Timer555

Since early 1971, the 555 remains the most versatile timer ic. It is possible to consult the large amount of literature online, to understand his functions. But I would provide here a synthetical one: At powe on, C2 ensure the internal Flip Flop reset, who brings the output port to be High. Meanwhile, C1 begins to charge through D2 and the R2+RV2 series. Once the 2/3 Vcc threshold is reached, the upper comparator sets R high, making the FF to switch. At this point the output goes Low, and the internal NPN transistor enters in conduction state, making C1 to discharge through D1 and R1+RV1 series. The discharge continues until the C1 voltage decreases under the 1/3 Vcc threshold. Now the lower comparator sets S high, restoring the initial conditions.

With the exception of fist charge phase, that begins from 0 volts (the next steps begins from 1/3 Vcc), the times may be calculated using the following formulas: Ton = 0,693*C1*(R4+R2+RV2), Toff = 0,693*C1*(R1+ RV1). They can be approximated, when R4 ≪ R+RV, to the following: T = 0,000693*C1*(1+R+RV) adopting Kohm e uF units. The table below supplies some values to be used for times between 7 and 700 seconds. Share the total resistance between the fixed and the variable resistors to obtain the desired intervals. Approximate to the upper value for commercial trimmers, to avoid intervals cutting.

In our case the values choosen for C1 and the resistors make the timer to work in the following time intervals:
Ton : 15 – 110 s
Toff : 90 – 330 s

It may occurs a Relè with NO and NC pin inverted. In this case use the following layout:

This version has no time variation due to compoennts' temperature. The circuit adopts a microcontroller, and uses its precise internal clock to manage the timing. The times setting occurs at every phase reading the trimmers levels vua two 10 bit A/D channels. It makes possible to adjust the timing during operations. The maximal times are set via code to 2 (Ton) and 10 (Toff) minutes, but the code is supplied in order to allow the user to change the values if needed.

/***********************************************************************************

     Cyclic timer
     Ton and Toff setting uses two trimmers via ADC pins. See schematic

     MCU:             PIC12F675
     Oscillator:      Internal, 4.000 MHz
     Compilator:      mikroC v8.2.0.0

     Author: stefano.purchiaroni@email.it

     Changes log:
     - 03.08.2017 : Creation of the program

***********************************************************************************/

#define TonMax     120     // Max ON state time (s)
#define ToffMax    600     // Max OFF state time (s)

void SetOut(int outval, int tmax, int channel) {
// Set output according to specified value for a time got from the indicated analog channel
int i, t;
GPIO.F2 = outval;
t = ((long)Adc_Read(channel) * tmax) / 1024;
for (i=0;i<=t;i++) Delay_ms(1000);
}

void main() {
CMCON = 0b00000111;     // Comparator module off
ANSEL = 0b00001010;     // Configure ANalog pins (1=An)
TRISIO = 0b11111011;     // Configure pins directions (0=Out)
do {                     // Endless loop
    SetOut(1,TonMax,3);    // Turn output on and wait Ton=AN3 seconds
    SetOut(0,ToffMax,1);   // Turn output off and wait Toff=AN1 seconds
} while(1);
}

A friend needed to time the crib's bells. So I realised this other version employing a 12F683 and a BD139 transistor in place of the relay. Also the trimmers are two smaller ones, to minimize pcb size to about 2.5 centimeters side. The load shall remain under 0.5 Ampere (1 A max, using an heat sink) and the power shall stay under 24V. Click here to download the Hex file to load on the PIC12F683.

Cyclic timer with independent On Off times regulations