Pulse Width Modulation DC Motor Control

Often, people attempt to control DC motors with a variable resistor or variable resistor connected to a transistor. While the latter approach works well, it generates heat and hence wastes power. This simple pulse width modulation DC motor control eliminates these problems. It controls the motor speed by driving the motor with short pulses. These pulses vary in duration to change the speed of the motor. The longer the pulses, the faster the motor turns, and vice versa.

Parts

• R1 1 1 Meg 1/4W Resistor
• R2 1 100K Pot
• C1 1 0.1uF 25V Ceramic Disc Capacitor
• C2 1 0.01uF 25V Ceramic Disc Capacitor
• Q1 1 IRF511 MOSFET IRF620
• U1 1 4011 CMOS NAND Gate
• S1 1 DPDT Switch
• M1 1 Motor (See Notes)
• MISC 1 Case, Board, Heatsink, Knob For R2, Socket For U1

Notes

• R2 adjusts the speed of the oscillator and therefore the speed of M1.
• M1 can be any DC motor that operates from 6V and does not draw more than the maximum current of Q1. The voltage can be increased by connecting the higher voltage to the switch instead of the 6V that powers the oscillator. Be sure not to exceed the power rating of Q1 if you do this.
• Q1 will need a heatsink.
• Q1 in the parts list can handle a maximum of 5A. Use the IRF620 for 6A, if you need any higher.
• This circuit is not a true pulse width modulation control. Because only the frequency of pulses varies, it is really pulse frequency modulation. This works, though not as well as true PWM.

1KHz Sine wave Generator

Simple circuitry, low distortion, battery operated
Variable, low impedance output up to 1V R

Circuit description:

This circuit generates a good 1KHz sinewave adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. A small filament bulb ensures a stable long term output amplitude waveform. Useful to test the Precision Audio Millivoltmeter, Three-Level Audio Power Indicator and other audio circuits posted to this website.


Parts:

• R1____________5K6 1/4W Resistor
• R2____________1K8 1/4W Resistor
• R3,R4________15K 1/4W Resistors
• R5__________500R 1/2W Trimmer Cermet
• R6__________330R 1/4W Resistor
• R7__________470R Linear Potentiometer
• C1,C2________10nF 63V Polyester Capacitors
• C3__________100µF 25V Electrolytic Capacitor
• C4__________470nF 63V Polyester Capacitor
• Q1,Q2_______BC238 25V 100mA NPN Transistors
• LP1___________12V 40mA Filament Lamp Bulb (See Notes)
• J1__________Phono chassis Socket
• SW1__________SPST Slider Switch
• B1_____________9V PP3
• Clip for 9V PP3 Battery

Notes:

• The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion.
• Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one.
• Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation.
• Set R5 to read 1V RMS on an Audio Millivoltmeter connected to the output with R7 rotated fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak amplitude on the oscilloscope.
• With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10KHz but R5 requires adjustment.
• High gain transistors are preferred for better performance.

Low-distortion Audio-range Oscillator

Generates very low-distortion sine waves up to 1V RMS
No thermistors required - No settling time

Producing low-distortion sine waves, this oscillator operates over the range 16 to 22000 Hz.
The circuit is based on two articles that have appeared earlier in Wireless World - Roger Rosens' "Phase -Shifting Oscillator", February 1982 pp. 38-41, and J. L. Linsley Hood's "Wien-Bridge Oscillator with low harmonic distortion" from May 1981 pp. 51-53.

This design features the simplicity of the Rosens' circuit but avoids the use of a thermistor. Instead, oscillator stability is controlled by means of a common photo-resistor driven by a LED, as suggested in the Linsley Hood article. There is no settling time when the oscillator's frequency is changed and no bouncing of the output waveform. Use of an expensive and sometimes difficult to obtain thermistor is avoided.


