0 Extend Timer Range For The 555

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 solution 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.

 Extend Timer Range Circuit 

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 MΩ) 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.

Circuit Source:  CircuitsProject

Detail: Extend Timer Range For The 555

0 Real Time Clock Using the PIC16CXXX

A very simple real time clock electronic project can be designed using the PIC16CXXX microcontroller family , designed by Microchip Technology . This real time clock electronic project uses the Timer1 module, from a mid-range PIC16CXXX microcontroller, to control a low-power real-time clock. Timer1 was chosen because it has its own crystal which allows the module to operate during sleep.

Upon power-up, the device is initialized with the display starting at 12:00 PM, and Timer1 is configured to generate an interrupt (every second). The Timer1 overflow interrupt wakes the device from sleep. This causes the time registers (HRS, MIN, SECS) to be updated. If the SECS register contains an even value (SECS<0> = 0), the colon (":") is not displayed. This gives a visual indication for each second. Then the device returns to sleep.

Real Time Clock Circuit Diagram

For setting the clock are used three keys : SELECT_UNITS Key (S1) selects which units are to be modified (hours, minutes, off), the INC Key (S2) increments the selected units and CLR_MIN Key (S3) clears the minutes and seconds (useful for exactly setting the time ) .

This simplify design use a standard Hitachi LCD display module and some other electronic parts .

The RA2:RA0 pins are the control signals to the LCD display, RB3:RB0 acts as a 4-bit data bus, and RB7:RB5 are the input switches. The OSC1 pin is connected to an RC network, which generates an approximate 4 MHz device frequency. Because Timer1 operates asynchronously to the device, the device's oscillator can be configured for RC mode.

Timer1’s crystal is connected to the T1OSI and T1OSO pins. A good choice for a crystal is a 32.786 kHz (watch) crystal.

This electronic project and source code was designed by Mark Palmer Microchip Technology Inc.

 Download Source code

Detail: Real Time Clock Using the PIC16CXXX