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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.
- 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.
- 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.
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