Parts:

• P1_____________10K Log. Potentiometer (Dual-ganged)
• P2______________2K2 Linear Potentiometer
• R1,R2,R4,R5_____3K3 1/4W Resistors
• R3,R6_________820R 1/4W Resistors
• R7_____________10K 1/2W Trimmer Cermet
• R8_____________22K 1/4W Resistor
• R9_____________Photo resistor (any type)
• R10_____________8K2 1/4W Resistor
• R11,R12,R14,R15_3K3 1/4W Resistors
• R13_____________2K7 1/4W Resistor
• R16--R20________3K3 1/4W Resistors
• R21____________56K 1/4W Resistor
• R22____________68K 1/4W Resistor
• R23_____________1K 1/4W Resistor
• C1,C6_________220pF 63V Polystyrene Capacitors
• C2,C7___________8n2 63V Polyester Capacitors
• C3,C8__________82nF 63V Polyester Capacitors
• C4,C9_________150nF 63V Polyester Capacitors
• C5,C10________680nF 63V Polyester Capacitors
• D1--D4______1N4148 75V 150mA Diodes
• D5_____________LED 5mm. Red
• IC1,IC2_____NE5532 Low noise Dual Op-amps
• IC3__________TL084 Quad BIFET Op-Amp
• SW1__________2 poles 3 ways rotary switch

Technical data:

Output voltage: Sine wave, 1V RMS max.
Total harmonic distortion @ 1V RMS output:

Frequency Reading
100Hz = 0.0035%
300Hz = 0.0028%
1kHz = 0.002 %
3kHz = 0.002 %
10kHz = 0.001 %

Notes:

• Any common photo-resistor and 5mm. red LED can be used, provided they are in close contact and enclosed in a light-proof small box. I used the metal screen of a small IF transformer for AM transistor radios sealed with black insulating tape.
• The 10K trimmer must be set to obtain a 1V RMS output.
• The circuit must be supplied by a + and - 15V dual regulated supply. Common 7815 and 7915 regulator ICs should be used for this purpose.

Simple Square wave Generator

Three switchable frequencies: 100Hz, 1KHz, 10KHz
1.5V battery operated, minimum parts counting

This simple circuit generates a good and stable 1V peak-to-peak square wave at 100Hz, 1KHz and 10KHz using a single 1.5V cell as power supply.

An useful feature of this circuit is that frequency changes can be obtained by switching only one capacitor at a time. Current consumption is about 600µA.

Parts:

• R1____________560K 1/4W Resistor
• R2____________680R 1/4W Resistor
• R3______________2K2 1/4W Resistor
• R4____________150K 1/4W Resistor
• C1_____________12nF 63V Polyester Capacitor
• C2______________1n2 63V Polyester Capacitor
• C3____________120pF 63V Polystyrene or ceramic Capacitor
• C4,C5__________10µF 25V Electrolytic Capacitors
• Q1,Q2________BC549C 25V 100mA NPN High-gain Low-noise Transistors
• SW1____________SPST Slider Switch
• SW2____________1 pole 3 ways Rotary Switch
• B1_____________1.5V Battery (AA or AAA cell etc.)

Notes:

• If a precise 50% duty-cycle is needed, trim R1 and monitor the output wave form by means of an oscilloscope.
• A good 500mV peak-to-peak square wave is provided even at 1V supply.

Self-powered Sine to Square wave Converter

Converts sine to square waves without a power-source
Useful as a test instrument for audio purposes

This circuit is intended to provide good square waves converting a sine wave picked-up from an existing generator. Its main feature consists in the fact that no power-source is needed: thus it can be simply connected between a sine wave generator and the device under test.

The input sine wave feeds a voltage doubler formed by C1, C2, D1 & D2 that powers the IC. IC1A amplifies the input sine wave, other inverters included in IC1 squaring the signal and delivering an output square wave of equal mark/space ratio and good rise and fall times through the entire 20Hz-20KHz range.


Parts:

• R1_____________1M 1/4W Resistor
• R2___________100K Linear Potentiometer
• C1,C2________100µF 25V Electrolytic Capacitors
• C3____________10nF 63V Polyester Capacitor
• D1,D2_______1N4148 75V 150mA Diodes
• IC1___________4069 Hex Inverter IC

Notes:

• Best performances are obtained with an input sine wave amplitude from 1V RMS onwards.
• Output square wave amplitude is proportional to input amplitude.
• Minimum sine wave input amplitude needed for good performance: 750mV RMS.
• Output square wave amplitude with 1V RMS input: 3V peak to peak, with R2 set at max.
• Minimum output square wave amplitude: 2V peak to peak, with R2 set at max.
• Substituting the two silicon diodes with germanium types (e.g. AA118, AA119), the minimum input threshold can be lowered.