From: davidr@pobox.com (Dave Rahardja)
> Okay. I've made a self-oscillating Schmitt Trigger rig like so:
[deleted]
> Because of the inherent capacitances of the transistors, this
> circuit naturally generates a nice 1.4V+ pulse wave output at almost
> exactly 1MHz. Now, is there any way that I can scale the frequency
> down to about 1kHz? I've tried caps and inductors, etc, etc, but no
> success! Any suggestions?
> Is there a way to delay the feedback from the output transistor?
>Inductors don't work! (at least they didn't for me)
You need some more resistors to set the upper and lower trigger levels.
In this circuit if R1=R2=R3 the trigger levels will be about 1/3 and
2/3 of the supply. The other resistors need to be about 1/10 or less
of R1. The frequency is about Rf*C and the symmetry will depend on the
gain of the NPN pair (capacitor supplies base current on discharge but
not charge). You can adjust the symmetry with another resistor and diode
in parallel with Rf if you need an exact square wave.
+V
+--------+------+-----------+
| | | |
| \ \ |
| / / R1 |
| \ \ E
| | | |/
| +--------------B| PNP
| | | |\
C C | C
|/ \| | R2 |
+-----+--B| NPN |B- +---\/\/\---+-----+---> out
| | |\ /| | | |
--- | E E \ \ |
C --- | ----+--- / / |
| | | \ R3 \ |
| | \ | | |
| | / | | |
| | \ | | |
| | | | | |
+---- | ----------+----------+-----------+ |
| | Rf |
Gnd +--------------------\/\/\---------------+
Bill
Date: 29 Feb 1996 21:23:41 -0500
Original Subject: Re: Schmitt Trigger Oscillator help!