Extended tuning range for RC-oscillators like the 555
Increase potentiometer frequency control span with a factor of 10 or more by wiring it up differently. This approach can also be used to extend the range of a pulse width/delay control. Example circuits are shown. Advantage compared to standard circuits: The increased span adds lower frequency range without sacrifying control resolution at the high end.
Limitation of the standard approach:
The oscillator frequency is set by the RC value. In standard circuits the R is made variable using a potentiometer in series with a smaller fixed resistor that limits the loading of the chip used. Drawback is that when trying to create a large span like 1:100 you run into problems. Example: a potmeter of 100k and a series resistor of 1k. Then the highest frequency is already lowered by 50% when lowering the potmeter by just 1% which makes it hardly usable.
Improvement by the new approach: (see circuits and demo movie):
Instead of using the potmeter as a 2-wire varable resistor, it is used as a 3-terminal device that both changes resistance and fades over to an additional larger capacitor connected at it's high resistance endpoint. This way the frequency tuning range can be enlarged by a factor of 10 or more towards lower frequencies without loosing resolution at the high side of the frequency range. For example: if we use the potmeter 100k again we can add a 10k series resistor, a 10C added endpoint capacitor, and get the 1:100 range. In this case the highest frequency is lowered by 50% only after lowering the potmeter by 10%.
Circuit examples:
a 555 oscillator with a 30Hz to 7kHz potmeter tuning range (see and listen in demo movie)
a PSPICE simulation of a schmitt-trigger inverter oscillator with potmetersettings for 1:1000 tuning range.
Remarks:
-In this 555 circuit the discharge pin is not used so it could have other functions. In the demo movie it drives an 8 ohm speaker with 680ohm series resistor connected to the supply.
-Note that the voltage on the capacitor is no longer a triangular wave anymore for the whole tuning range. This is no problem when making square wave/pulse signals, only if you need this triangular wave also.
-In theory, using a anti-log potmeter could also improve the standard circuit but these are very hard to find.
The oscillator frequency is set by the RC value. In standard circuits the R is made variable using a potentiometer in series with a smaller fixed resistor that limits the loading of the chip used. Drawback is that when trying to create a large span like 1:100 you run into problems. Example: a potmeter of 100k and a series resistor of 1k. Then the highest frequency is already lowered by 50% when lowering the potmeter by just 1% which makes it hardly usable.
Improvement by the new approach: (see circuits and demo movie):
Instead of using the potmeter as a 2-wire varable resistor, it is used as a 3-terminal device that both changes resistance and fades over to an additional larger capacitor connected at it's high resistance endpoint. This way the frequency tuning range can be enlarged by a factor of 10 or more towards lower frequencies without loosing resolution at the high side of the frequency range. For example: if we use the potmeter 100k again we can add a 10k series resistor, a 10C added endpoint capacitor, and get the 1:100 range. In this case the highest frequency is lowered by 50% only after lowering the potmeter by 10%.
Circuit examples:
a 555 oscillator with a 30Hz to 7kHz potmeter tuning range (see and listen in demo movie)
a PSPICE simulation of a schmitt-trigger inverter oscillator with potmetersettings for 1:1000 tuning range.
Remarks:
-In this 555 circuit the discharge pin is not used so it could have other functions. In the demo movie it drives an 8 ohm speaker with 680ohm series resistor connected to the supply.
-Note that the voltage on the capacitor is no longer a triangular wave anymore for the whole tuning range. This is no problem when making square wave/pulse signals, only if you need this triangular wave also.
-In theory, using a anti-log potmeter could also improve the standard circuit but these are very hard to find.
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