Switching ON

Since the Body Detector is intended to detect any and all body presence, an On/Off switch that is mounted close to the circuit could in some instances present a problem. At the same time, to include any delays in triggering might be self-defeating, since some applications will require instant triggering such as a "turnstile counter" or an anti-tamper alarm.

A key switch was thought to be the most obvious solution for switching off and may be located some distance from the circuit. This may be inserted in place of (in series with) S1a. Best of all, any delays in triggering should be included in the external circuit. The author mounted the on/off switch on the case for the purpose of neatness and easy setting up. In most applications, this did not cause the circuit to trigger when switching off. 

However, solder pins have provided for compensation capacitors (Cx) at poitions E8 and E9 on the oscillator board. Their insertion may be left until the circuit is complete and is found to be working satisfactorily. 

Preliminary Tests

Meaningful testing can only be carried out once the oscillator board has also been completed and connected up. For the time being, it may establish that regulator IC4 is supplying the correct voltage. Attach a 9v PP3 battery to the battery clip, switch S1 to any position other than Off and measure the voltage across capacitor C15. This should be close to 5v. The regulator i.c should remain fairly cool and supply current should not rise above 15mA.

If any specified components for the Body Detector cannot be sourced at this stage, it is important that equivalents should have low temperature coefficients particularly capacitors C1 and C4 which should if possible have a zero temperature coefficient. The multiturn  potentionmeter VR1 may be pricy. however, these devices may sometimes be obtained cheaply as surplus goods. Alternatively, use a cheap 470ohms or 1 kilohms potentiometer, although this will not offer the same high degree of precision when it comes to calibration. 

OSCILLATOR BOARD

Having completed the preliminary checks, we can now tackle the construction of board B, which includes the h.f.o.s, the l.f.o and monostable. We shall also be casing the unit and calibrating it. Taking the second piece of strip-board, again having 18 holes by 34 cooper strips, create the breaks in the underside copper tracks with with a drill bit or other appropriate tool. Details of the topside component layout together with the underside details. 

Solder in position the wire links and solder pins, then the dual-in-line sockets then the resistors and multiturn presets continuing with the capacitors. Be careful to observe the correct polarity of the electrolytic capacitors and the correct orientation of IC1 and IC2, when inserting them into their holders. Pin 1 of IC1 and IC2 lies close to the small indentation on one of their encapsulation. 

Next, prepare seven sheathed wires 15cm long and solder them to potentiometer VR1, the sensor solder tag and sections S1b and S1c of the mode switch. Finally attach the leads from VR1 to the solder pins on the topside of the strip-board, the wire from the sensor solder tag and then the eight colour-coded wires from board A. Jack socket SK2 is included for switching small external loads- a second jack socket may easily be added. solder pins have been provided for this purpose at the opposite side of relay RLA at board position R9 and R13 on board A. the specified relay is rated at 60W 250V a.c and would therefore also be capable of switching small a.c resistive loads.

Construction

The Body Detector is built up on two pieces of strip-board each having 18 holes by 34 cooper strips. We start construction with board A. This holds the regulated power supply, the digital mixers (IC3), the inverter and the relay.  Details of the topside component layout together with the underside details are shown in figure above. All components should fit into place without difficulty, provided that miniature radial capacitors are used. 

 

Commence construction by cutting a standard piece of strip-board down to size using a hacksaw. Create the breaks in the underside of the strip-board with a handheld drill bit or other appropriate tool. Solder in position the wire links and solder pins, then the dual-in-line socket, then the resistors, the relay and the diodes continuing with the capacitors, transistors and voltage regulator IC4. The polarity of the piezoelectric sounder WD1 is unimportant. 

Be careful to observe the correct polarity of the electrolytic capacitors and the correct orientation of the regulator, the transistor, diodes, L.E.D, relay and IC3. Pin 1 and IC3 lies close to a small indentation on one end of the encapsulation. The cathode (k) of L.E.D D1 has the shortest lead and the cathodes (k) of diodes D2 and D3 are banded.

Prepare seventeen sheathed wires 15cm long- eight of which are colour-coded. The colour-coded wires attach to the oscillator board (board B) later. Solder wires to Mode switch S1, power socket SK1 (power in), jack socket SK2 (out), L.E.D. D1 and two solder tags which each attach to a Test bolt as shown. Finally attach the leads from S1, SK1, SK2,D1 and the test bolts to the topside of the strip-board and connect the colour-coded wires to the solder pins as indicated in figure 7. 

Check that all the wire links and components are correctly in place. Check that the track breaks are all there and in the correct positions. and that there are no solder bridges on the board. The author routinely runs a thin, sharp screwdriver down between all the strip-board tracks.

Schematic Diagram of Body Detector

The full circuit diagram for the Body Detector is shown in figure above. IC3 is a CMOS 4520 dual binary counter, which is wired as a dual binary mixer. Many mixers in similar applications employ a charge pump to detect a difference frequency. However this tends to be an art as much as it it science. The 4520 dual binary counter enables precise digital detection, potentially to an accuracy of about 1Hz at frequency up to 5MHz.

Benchmark high frequency oscillator (h.f.o) IC2a clocks binary counter IC3a, while sensor-h.f.o. IC1a resets the counter at around the same frequency. These two inputs far from simply cancelling each other out, produce a waveform as in figure 6a, when a larger difference frequency is present and as in figure 6b when the difference frequency is close to the null point. It then remains merely to detect the troughs in the waveform when exceed a specific duration (e.g 50ms). This is accomplished through binary mixer IC3b.

