Extend Timer Range For The 555


Posted on Feb 4, 2014

Anyone who has designed circuits using the 555 timer chip will, at some time have wished that it could be programmed for longer timing periods. Timing periods greater than a few minutes are difficult to achieve because component leakage currents in large timing capacitors become significant. There is however no reason to opt for a purely digital s


Extend Timer Range For The 555
Click here to download the full size of the above Circuit.

olution just yet. The circuit shown here uses a 555 timer in the design but nevertheless achieves a timing interval of up to an hour! The trick here is to feed the timing capacitor not with a constant voltage but with a pulsed dc voltage. The pulses are derived from the un smoothed low voltage output of the power supply bridge rectifier. The power supply output is not referenced to earth potential and the pulsing full wave rectified signal is fed to the base of T1 via resistor R1. A 100-Hz square wave signal is produced on the collector of T1 as the transistor switches. The positive half of this waveform charges up the timing capacitor C1 via D2 and P1. Diode D2 prevents the charge on C1 from discharging through T1 when the square wave signal goes low. Push-button S1 is used to start the timing period. This method of charging uses relatively low component values for P1 (2. 2 M) and C1 (100 to 200 µF) but achieves timing periods of up to an hour which is much longer than a standard 555 circuit configuration.




Leave Comment

characters left:

New Circuits

.

 


Popular Circuits

10MHz DDS Sine/Square Function Generator AD9835
Class-A audio Amplifier circuit
Frequency comparator II
Responsive Wearable Technology
Saturating a N-Channel MOSFET
solar watering system
video signal amplifier circuit diagram
12 volt switching power supply using LTC3810-5 switching regulator
3-6V Xenon Strobe Light
High-performance quadrature sine wave type oscillator circuit
A long light power control circuit
Random password keyboard circuit



Top