Variable frequency 555 circuit

[jonj44]

Hello all,

I seem to recall having a schematic for one roughly 20 years ago, but there have been several hard drive crashes and at least one plumbing "flood" since then.. and I can't seem to re-locate it online.

My goal is to externally trigger square wave pulses (of variable frequency say from 20-22000 Hz) with a 555 timer circuit using a function generator/laptop/tablet/etc. Most of the schematics I have found show 555 pins TRIG 2 & 6 joined together for astable square waves.

Is this an internal IC connection or just a common way of externally wiring the 555 for astable mode?

Ideally, I would like to keep my output voltage fairly close to the maximum 15 VDC input (and I'm wondering how duty cycle would impact this goal). Also, I am looking to software control a 555 square wave oscillator, not "set" a hardware output frequency.

 

[Ramussons]

Does something here help?

‘555’ Astable Circuits

Use the 555 timer IC in a variety of useful applications.

www.nutsvolts.com

 

[rjenkinsgb]

Not really practical, sorry.

The 555 is basically two comparators and an SR latch. The output toggles low when the high threshold input is reached, and high when the input drops below the low threshold.

The charge/discharge time to reach the high or low thresholds is totally dependant on the external components.

The only internal "variable" is by changing the bias voltage on the resistor divider that the comparators use for the thresholds; the upper comparator point in the divider is brought out on pin 5.

That allows some change in frequency but at a guess possibly 4:1 or so, not 1000:1
The control voltage is not linear.

555 internal schematic:

To actually use a 555 over that 1000:1 range, you would need to select different external components using a bank of analog switches, each giving that 4:1 range, so six or seven ranges, each range individually calibrated and with logic to switch between them. It's very unlikely it would ever be long term stable, even if you could get it to work initially.


To get the type of frequency range with complex external switching using analog parts takes such as a purpose made voltage controlled oscillator or such as the VCO designs for modular music synthesisers.


A single MCU could do the whole lot far more accurately with minimal components.

And, as you want to control it via an internet connection, you would need an MCU to do that. The audio could be generated directly in the same device.