Hello,
First time posting on the forums, and am hoping I can get some design help for a pretty straight forward 555-based circuit, but I have some tight constraints on how I need it to operate.
The goal of the intended circuit is to use a Flexiforce pressure sensor to modulate the frequency of a 50% duty (t1 = t2) 555 pulse generator. I hope to record the output with the line-input of a soundcard, and so I need to DC offset the output to +/- 0.5. Basically, I want to generate a frequency modulated signal that can be recorded by a computer sound card where frequency will scale with the force applied to the sensor.
Moreover, I need to get a decent estimate of force applied to the sensor at a timescale with approximately 1ms resolution, so the 555 frequency needs to be >2kHz... I'd like to have it scale between 8kHz and 24kHz to avoid any sampling problems. According to the Nyquist theorem, this gives me a padding factor of 4 orders of magnitude for resolution at the low end, and high end (I will sample the signal at 96kHz with the sound card).
As I understand it, I can eliminate R1 because t1=t2 for an equal high/low time. So, the resistivity across R2 has to be modulated by the force on the force sensor. The flexiforce sensors have >5MOhm at rest, and when pressing as hard as I can on the sensor it reduces to 2MOhm. This is obviously not compatible with the requirements for R2 resistivity in a 555 circuit. By my calculations, with a 0.1uF capacitor, to get an output between 8kHz and 24kHz, I need to modulate R2 value between 600Ohm and 1.8kOhm.
Question 1: How can I use resistivity of the flexiforce sensor to scale resistivity across another component?
I'm assuming that using the flexiforce sensor to modulate current flow though Base to Emitter of a transistor could potentially modulate the resistance across the Collector to Emitter. So, I presume the solution would be replacing R2 with the Collector/Emitter ends of a transistor. but how do I do the calculations required to keep this between my goal of 600 and 1800 Ohms? Maybe I'm way off entirely with this idea though.
Furthermore, what would be the best way to offset the output to +/- 0.5V. My device needs to be portable, and will use a 9V batter and 5V regulator. I have some negative charge pumps to generate -5V, but if I can avoid this, and instead use a virtual ground (0-Rx-2.5v-Rx-5v) I'd rather this.
Any suggestions? Thanks in advance.
Brian
First time posting on the forums, and am hoping I can get some design help for a pretty straight forward 555-based circuit, but I have some tight constraints on how I need it to operate.
The goal of the intended circuit is to use a Flexiforce pressure sensor to modulate the frequency of a 50% duty (t1 = t2) 555 pulse generator. I hope to record the output with the line-input of a soundcard, and so I need to DC offset the output to +/- 0.5. Basically, I want to generate a frequency modulated signal that can be recorded by a computer sound card where frequency will scale with the force applied to the sensor.
Moreover, I need to get a decent estimate of force applied to the sensor at a timescale with approximately 1ms resolution, so the 555 frequency needs to be >2kHz... I'd like to have it scale between 8kHz and 24kHz to avoid any sampling problems. According to the Nyquist theorem, this gives me a padding factor of 4 orders of magnitude for resolution at the low end, and high end (I will sample the signal at 96kHz with the sound card).
As I understand it, I can eliminate R1 because t1=t2 for an equal high/low time. So, the resistivity across R2 has to be modulated by the force on the force sensor. The flexiforce sensors have >5MOhm at rest, and when pressing as hard as I can on the sensor it reduces to 2MOhm. This is obviously not compatible with the requirements for R2 resistivity in a 555 circuit. By my calculations, with a 0.1uF capacitor, to get an output between 8kHz and 24kHz, I need to modulate R2 value between 600Ohm and 1.8kOhm.
Question 1: How can I use resistivity of the flexiforce sensor to scale resistivity across another component?
I'm assuming that using the flexiforce sensor to modulate current flow though Base to Emitter of a transistor could potentially modulate the resistance across the Collector to Emitter. So, I presume the solution would be replacing R2 with the Collector/Emitter ends of a transistor. but how do I do the calculations required to keep this between my goal of 600 and 1800 Ohms? Maybe I'm way off entirely with this idea though.
Furthermore, what would be the best way to offset the output to +/- 0.5V. My device needs to be portable, and will use a 9V batter and 5V regulator. I have some negative charge pumps to generate -5V, but if I can avoid this, and instead use a virtual ground (0-Rx-2.5v-Rx-5v) I'd rather this.
Any suggestions? Thanks in advance.
Brian