This design utilizes an LM311 comparator as the active circuit element. Comparators are simply operational amplifiers optimised for switching (as opposed to linear) operation. In fact, any gain stage (eg. op-amp, logic gate, discrete transistors, etc.) could have been used.
The output of comparator U1 (pin 7) can assume two possible voltages, depending upon the state of its non-inverting (pin 2) and inverting (pin 3) inputs. The output is nearly at positive supply level when the voltage at the non-inverting input exceeds that at the inverting input, and nearly at ground when the input potentials reverse.
Resistors R1, R2 and R3 form a voltage divider between the positive supply, ground, and the comparator output. Both comparator inputs present a very high impedance, therefore pin 2 will not significantly affect voltage division. Because the DC supply and ground are fixed (we hope!), the divider network can produce only two possible voltages at the U1 non-inverting input. This voltage will depend upon the state of the U1 output such that when pin 7 changes state, pin 2 will step up or down by a voltage equal to 1/3 of the change at pin 7 (i.e. between roughly 1/3 and 2/3 of the DC supply voltage).
A change of state at pin 7 will also produce a change at the inverting input, although not immediately. The voltage at pin 3 will exponentially approach that at pin 7, with a time constant determined by capacitor C1 and either R5+R4 or R5 alone. If pin 7 is high, diode D1 will be reverse biased and C1 will charge through R4+R5; if pin 7 is low, D1 will conduct and C1 will discharge through R5+D1. A forward biased diode has very low effective resistance, so a much shorter time constant results.
Oscillation occurs because the U1 output state is always that which will cause C1 (at U1 pin 3) to charge or discharge towards the voltage at pin 2. When the voltage ramp at pin 3 crosses the fixed voltage at pin 2, the comparator changes state and the process begins again in the other direction. The negative feedback loop delay determines the timing of each transition.
For example, if pin 7 is at 15V, pin 2 will be at 10V and pin 3 will be climbing exponentially towards 15V. However, when pin 3 reaches 10V the comparator changes state. Pin 7 drops to 0V, pin 2 becomes 5V, and C1 (at pin 3) begins to discharge from 10V towards ground. This continues only until pin 3 reaches 5V, when the comparator once again changes state.
The frequency of oscillation can be changed by adjusting the C1xR4 product. The duty cycle (ratio of on time to period of oscillation) may be altered by varying the R5/R4 ratio. Note that without diode D1 the duty cycle will always be 50%.
The comparator has an open collector output. That is, pin 7 is simply the collector of an NPN transistor (the emitter is connected to ground at pin 1). R6 is a pull-up resistor which supplies the current required to permit pin 7 to (nearly) reach the positive supply voltage when the NPN is off (output high state). When the NPN is on (output low state) current flows through R7 to illuminate light emitting diode D2. The value of R7 therefore controls the LED brightness.
C2 is a power supply bypass capacitor which presents a low impedence to high frequency transients on the positive supply, thereby bypassing them to ground and minimising adverse effects on comparator operation. Bypass capacitors are normally used at all integrated circuit power supply pins but, for simplicity, they are sometimes omitted from the circuit diagram.
ANU Engineering - ENGN2211