PWM Using 555 Timer IC
Pulse-width modulation (PWM) using two 555 timer ICs is an ideal and easy way to control a motor which could be inside a drill, or a cassette tape recorder. The theory is that when you need pulses of a precise fixed frequency, yet variable width, then two 555 timer ICs are required. The first 555 IC is usually in an astable multivibrator configuration, behaving as an oscillator producing a continuous train of clock pulses of the desired frequency. Its output feeds the trigger pin of the second 555 IC in a monostable multivibrator configuration. This second stage adjusts the pulse width according to the "Speed Control" variable resistor. This way, the circuit produces width-modulated pulses but at a fixed frequency determined by the first IC.
The principle to understand here is that it is possible to feed the output pulses from the first 555 timer IC, to the trigger pin of the second timer IC. The astable and monostable stages are standard configurations without any changes.
Power Supply
Remember that the 555 IC operates between 4.5 V and 16 V, whilst 18 V is the absolute maximum. In addition, the output pin can source or sink up to 200 mA. These parameters determine the size of the motor you can drive directly by the 555-timer output pin. As you can see, it is not going to be a very powerful drill if the PWM circuit and the motor share a common supply rail.
For voltages larger than 16 V and higher currents, you may wish to use a power transistor such as a Darlington, then you can drive a bigger motor load that requires greater current and voltage. However, it will require its own power supply rail.
Example and Calculations
In this example, let us assume that the drill motor requires a fixed pulse rate of 10 Hz. In this case, the period 1/f, is 0.1 sec. In this design, I have decided that when varying the width of the pulse, the maximum value of the width would approach 0.1-seconds, but it can never be greater than or equal to 0.1-seconds. If it were to become greater, then frequency can no longer be a constant! The minimum width can be very small approaching zero, but can never be equal to zero or less than zero because that too would introduce instability, and practically the motor will not start!
ASTABLE Calculations - Finding C
Given Frequency of 10 Hz, RA=1000 Ω, and RB=100,000 Ω, calculate C1.
This is the standard formula for 555 astable circuit.
We transpose the formula for C1, which is the timing capacitor in my circuit diagram.
Using this standard formula, we get the period T, given the frequency 10 Hz.
We plug all the values into this formula to find C.
This is the intermediate stage working.
This is the answer.
MONOSTABLE Calculations - Finding R
Given timing capacitor C=150 nF, and T=0.1-seconds, find R.
This is the standard formula for 555 monostable circuit.
We transpose it to find the value of R.
We plug all the values into the formula.
This is the answer in ohms.
Considerations
When the potentiometer wiper (speed control) is at the extreme end, it poses zero ohms, which is an undesirable condition. If we look at this small fragment showing the internal circuit of the 555 IC, pin 7 is the discharge pin, which is an open collector of transistor Q14. The emitter of this transistor goes to ground, therefore if pin 7 were to connect directly to the positive voltage rail; it would short the power supply through the potentiometer wiper and in addition destroy the carbon track of the potentiometer. Hence, a 1 kΩ resistor prevents pin 7 from shorting to the positive rail.
Considering the monostable circuit formula T=1.1×R×C, mathematically, we can see that when R=zero ohms, the period T would become zero seconds, and since the frequency 1/T, becomes infinity, this brings instability within the circuit, hence the 1 kΩ resistor also prevents this condition. This was just a circuit idea though, and if you can help improve it then feel free to contact me.