- The 555 or 7555 timers will oscillate in a stable manner.
- However, if we add an external capacitive sensor it becomes possible to vary the oscillation frequency. In this circuit the square wave is integrated by the triple RC network, while lC2, used as a comparator (with a variable reference value), uses the changes in the integrated voltage to alternately make and break the relay. Thus when you move close to C the relay makes; if you remain stationary the relay breaks.
- By changing the existing frequency of the signal the average integrated value remains the same but, at the instant when the frequency is changed, a positive or negative voltage peak will appear due to the momentary change in the average waveform of the signal.
- Failing this the operation of the circuit would often be disrupted by false detections. Coarse l and fine adjustment is provided, using P1 and P2, to reduce the risk of incorrect switching. Note: The numbers in parentheses are the pins if an LM3l1 is used in place of the CA3130.
- Without this refinement the size of the sensitive plate must be such that the frequency of oscillation be at least , several kHz.
- It may seem a bit basic but it is a valid idea and it is worth ) looking at it in greater detail.
- Take a square wave signal with a given frequency and integrate it. This gives a stable continuous average voltage.
- To obtain better results you could take the signal after integration and differentiate between negative pulses (the frequency decreases as the value of C increases: when the sensor is approached) and positive pulses (the frequency increases again if the sensor is no longer affected) and compare them.
- This is the principle upon which our switch is based.
Wednesday, September 24, 2014
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