into
thevibrator. The 741 op amp is known as the 'workhorse of
industry' because it can be configured into many different
functions by simply rearranging a few external components and the
manner in which they are connected to the device pins. For our
purposes, the op amp is setup in a comparator-schmitt trigger
configuration which forms the r-f detector part of the circuit.
The 741 incorporates balance(null) circuitry that ensures equal
conduction of the inherent differential input amplifiers. It
allows the use of a balance potentiometer to bias the chip so
that you get 'zero out' for a 'zero in' condition. In our case,
we utilize the balance pot, R6, as a "sensitivity
adjustment" to lower the threshold to the trip point, so
that we are able to detect a relatively small signal induced into
the antenna wire. In fact, the RF-V has responded to as little as
20mW at six feet away from the antenna lead. By adjusting the
sensitivity, you can use the RF-V as a sniffer to pinpoint hidden
___Adding some resonant components would make the detector frequency selective. We could install preamp circuitry which would permit us to hear the r-f generated by an angry gnat rubbing its hind legs together. People have suggested many desirable add-ons, but increasing complexity only adds to the cost and size; both of which negate the intent of this design. I could provide you with a design that is 1/2 the size of the RF-V case, limits detection to the VHF & UHF bands, and needs no external antenna lead. If you're interested, send me a cashiers check in the amount of $1200 and be willing to sign a proprietary agreement. You see folks, in a phrase, "it's all economics." Believe me when I tell you that there are not that many engineers in this country who are willing to create economical designs for public consumption. They are able to glean the maximum yield from simplistic circuits and comprise a design which rivals the performance of many high priced commercial units. This is done for the shear satisfaction of building the better mousetrap. For me, there are not enough buyers to support even a part-time endeavor in this work. My reward is in the knowing that I have provided a quality plan to someone whom can replicate the device and who realizes its worth. Maybe it will spark an interest, and the builder will continue the tradition. Well, enough of this noise. Thank you for your forbearance. Now, please step back while I jump off of this 'soap box'.
___Detailed Description: IC1 is fundamentally used as a comparator. With the addition of R2, positive feedback is applied to non-inverting input pin 3. R1 drops the feedback level and then applies it to the inverting input pin 2. This sets up a span range between inputs and converts the circuit function into a schmitt trigger. R6 null potentiometer is initially adjusted to unbalance the input bias currents such that IC1 trips on, placing output pin 6 low. This in turn will cause Q1 to conduct, thereby energizing LED1 and the vibe motor. Now R6 is 'backed-off' until IC1 just resets (with the LED and motor off). At this setting of R6, the input amplifiers are unequally conducting with the non-inverting input (pin 3) being slightly more positive than the inverting input (pin 2). This is the reset state, with output pin 6 being high, thus keeping Q1 turned off. As you gingerly move R6 closer and closer to the trip-point level, the imbalance between the input bias currents is lessened. Essentially this increases the 'sensitivity', since now less input signal is required to cause pin 2 to become slightly more positive than pin 3, which then sends output pin 6 low, again turning on Q1. Realistically R6 acts as an offset pot which narrows the margin to trip-on IC1.
___In summary; when an input r-f signal is induced into the antenna lead wire, the level is limited by D1/D2 to keep from saturating the inputs. C1 has low impedance to the r-f signal and couples it to the op amp inputs. It is applied directly to pin 2, but delayed to pin 3 by the time constant effect of R1/C2. On the positive alternations, the r-f level aids the voltage at pin 2 such that IC1 trips-on. As long as the r-f is present with sufficient amplitude, the op amp remains in the set state. When the r-f level drops or disappears, then the established balance currents return the op amp to the reset condition.
___R3, R4, and R5 are current limiters. D3 ensures that the vibe motor doesn't conduct any armature current until Q1 turns on enough to forward bias D3. This precludes the battery from being drained by the op amp responding to low-level noise or R6 settings that may cause the op amp output to allow Q1 to weakly turn on. When this happens, the low impedance path through the dc motor would prematurely kill the battery. We wouldn't realize this occurring because the current flow wouldn't be enough to illuminate the LED or cause motor rotation. So you see, some things aren't intuitively obvious to the casual observer.
___Of course you can shorten the antenna lead. It doesn't affect circuit design parameters in any manner. It's just that you can "trap more bugs with a larger net". The longer the antenna wire, the more apt you are to detect low power transmitters. 'Mr. Businessman' usually puts the RF-vibe in his vest pocket with the antenna lead running down inside the pants leg. Sometimes paranoia sets in when a faulty lighting ballast or a r-f belching computer system triggers the detector. Most often, you can readjust the sensitivity in successive increments and walk right to the source. It's quite a feeling of empowerment when you discover that the signal is emanating from a live 'body'. One firm reported that the little RF-vibe saved them from making a costly mistake during high power negotiations.
___In some RF-V units, the dc vibe motor commutator produces enough r-f chatter to keep the op amp in the set state once it's triggered. If this occurs, R6 is then adjusted to re-set everything. The inherent hysteresis and pseudo-sensitivity are affected by varying the values of R1, R2, and C2. You can experiment and 'prune' to meet your specific needs, but the circuit depicted in this plan will give you an overall reliable operation.
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