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.