Timer 555
Better timing

Fig. 10 Better Timing:

Better and more stable timing output is created with the addition of a transistor and a diode to the R-C timing network. The frequency can be varied over a wide range while maintaining a constant 50% duty-cycle.

When the output is high, the transistor is biased into saturation by R2 so that the charging current passes through the transistor and R1 to C.

When the output goes low, the discharge transistor (pin 7) cuts off the transistor and discharges the capacitor through R1 and the diode. The high & low periods are equal. T

he value of the capacitor (C) and the resistor (R1 or potmeter) is not given. It is a mere example of how to do it and the values are pending on the type of application, so choose your own values.

The diode can be any small signal diode like the NTE519, 1N4148, 1N914 or 1N3063, but a high conductance Germanium or Schottky type for the diode will minimize the diode voltage drops in the transistor and diode.

However, the transistor should have a high beta so that R2 can be large and still cause the transistor to saturate. The transistor can be a TUN (europe), NTE123, 2N3569 and most others.