Hi
I am currently searching for an resonant circuit to act as a signal conditioner for a capacitive humidity sensor. For starters i started playing with the all time favorite 555 timer (specifically the cmos intersil 7555 ) in astable mode. However one of the requirements of my project is 50% duty cycle output square pulses. I used the circuit with the single resistor in order to achieve that and apart from the fact that i didn't get exactly 50% duty cycle (48%), stray capacitances of other circuits connected in the output resulted in a change of frequency, thus errors in my measurement.
Getting to the point: i understand that the basic circuit for astable operation of the 555 requires 2 resistors (R1,R2) and a capacitor. The capacitor charges through R1,R2, discharges through R1 and the threshold and trigger pins manage the output based on the capacitor voltage. My first thought after understanding this was why couldn't we make our lives simpler by connecting a resistor right after discharge pin and get a high output calculated by ln(2)R1C and low by ln(2)R2C. Intuitively, by setting R1=R2=R i can get a 50% duty cycle without adding circuitry to the output. Anyway, i build the attached () circuit but the result was a failure. It cannot produce any pulses at all
My questions:
1) What is wrong with the attached circuit? (which seems allright to me)
2) Can anyone propose a low power (<1mA@2V or less) signal conditioner for capacitive sensors with frequency output (preferably above 20-30Khz)?
Thank you in advance
Lefteris
I am currently searching for an resonant circuit to act as a signal conditioner for a capacitive humidity sensor. For starters i started playing with the all time favorite 555 timer (specifically the cmos intersil 7555 ) in astable mode. However one of the requirements of my project is 50% duty cycle output square pulses. I used the circuit with the single resistor in order to achieve that and apart from the fact that i didn't get exactly 50% duty cycle (48%), stray capacitances of other circuits connected in the output resulted in a change of frequency, thus errors in my measurement.
Getting to the point: i understand that the basic circuit for astable operation of the 555 requires 2 resistors (R1,R2) and a capacitor. The capacitor charges through R1,R2, discharges through R1 and the threshold and trigger pins manage the output based on the capacitor voltage. My first thought after understanding this was why couldn't we make our lives simpler by connecting a resistor right after discharge pin and get a high output calculated by ln(2)R1C and low by ln(2)R2C. Intuitively, by setting R1=R2=R i can get a 50% duty cycle without adding circuitry to the output. Anyway, i build the attached () circuit but the result was a failure. It cannot produce any pulses at all
My questions:
1) What is wrong with the attached circuit? (which seems allright to me)
2) Can anyone propose a low power (<1mA@2V or less) signal conditioner for capacitive sensors with frequency output (preferably above 20-30Khz)?
Thank you in advance
Lefteris