I have a question on basic electronics. I'm looking at a very simple CMOS RC Circuit around a Schmitt Inverter which creates square waves. Something like this.


simulate this circuit – Schematic created using CircuitLab

Now the frequency of the square waves is a function of the value 1/(R1*C1). Hence the frequency is inversely related to the value of the resistance. High resistance = lower frequency

Is there any configuration of such an RC circuit where the relation is direct instead of inverse? Higher resistance = higher frequency? I was thinking along the lines of having a second resistance and getting to the ration of R1/(R2*C), but I can't figure out the circuit.

My use case for this is when R1 is replaced with a LDR. LDRs increase in resistance when light level goes down, but I would like the square wave frequency to increase instead when the light level goes down.


You could consider using a 555 timer circuit that uses a constant current source to charge the voltage up on the charge capacitor. The constant current source requires normally 3 resistors to set the current and one of these resistors, when increased will linearly (to a point) create greater current as its value increases. A greater constant current (higher resistance) means the capacitor charges more quickly hence, F is proportional to R.


Note that increasing R1 causes the charging current to be greater thus increasing the speed at which C1 charges and hence frequency rises. Circuit taken from here

  • \$\begingroup\$ Yes. Indeed. Unfortunately this is a design that's based around and RC Inverter design and I can't change the chip at this point of time. \$\endgroup\$ – notthetup Nov 10 '14 at 8:28
  • \$\begingroup\$ @notthetup then you are probably out of luck unless you are prepared to add an extra transistor to shunt-discharge the cap when the schmitt trigger output rises high but then you are basically constructing a 555 timer in hardware. \$\endgroup\$ – Andy aka Nov 10 '14 at 10:25
  • \$\begingroup\$ @notthetup: What can you change? \$\endgroup\$ – EM Fields Nov 10 '14 at 12:58
  • \$\begingroup\$ I can add/remove passives, and maybe transistors. I am able to rewire the inverter as well. I just need to stick with the inverter. \$\endgroup\$ – notthetup Nov 10 '14 at 14:01

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