This astable timer circuit is a simple square wave generator based on a CMOS version of the 555 timer that can maintain a nearly constant 50% duty cycle (+/- 1.0%) over a wide frequency range. This requires a transistor and a diode in addition to the other timing resistors and capacitor normally used.
HOW IT WORKS:
When the timer’s output is high, transistor (Q1) is forced into saturation (transistor on) by resistor (R2, 5.1k). This results in current passing through (Q1) and also (R1 + 3.3k resistor) to charge timing capacitor (C1).
When the output goes low, the discharge transistor, (open collector, internal to the 555 at pin 7) cuts off the base of Q1 (transistor off) and allows the timing capacitor to discharge through (R1 + 3.3k resistor) and diode (D1).
Since the resistance path is equal (or nearly so) in both the charging and discharging paths, the timing periods of both the high and low outputs are also equal.
Although a regular bi-polar 555 timer can be used with good results at lower frequencies, I have found that using a CMOS 555 gives much better results over a wider and higher frequency range.
Transistor (Q1) can be any general purpose NPN transistor such as a 2N3904. Diode (D1) can be a 1N4148 or 1N914.
Resistor (R2) should be a value between 1.5k and 10k. The exact value of (R2) can be selected to give the best duty cycle symmetry with the timer running at maximum frequency. Maximum frequency is about 650 KHz (dependent on value of timing capacitor) .
The timing capacitor should be a quality Mica, Mylar or Tantalum type. Minimum timing capacitor value is about 100 pF.