There are several applications at home or industry for long delay timers (ON or OFF delay). Time delays on the order of seconds can be generated using 555 timer circuits, provided the timing capacitor values do not exceed the limit specified by 555 datasheets. When the time delay needed goes to minutes and hours, these circuits will not help.
Figure 1’s circuit can generate time delay of seconds to hours without using high value capacitors. This simple, inexpensive circuit is basically an oscillator, divide by 4096 counter and followed by another divide by 10 circuit. The equation below can be used to obtain the time delay, T, in seconds.
Where F is the U1 (555) oscillator frequency in Hz.
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| Figure 1. | The above circuit produces an OFF delay of 30 minutes. The switch SW1 can be thrown on the other side at starting to get a 30 minute ON delay. The capacitor C2 can be changed to get various timings. The required load can be connected in place of LED D1. |
The circuit draws very little current and is therefore suitable for battery powered gadgets.
Circuit in Figure 1 generates a time delay of 25 minutes for the component values selected. In one position of switch SW1, an “ON” delay is generated. In another position, an “OFF” delay is generated. The load can be connected between Q1 and the power supply (across LED + R6), which can be switched “ON” or “OFF” after the time delay. U1 (555) is connected as an astable multivibrator where its output is given to U3 (4020 counter), its Q12 output is fed to U4 (4017 counter and decoder). U4’s Q9 output goes “High” (up until this point, it was “LOW”), after receiving 10 cycles from Q12 of U3. It is inverted by U2C and given to U2A. This “LOW” output inhibits pulses reaching U3 causing the counters to stop after the required time delay. The Q9 output of U4 and output of U2C are connected to the SW1 switch to select an “ON” delay or “OFF” delay mode. In “ON” delay mode, the load/LED gets energized after the time delay. In “OFF” delay mode, the load / LED is energized as soon as timer is switched “ON” and gets de-energized after the time delay.