The mixed signal (the difference frequency) from IC3a is fed to the reset pin 15 of binary mixer IC3b. The low frequency oscillator (i.f.o) IC1b feeds the clock input of binary mixer IC3b. The clock input is completely cancelled out by the reset pulses, unless the duration of the troughs at the reset pin falls below the frequency of the clock input. In this case, the clock pulses break through. With the component values shown, the frequency of the i.f.o is fixed at around 500Hz- that is 500Hz away from the null point. 

:: TIME DELAY

  

At this stage, the output of binary mixer IC3b at pin 12 is not particularly useful and first needs to be inverted before triggering monostable timer IC2b. This is accomplished with the help of transistor TR1. With the component values shown, monostable IC2b may be adjusted over a useful 150ms to more than 30seconds by means of preset VR4. If different timing periods are required, capacitor C12 may be altered accordingly. 

The output of monostable IC2b at pin 9 provides current for switching transistor TR2, which in turn controls relay RLA. A variety of miniature relays would be suitable here, provided that the norminal operating power does not exceed 500mW. Diode D2 suppresses back e-m-f when the circuit is broken.

A delay is provided at switch-on in the form of capacitor C11 and resistor R9. this arrangement produces a negative pulse for a few seconds at IC2b's reset pin, so that the user sufficient time to step out of range before the Body Detector is activated. The delay is reactivated in the Sleep position setting of rotary switch S1. 

Low dropout regulator IC4 is used to ensure a steady supply voltage. Any similar regulator may be used on condition that it is rated 150mA or higher. With the specified low dropout regulator, the unit's power consumption is typically 13mA or standby and up to 100mA when triggered. 

An alkaline PP3 battery should thus give two days continuous service. the battery option is provided mainly for freeing up the unit during testing, and for demonstration purpose. The option of an external d.c power  supply (7v to 26v) is included. The circuit is reverse-polarity protected through diode D3 although the regulator itself is virtually indestructible. 

Block Diagram

  Sensor H.F.O  IC 1a (ICM7556IPA)  

•  is an RC oscillator, so that when its metal sensor is approached, C increases and frequency drops, creating a frequency difference between the two H.F.O oscillators. 

•  reset the counter at around the same frequency.

 L.F.O IC1b (ICM7556IPA) 

• the low frequency oscillator.
• detect only the smallest different frequency. 
• feeds the clock input of binary mixer IC3b.  

 Benchmark H.F.O  IC2a (ICM7556IPA)   

• improve the circuit's stability, so that the unit has intelligent frequency compensation (as opposed to temperature compensation, which merely reacts to environmental conditions). 
• clock binary counter IC3a.    

 Two binary mixer, IC3a & IC3b (HCF4520BEY DUAL BINARY COUNTER) 

•  wired as a dual binary mixer in this circuit.
•   employ a charge pump to detect a difference frequency.
• will detect frequency variations to within a small fraction of one per cent. it has a high degree of accuracy as well as flexibility. 
• it could have a wide range of possible applications, such as a detection of body capacitance for this project.
•  these mix a signal from high frequency oscillator, IC2a with a benchmark frequency produced by IC2a

Inverter (TR1, 2N3904 npn low power transistor) 

•  the output of binary mixer IC3b at pin 12 is not particularly useful and first need to be inverted before triggering monostable timer IC2b. this is accomplished with the help of transistor TR1. 

Negative pulse at switch ON (C11 & R9) 

•  A delay is provided at switch ON in the form of capacitor C11 and resistor R9.
• This arrangement produces a negative pulse for a few seconds at IC2b's reset pin, so that the user has sufficient time to step out of range before the body detector is activated.  

Monostable and Relay, IC2b (ICM7556IPA low power dual timer)

•  the output of monostable IC2b at pin 9 provides current for switching transistor TR2, which in turn control to relay.
• monostable IC2b may be adjusted over a useful 150ms to more 30second by means of preset VR4. 

Circuit Application

Due to its high sensitivity and good stability, the Body Detector may be attached to a wide variety of metal objects in the process sensitizing the entire object concerned. Although in theory the Body Detector is dependent on the electric field which surrounds the human body, in effect it acts as though an invisible field were created around the object concerned similar to the "invisible" defence shields seen in the latest star wars movie.

From a practical point of view, the sensor may include any object from the size of a pin to about 70kg in weight (e.g lightweight motor-scooter). However the greater the weight of the metal sensor, the less the sensitivity of the circuit. the more critical the tuning and the more it becomes susceptible to temperature variations especially. 

If attached to lighter metal objects such a sheet of tin-foil, the Body Detector may be turned to detect a person's presence up to 80cm away. At several centimeters distance, the circuit is sufficiently stable to avoid spurious triggering over a wide temperature range. In one test,  bicycle was moved from shade to full sun and back into the shade during the course of a day, maintaining reliable triggering. In another test, a 300mm square of tin-foil was tested successfully without the need for readjustment between 10degree circus and 0 degree circus and would in fact have exceeded this. This compares very favorably with variations in room temperature which typically amount to no more than 10 degree circus.

Limitation of Body Detector

The limitation of Body Detector is : 

1) it can only detect a presence of human at up to 80cm away only.

2) the sensor may include any object from the size of a pin to about 70kg in weight only.

3) the greater the weight of the metal sensor, the less the sensitivity.

4) over a modest temperature range, e.g 10*C to 25*C. 

5) in a single application only.