Variable frequency square wave (need help)

[jrz126]

I need some help with a (hoopefully) simple circuit for the leds I have mounted in my roof of my car. Heres alittle background first...
The leds are controlled by 8 bit shift registers with transistor arrays connected to the 8 outputs to provide the high current. I then control the shift in bit and clock bit using a couple 4017 Counters (used as a frequency divider). The clocks of the counters are controlled by a LM3915 VU meter IC which is connected to my subwoofer amp. So the LEDs change with the music.

I'm looking for a circuit that has a 5VDC square wave output. which changes frequency over time. (ie. it starts running at say 10 Hz then it increases slowly to say 50 Hz. over a period of 15 seconds, then it goes back to 10Hz and repeats).

I know a PIC would be excellent for this job, but I dont have any idea how to program one. (I'll be taking a microprocessors class next fall though ). So right now I'm hoping for a simple solution.
If you need any more information or have any extra questions please ask. I'd greatly appreciate the help.

 

[audioguru]

Use an LM331 VCO with its frequency controlled by the rising and falling voltage of the timing cap of a very low frequency of an astable 555, or the ramping voltage output of a very low frequency triangle-wave generator.

 

[Dr.EM]

I've got something similar going. I'm using a 555 as an oscillator, and by connecting the voltage control pin to a triangle wave generator (made using a pair of op-amps as a ramp generator and schmitt trigger in a loop). I'm getting rising a falling frequencies, but if you used bigger caps and resistor, i'm sure it'd do what you need it to.

 

[audioguru]

Hi Dr. EM,
I haven't tried it. Does a 555 change its frequency 5 :1 with its voltage control pin?

 

[Dr.EM]

Oh dear, maths :lol:

I'm sure you could make it anyhow, by varying the amplitude of the triangle wave generator, which is easily done with a pot.