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Extend Timer Range For The 555

Anyone who has designed circuits the utilization of the 555 timer chip will, at some time have wished that it may be programmed for longer timing periods. Timing periods better than a few minutes are difficult to reach as a result of component leakage presents in huge timing capacitors turn into vital. There is alternatively no motive to opt for a only digital answer simply but. The circuit proven here uses a 555 timer in the design but nonetheless succeed ins a timing interval of as a lot as an hour! The trick here is to feed the timing capacitor now not with a relentless voltage however with a pulsed dc voltage. The pulses are derived from the un smoothed low voltage output of the ability provide bridge rectifier.The power supply output is just now not referenced to earth possible and the pulsing full wave rectified sign is fed to the bottom of T1 by way of resistor R1. A 100-Hz sq. wave signal is produced on the collector of T1 as the transistor switches.

The sure 1 of 2 of this waveform charges up the timing capacitor C1 via D2 and P1. Diode D2 forestalls the charge on C1 from discharging thru T1 when the sq. wave signal goes low. Push-button S1 is used to start out the timing length. This methodology of charging uses reasonably low component values for P1 (2.2 MΩ) and C1 (100 to 200 µF) but reachs timing durations of up to an hour which is for much longer than a typical 555 circuit configuration.