LM555 as variable frequency oscillator

**xyz9915** · 06-16-2009, 01:20 AM

Hello!
I need a circuit in which the frequency of LM555 astable multivibrator can be controlled externally I mean by the help of transistor etc. So, please guide what modification may be done in the attached circuit

**Curiosity** · 06-16-2009, 01:48 AM

What's the ultimate goal?
What will it control?
What will control it?
A set of predefined speeds or variable?

**xyz9915** · 06-16-2009, 09:57 AM

Actually I want to control the frequency by the help of R2R (ladder network), where the digital input value will increase/decrease the DC voltage of R2R network, so the frequency may be varied from 1Hz to 5Khz (i.e. My desired frequency). So what should I do?

**Curiosity** · 06-16-2009, 01:07 PM

That's a pretty wide range. Tone generator? You could always use a potentiometer for manual selection or digital pot for some other input. For momentary R2R, do it like steering wheel controls (each pushbutton and resistor in series). You probably already thought of that. Switching in sequence, maybe a NAND gate (google LED chaser circuit).