SgtWookie
01-24-2008, 03:52 AM
There have been a large number of requests for an easy-to-build PWM control circuit. You can always use a 555 or 556 timer circuit, but here's something you might build out of components from your "junk box."
Attached is a schematic that uses a CMOS 4093 Schmitt input NAND gate in conjunction with a potentiometer, 1 resistor, a cap and a couple of 1N914 diodes to create a roughly 2% to 98% pulsewidth duration, at around 1kHz. You could use any CMOS gate that has a Schmitt input and an inverting output. You can use just about any general purpose silicon diodes instead of the 1N914, as long as they have a PIV greater than 10V and current of more than 3mA (NOT Zener diodes). The circuit should work with Vdd from about +5v to +16v (if using power MOSFETS, you'll likely need to use a Vdd of at least 10V to fully turn on the gate; check your datasheet to be certain.)
Select R2 to limit the maximum current in the RC network to around 2mA; this will depend upon your Vdd. Multiply your Vdd times 500; that's the minimum resistance you need in Ohms.
With the components shown, center frequency is near 1kHz. Decreasing C1 and/or R1 will increase the frequency. It's best to reduce C1, as reducing R1 will adversely affect the PW control at the extreme limits.
Increasing R1 much above 500k Ohms will likely result in stability problems. To lower the freqency, increase C1 instead.
If you decide to use something other than a FET/MOSFET for the output driver, be sure to use a 10K current limiting resistor to prevent overloading the output of the gate.
And if you need more PWM circuits - there are three more gates in that 4093 that could be used for three more such circuits. :)
"Quickie" fader:
Connect the output of U1A to both inputs of one of the spare 4093 gates (after removing the grounds, of course) and add another MOSFET to the output of that gate. You now have a complimentary PWM driver by adding only one physical part; the MOSFET. Since the 4093 NAND gate inverts the signal, when one MOSFET is turning ON, the other will be turning OFF, and vice versa. If you were using the MOSFETS to control one or more LEDs, those controlled by one of the MOSFETS would increase in brightness while those controlled by the other MOSFET would grow dim.
Attached is a schematic that uses a CMOS 4093 Schmitt input NAND gate in conjunction with a potentiometer, 1 resistor, a cap and a couple of 1N914 diodes to create a roughly 2% to 98% pulsewidth duration, at around 1kHz. You could use any CMOS gate that has a Schmitt input and an inverting output. You can use just about any general purpose silicon diodes instead of the 1N914, as long as they have a PIV greater than 10V and current of more than 3mA (NOT Zener diodes). The circuit should work with Vdd from about +5v to +16v (if using power MOSFETS, you'll likely need to use a Vdd of at least 10V to fully turn on the gate; check your datasheet to be certain.)
Select R2 to limit the maximum current in the RC network to around 2mA; this will depend upon your Vdd. Multiply your Vdd times 500; that's the minimum resistance you need in Ohms.
With the components shown, center frequency is near 1kHz. Decreasing C1 and/or R1 will increase the frequency. It's best to reduce C1, as reducing R1 will adversely affect the PW control at the extreme limits.
Increasing R1 much above 500k Ohms will likely result in stability problems. To lower the freqency, increase C1 instead.
If you decide to use something other than a FET/MOSFET for the output driver, be sure to use a 10K current limiting resistor to prevent overloading the output of the gate.
And if you need more PWM circuits - there are three more gates in that 4093 that could be used for three more such circuits. :)
"Quickie" fader:
Connect the output of U1A to both inputs of one of the spare 4093 gates (after removing the grounds, of course) and add another MOSFET to the output of that gate. You now have a complimentary PWM driver by adding only one physical part; the MOSFET. Since the 4093 NAND gate inverts the signal, when one MOSFET is turning ON, the other will be turning OFF, and vice versa. If you were using the MOSFETS to control one or more LEDs, those controlled by one of the MOSFETS would increase in brightness while those controlled by the other MOSFET would grow dim.